Once again, Need way more information.

Got pictures?
What type, make and model of engine?
What prompted the reconditioning?

Were you present during the running in?
Please Describe the running in procedure.
Was there always oil in the sump?
Were there issues with the cooling system, and Did it overheat?

Was any non-destructive testing (crack checking) done on the rod(s) and piston(s) ?

What was the torque value used on the conrod bolts?
What were the bearing clearances, piston clearance, and piston ring end gaps?
What size rings did you install?

Engine is MGA Twin Cam 1958 built
Recon due to holed piston.
Alas I was not present at running in!
Breakage was sudden after leaving traffic lights and very sudden.
This coincides with the clean oil, no small debris. Sump was full. Water OK, no overheating.
No testing of rod. New OEM rod bolts 3/8" UNF Torque 55 lbs/ft(6.9 Kgm) Bolts are 55 Ton tensile .
Rod bolts were snapped trough straight,no sign of bending before breakage.
Bearings were free of seizure and running marks.
Pistons are NOS, clearance .004".No Marks on the sides,top two rings still on and more or less undamaged.
To me these are sure signs of overrevving but I'd want advice before confronting.
Thks,Dirk

It's a tractor engine, not meant for racing.

Is the owner a fanatic about matching numbers and such?

If not, maybe you can talk him into something more modern,
like a hopped up Pinto engine, or a Japanese four.
On balance, a stronger engine is guaranteed to break
something in the driveline, and upgrading that will
allow the engine to break something else, etc.

Maybe you can talk him into installing a rev limiter,
just to curb the occasional burst of enthusiasm.

Mike Halloran
Pembroke Pines, FL, USA

Ouch.

I'd like to see more close ups of the rod bolts, especially the fracture surfaces.

Thanks for the pictures.

The 55 lb/ft installation torque you note seems high for a 3/8-24 UNF bolt having 110 ksi UTS, especially if the threads were lubed at assy. You might want to do a quick calculation to check if the combined tension and torsion forces at installation would be enough to overstress these bolts.

http://mgaguru.com/mgtech/books/tdb/tdtc_b_engine....
second page says 600 lb-in for stock twin cam rod bolts, but with no mention of bolt size, pitch, or grade.

110 ksi UTS is more like a grade 5 bolt spec.
Hardness tests and some tightening experiments of surviving bolts, preceded and followed by length checks might help understand if the bolts were adequate despite the claimed UTS.

I'm also concerned that the bolts might have bottomed in the blind holes.

110 ksi UTS is not even grade 5. I did a quick check for 55 lb/ft torque on a 3/8-24 UNF bolt with lubed threads and it exceeded 110 ksi at the thread root. Given that both bolts appear to have experienced similar failure modes, where they parted right at the thread root section closest to the mating internal thread, and right where you would expect to see a stress concentration, it seems like this might be the result of excessive preload.

Hi all,
first thks. for reactions.
I tried to make some closer fotographs.
No bottoming,holes go straight through.Torque is factory one.New ones are not marked,an old one shows 55T65;I assume this is 55t/sq.in.,which is not bad.The conrod and piston assy is quite heavy;combined with a long throw(89 mm)will put a lot of pull when overrevving.MGA twin cam conrod bolts were a failure cause in the past for race engines.Would have loved to put ARP in but they do not make them in 2".
Dirk

Grade 8 3/8 UNF recommended torque is 49 lb ft, dry, half that lubricated. Grade 8 bolts are 75 tons per sq inch. You fitted 55 tons per sq inch bolts and then torqued them to 55 lb ft.

They broke.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Actually it was 50 Lbs(I used the 6.9 Kgm setting)Bolt was dry as out of pack.
I cannot believe that using the factory spec would cause this.I will try some of the remaining bolts for strech/breakage
Dirk

Well,I did some tests with oiled bolts:
Static length was 61.47 mm,
At 70 LBS/ft 16.59(overloaded 20 lbs/ft to spec.
At 80 Lbs(serious overload)61.62 mm
When released, bolt measured 61.49,just within elongation spec?
At 90 Lbs/ft,bolt broke. Other one broke at 85 when torqued up slowly,showing narrowing and torque damage

I showed the bolts to a metallurgist and he immediately used the words: IMPACT SHEAR, without knowing what happened.
I always wondered why the conrod bolts would have gone first,as there is clear evidence of the piston broken in tension.
Shure enough,some searching on the internet brought the following: "Impact shear of conrod bolts often happens when there is a blockage of the rod in an engine due to breakage of other components,like camshaft."
This leads to the following:first the piston broke in tension(ewhaust phase). Top of piston is thrown against head,valve blocage and hence OHC broke.Next crank throws rod against remains of lower side of piston,smashing it to pieces and bolts break on impact.
I think this is a more logical line of thought.
Dirk

If I was betting on it, I'd bet the rod bolts broke and then the rod was twisted when the crank came around and hit it which smashed the piston.

I do agree with the others that using 50 lb-ft is too high for a 55 ton-force or 110 ksi bolt.

I'm in the "piston failure" camp. Probably put it back in first gear instead of third during the traffic light drags.

je suis charlie

Just a couple other things I noted.

The pictures of the rod cap parting faces seem to show a fair amount of fretting. This would indicate they were separating and smacking back together for more than just a couple cycles. If the engine was over revved the fretting may have occurred after the bolts yielded from inertia loading. Or it may have been the result of the cap not being clamped tightly during assembly. As Tmoose noted, it is possible the bolts may have bottomed out in the blind tapped holes of the rod beam leaving a gap at the rod split face. Even a small gap between the cap and beam faces could produce significant impact loads on the rod bolts around TDC of the exhaust stroke.

The picture of the piston shows excessive scuffing wear at the inboard edges of the wrist pin bores. This is not normal wear and indicates significant bending of the wrist pin and/or piston structure along the bore axis, prior to the fracture failure.

I have had this before on one of my own engines,with Carillo rods,ARP bolts. Piston was Powermax from Triumph Bonneville.
Thought I was in 4 th. gear,but I was in 3rd! Although much lighter piston and rod,the piston broke in two exactly the same.
Rod was twisted also the same,bolts (ARP2000) did not break.Block was also ruined. That was at around 9000 rpm.....
Dirk

Need to see all the parts.

BMC B block - rod cap is angled, not a horizontal cut. Rods not fitted backwards by chance? This would put tension on the rod bolts on the power stroke.

I fail to understand why most of You want to point to the rod bolts.
1)OEM bolts,torqued to factory figure.
2)No bottoming of bolts,angled right way.
3)How do You explain that the piston broke in half after the rod let go as there is minimal damage to the cyl. head and top of piston.
If the rod let go first there should a lot of damage to the top of the piston and cyl. head
4) The top part of the piston shows no damage by the rod smashing into it,break line is practically untouched.
5)It is the bottom part of the piston that caused the blockage of the rod.(Nothing left,gone in smitheriens)
I'll have some left over bolts further tested and will keep You all posted.
Dirk

Are the other 3 pistons intact?
I'd be inclined to Zyglo or dye penetrant check them.

Could you post close up pictures of all the ring end gaps?

The reason we look at the bolts is you claimed they were "Bolts are 55 Ton tensile", torque specs for which are readily available. If they aren't 55 ton material, then sure there could well be other explanations.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

I assumed they were 55 t/sq in. as I said the OLD ones were marked 55T65.This being old English markings I am going to send the remaining ones to be tested further to make sure.
Dirk

Alas cannot get close enough for the rings but no signs of touching neither top or second one.I chequed the gaps which were all within.009-.011"
which is right for 3" bore.I included foto's of the top and side of piston and of the block inside.
Bottom of side opposite of thrust is gone,nearly no marks on cylinder upwards.
Dirk

Looks like it is not an uncommon problem with these engines (I assume the correct octane rated fuel was used as it is no longer easily available in the country of the engine's birth?)

http://content.time.com/time/specials/2007/article...

I have seen failed con-rod bolts due to one not being clamped the same as the other one - it's the correctly loaded one that fails in fatigue due to the "flexing" of the joint. Fatigue evidence was always very evident though.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

The rod appears to be an asymmetric design. I have seen one example where a similar rod was installed in the wrong direction and put into service! (an Allis-Chalmers WC)

Dirk, sorry to see the damage...have had a lot of experience with these engines ..I am that old .........from new, they were susceptable to breaking. question time ....
Those pistons do not look new
There appear to be scuff marks that might indicate partial seizure
The con rod bolts were they new, is the source of the bolts known and reliable.. they do not look new.
At first glance at pics I said " over tightened" how ever when some of those very bright contributors started and then I looked at the rest of the excellent photos....how old are the con rods.
What was the compression ratio?
I ask these questions based on my own experience when these engines were new ... did they give trouble, and all the symptoms you indicate we had then [1960]. C/R for the fuel available at the time a problem. Detonation, holes in the piston. I was very surprised to see what appeared to be an original lock tab for the big end bolts!! This was a big no no in our books from a long time back. The theory being that the lock tabs were made from mild steel and would compress when the bolts were torqued and would then relax a little releasing the preload on the bolt. Don,t know if any of this helps hope you get to the bottom of it. If money is not a problem Carrillo type rods, modern day pistons, steel main caps, that would be the way for me.
All the best Golfpin

Look at the wrist pin journal/hole on the left side of the piston. Why are there score marks worn in on the outer side of the left wrist pin circlip groove?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Lets say you made a mistake that lots of people make and either didnt seat the wrist pin circlip 100% properly, or you reused an old circlip, or just completely forgot it.

The wrist pin would float to the left side = flex a lot more because the other side isn't constraining flex like it should = leave scoring. Not only that, but it would fatigue the side that cant distribute the load throughout itself evenly because the pin is not riding completely in it. Once it has stress fractures, it will break on the part of the hole that has the least amount of material strengthening it. After that, the pin is going to violently destroy the other side and then smash a bunch of stuff.

Plausible?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

It is usually pretty obvious when a circlip is pushed out of its groove by the pin.

je suis charlie

gruntguru - What does it look like if no circlip is put in at all? I have never personally seen that mistake happen. But I have also never seen brand new factory spec bolts sheared off that bad, or a brand new piston completely grenade from an over rev. If this had happened because of such an extreme over rev, how do the other cylinders in the engine look? The wear marks on the wrist pin holes seem worthy of investigation, especially the one outboard of the circlip groove! Id be more apt to ignore those wear marks if they didn't happen on the most dramatic failure point. Those wear marks were caused by something

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Did you find both circlips or at least evidence of both circlips in the wreckage?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

If a circlip was omitted, there would be telltale marks on the relevant face of the bore where the pin would have done some rubbing - long before the failure. Possibly difficult to identify amongst all the other bore damage.

je suis charlie

By the time the wrist pin shifted that far, it would not take much to rip it out of the piston

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Looking at the piston rings,although they could have moved after the unfortunate event,as you may know already Dirk,piston ring gaps are positoned 120 degrees from each other

The position of the ring gaps means nothing. Rings rotate during engine operation at different rates.

Hi all,
Back from a trip. Thks for the follow up.
to Panther 140:No marks from wrist pin on side of cyl.Other cylinders are perfect.
I can only repeat that a had this sort of breakage before with a Powermax piston.Rod bolts were ARP,so did not break,but the piston was broken in two at exactly the same place.Such shear breakage will occur in the exhaust phase(no compression to hold back the piston)
It was also the exhaust cam that was blocked!
Further small damages occurred when the driver tried to restart the engine before realising what happened.
Looking at the piston underside I'd think this is also showing clear shear!I am not a specialist but the grainy structure shows it.
Dirk

dirk489-

I looked over all of the (very nice) photos you provided of the failed piston, and I saw no indication of a shear-type failure. The only thing I could see in the photos seemed to indicate tensile type fractures that originated at the wrist pin bore edges.

Tbuelna,You are right:being a bloody foreigner,I meant tensile instead of shear...I still think the piston broke first as there is not so much damage to the fracture. The bottom half of the piston was smashed into smitheriens.It must have got blocked in the bottom part of the cylinder somehow to cause the rod to bend. Why the rod bolts broke then is still a question to me.
Dirk

I don't think it would be easy to bend a rod 90 deg like that without breaking the bolts.

je suis charlie

What is your theory for those asymmetric markings on the wrist pin bores?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

If the writs pin slid over, the tension on the piston would stay the same, but be much more focused on the aluminum surrounding the inside half of the bore. So each side of the piston would still get 50% of the tension, but the right side would only be using 25% of the strength it could be using. Then that side breaks and gives the other side 100% of the tension which would make it fracture more dramatically

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

With the claim the retaining clips were in place and with the wrist pin not contacting the bore, it makes one wonder if the wrist pins were simply too short.

I went to my garage for another check on the block:there are no scratches or marks at all on the cyl. wall,exept honing marks.
As for the asymmetrical marks:the owner tried to restart the engine to see what was wrong;there was a lot of debris that could have been touching.Most of the bottom half of the piston was thrown out trough the big hole...Piston pin was OEM new,looks right length to me.
Dirk

have you contacted the piston manufacturer? Do any of the other pistons have wear marks in the writs pin bores even between the clips?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Piston is Hepolite,made in 1958 and NOS. Difficult to make a claim...
No marks on the other pin bores,I measured the bore with a tripod mike, no deviations,wrist pins are quite heavy,very little movement axially possible even without circlips.

Are the pistons marked "Heplex" on the inside?

Hi Tmoose,
Pistons are marked in the four corners:
Hepolite
AHH402
641
AEHF59

have any of the other pistons been sent out to be analyzed? A brand new piston should not abruptly fail under any RPM that the engine can rev to on its own

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Based on the amount of damage shown in the photos to the block/conrods/pistons/etc, I can't imagine the engine is worth salvaging, unless it is an extremely rare model. Fixing the massive hole in the side of the block would require extensive weld repair and re-machining of all close tolerance features. The picture of the block appears to show the cylinder bores have already been over-bored and sleeved. And given how badly the conrod was deformed, it seems likely the crankshaft was also bent out of shape.

While this is an interesting topic of discussion, I think the only course of action is to find serviceable replacement parts.

tbuelna we are focused on finding a root cause for this failure! We need resolution of some sort. Mail me the parts please

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Just wondering,as the rod is angled,and the piston broke first,whether there could not be shear implicated.
When the rod is blocked there would be side pressure on the cap lips as well.They are not designed to take those high loads and could move a bit sideways to create shear on the rod bolts.I will examine the cap's lips again.
I talked to some other guys and the piston breakage on those older engines is not uncomman when overreving.

Dirk

i have experienced this problem. Root cause was casting sand which had congealed and remained in the piston internally. Worked loose and cause grinding effect on liner , eventually seizing piston on TDC and separating at gudgeon pin area.

Offshore Engineering&Design

What motor oil was used?
And how about the wrist pin? Any pics of it?
If it was going to be hot rodded should have used some good forged pistons. Is that an inclusion right hand upper
wrist pin hole? I think it was a bad material-casting problem.

Hi all,

Dicer: 20/50 oil,no trace of the wrist pin. Might have thrown out with the debris of the underside of the piston.

I have some pictures of a similar engine desaster,also on cyL nr.2.
Rods are Phoenix,bolts ARP 2000 at 45 lbs.ft. Pistons were a lot lighter(Triumph Bonneville Powermax) and also broke top/bottom in two.
Rod being 1/2" longer went in the waterways.Happened at about 9000 revs.

• http://files.engineering.com/getfile.aspx?folder=2937f9d3-6be8-4d79-8161-e5

In both cases were the pistons cast? What kind of load were these engines under when this happened? 9K free rev?

Both pistons cast.The 9000 one I did myself.... Thought I was in 4th gear but was in 3rd.With the longer and lighter rods the engine is much smoother.I noticed something was wrong when I saw the rev needle pass the ignition light......After that it was silence.

The first engine was in 1st gear as he was driving away from the lights and trying to pass left before another arriving car.
The MGA Twin cam was known for easy overrevving .
Dirk

"The 55 lb/ft installation torque you note seems high for a 3/8-24 UNF bolt having 110 ksi UTS, especially if the threads were lubed at assy. You might want to do a quick calculation to check if the combined tension and torsion forces at installation would be enough to overstress these bolts. "

I put this torque (55 LBf-ft onto 3/8-24 stud/nut) in my thread stress software, inputting nut to base friction coef = .18, nut to stud threads coef = 0.18, (lubricated threads and nut seat)comes up with stud princ. stress = 136ksi! Not lubed, friction coef = 0.3, gives less than 100ksi stress.

To FredRosse:I mentioned 45 ,lubed as given by manufacturer of rods,and I think ARP as well.I have a racer with Cunningham rods,same spec.
Dirk

Your close up of the failed bolts clearly shows that one of them has the tell tale "beach marks" indicating that it failed over more than one cycle and not catastrophic like the other. That bolt fracture went catastrophic after about 3 cycles based on the beach marks and the other bolt is a pure catastrophic tension break, likely from the rod cap leveraging it in tension after the first bolt broke. The first bolt that broke also has a pulled thread and a slight lean opposite the failure origin (center OD of 1st beach mark) indicating that the cap was bending about the remaining bolt, causing the extreme tension that failed the second bolt catastrophically. BUT, pictures alone can be a bit deceptive.

Hi

I have followed this thread with interest for quite awhile now and one thing that caught my eye is that the original bolts were torqued dry to 6.9kgm, well torquing bolts dry can leave a huge variation in bolt preload +/- 25%, so is it possible that preload variation allowed one bolt to see more of the load than the other?
If so this might well support the theory of the last poster who suggested one bolt failed first in fatigue and subsequently followed by the second bolt.

Just a thought.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

The reasoning is right,with the exception that the bolts were new and engine only did 50 miles in total. I would not expect fatigue marks on an engine which is basically still running in?I gave the rod to the University lab and they will examine it next term. Will keep You posted then.
Dirk

hi dirk

Thanks for the response.

I don't think something being new would exclude it from failing in fatigue and besides how many cycles would the piston do in 50miles, it may also be a small flaw in the bolt material itself which wasn't detected.
Anyway it will be interesting to see what the university says.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

There is something called "low cycle fatigue". And 50 miles of driving could easily put 1x10^5 or more fatigue load cycles on the rod bolts. If the failed bolt had a defect, was over-stressed at installation, or was over-stressed during operation, it could easily have failed in low cycle fatigue.

I have approx. 10 years in building engine's as a full time career at 5 different facilities covering oem rebuilds to 1000Hp forced induction applications. This would be the first time I've seen an over-tqd rod bolt cause a chain of events to damage a piston like that. I am curious how close the crankshaft counterweights come to the bottom of these pistons. Older engines had heavy piston castings and longer than average strokes. This is what mainly seemed to determine their connecting rod length, which varies a lot within engine models year to year and displacement sizes. This combination doesn't allow for much rod twist/bend, bearing play, or even part combinations before the bottom of the piston is contacting the crank counterweights and fracturing the bottom of the pin bores while in turn putting the rod bolts in excessive tension.
The only assumption I can make here is the rod bolts broke after the counterweights fractured the piston. The g-forces on a heavy piston are high and at TDC I'll bet the pin was pulled through the bottom of its bore after it was weakened. However the rod bolts broke after that is anyone's guess.

Also did you measure the cylinder bore diameter and out-of-round? I can't see the thrust surfaces of the cylinder well in the block pic. Are they scuffed at the bottom as bad as the piston is near the second ring land?
The most difficult task with cylinder honing is preventing taper in the cylinder bore. Due to main webbing, the honing stones are limited on their travel beyond the bottom of the cylinder, and if the stones used had previously broken from contact on another job, taper becomes very hard to control. Many of the engines I've seen done by other shops have excessive cylinder taper, but this would be the first time I've seen a piston fracture as a result.

This is not a con-rod bolt problem, it is only a typical problem with cast piston broken in pin support area.
With cast piston you cannot have the control of metal uniformity, you don't have control of how it was cast and how it was cooled.
I have 50 years in racing engine, racing, building engines and as tech scrutiner and I have seen many times this problem with cast pistons.
Use only forged piston, and rarely you will see a piston broken this way, they cost more but a broken engine like this cost many times more.

flavioct
I fully agree with you. Me too about 50 years playing with ICR engines of all types and sizes.

I wish we knew if there were manufacturing defects, or had some data from the engine leading up to the events. Did you send them the other rods and pistons to be analyzed?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

The alloys "ageing" (example Al alloys) are structurally unstable, they are in a state of transition towards a stable state of equilibrium.
Consequently, in the conditions of sliding viscous, it is likely that the diffusion of atoms, increased by temperature and stress, involves the reactivation of aging process, resulting in a fast reduction of mechanical strength and brittleness.
Remember this cast piston is only 57 years old

What kind of fuel do you run in an engine from 1958? If I remember correctly, thats a relatively high compression engine, is it not?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

To Panther 140:the rod and piston are at the University as a project,will get results in april/may.
Investigation of the other rods showed that the thread was torn out partially on several of them.
That can only happen when a very heavy pulling load was applied,which makes the overevving plausible.
These engines run at a 9.9 compression ratio,works just OK with 99 octane fuel ,30 ° advance.
The race engines I built have Cunningham conrods and Arias pistons,1680 cc giving 163 Din BHP at engine(120 KW)at 7500 rpm.
Dirk

Interesting, glad they got back to you with more information. I wonder what sort of hardening/annealing processes (intentional or not) happened to the bolts throughout their life

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Well,the proposed project wasn't taken up by one of the students. Pity, as the lecturer found it interesting.He will keep the parts ,maybe later on someone will take it up.
Thanks all for the interesting comments.
Dirk

I'd be very interested to hear what the University determines in this matter. I am not a metallurgists nor do I pretend to be one on TV... but a number of years ago I had an opportunity to examine some (new) connecting rod bolts that were sheared off within minutes of starting a recently rebuilt engine. Several metallurgical labs examined the parts as well as a University. As indicated by someone above, each independent reviewer knew immediately that the bolts failed due to impact shear. I believe they refer to the "image" or "signature" on the broken pieces as a "thumbnail". Even though the engine builder claimed the bolts were "new defective", the impact signature and no metallurgical defects in the fractured bolts, strongly suggested the tightening torque was insufficient allowing the bolts to loosen and impact the block, shearing them off.

In regards to the 55 ft/lbs. tightening torque, normally I too would agree that 55 lbs. is excessive for a 3/8-24 thread bolt, however there are many variables and that is why when possible bolt stretch is used to preload the bolts instead of rotational torque, which is simply a measurement of rotational resistance, not of clamp force/tension. Obviously in this case using bolt stretch is not possible with a blind hole. It is possible as noted by one person here who modeled the bolt, that 55 ft/lbs. would not exceed the stated tensile strength or hopefully the yield strength unless designed to do so. As much as 60% of the tightening torque of a lubricated bolt can be required merely to overcome the friction between the threads and bolt head contact surface. Thus a relatively small percentage of the tightening torque ends up tensioning the bolt. In addition slippery lubes like moly / cam lubes can require as much as 30% lower tightening torque than regular petroleum engine oil as they lower the coefficient of friction. Since the factory specifies 55 ft/lbs. torque on these oiled bolts, assuming an accurate torque wrench was used, then there is no reason why 55 ft/lbs. should have caused the bolts to fail. If the bolts were over-tighten I'd expect to see necking down of the shank/threads at the fracture, which doesn't appear to be the case from what I can see in the photos.

Unfortunately cast pistons have very unpredictable durability in many applications. In determining which came first the broken piston or the broken rod bolts, I'd be willing to venture the piston failed, blocking the rod and shearing the bolts off with the clear "signature" pattern that metallurgist all seem to recognize. It should be interesting to see what the University experts are able to determine.

Hi,

Very interesting topic.

On these older engine the pistons, con-rods & caps, were just put together, off the assembly lines bench tested off into cars.
During my time racing and tuning in 60's, every engine we rebuilt we balanced, the piston, the rods and cap separately, you be surprise how far these could be out. a very simple method is balance scales. interesting to check the remaining piston rods.

One question not asked, whether you know or not was the engine cold? not up to running temperature, remembering these are high tensile but brittle when cold. A combination of cold engine, unbalance pistons ect (

I also support the theory of a loose bolt, or something could have given cause to loosen slightly. What you don't notice when stripping an engine and rebuilding engines is, and many people fail to notice is the tiny lip you get on the faces of both the con-rod and cap and main bearing housing and cap. Its always a good idea to run a file over the faces and deburr the edges. found this out using plastigauge. I believe you can still get it.

Probable the drivers had a heavy right foot, when rebuilt put a rev limiter on.

Hepe you resolve this.

Regards

Engine was rebuilt with Arias pistons and indeed rev limiter!It was difficult to find decent rod bolts as not covered by ARP(Too long)
Dirk

Do you mean original rebuild or subsequent?

je suis charlie

I am sure I read somewhere that the MGA Twin Cam engine was prone to failure after being rebuilt - don't know why that would be.

Has anybody here ever encountered an out-of-round crank pin? Lets say you had a flat spot on your crankshaft and then torqued your perfectly round brand new con rod onto that. Think about the effects of that as the crank pin rotated within the con rod. It could be extremely tight at certain angles.

Was this a new crank and rods, or was the same crank used and new rods/bearings/bolts? The reason for rebuild? Metals can deform and that engine was known for knocking.



"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

Apart from what is written above, we started from the assumption that the engine has been rebuilt with new pieces and according to the usual techniques for this engine that for certain verses had one serious problem.
Everyboby has a MGATWIN CAM must know MGAguru.com and what is written in his site:

Apparently the single most important lesson to learn about the MGA Twin Cam engine was not figured out by the factory until after this model had ceased production. The engine had been plagued by burned pistons, quite mysterious at the time. The factory first retarded ignition timing, then lowered the compression ratio (both reducing power output). They finally gave up on production when the engine could not be made to run reliably.
Sometime later avid driving enthusiasts were to discover the cause of the problem to be a resonant vibration of the SU carburetors at certain select engine speeds. Such vibration would cause foaming of the fuel in the float chamber, which in turn causes a lean running condition, which in turn leads to low power output and burned pistons. The problem may be overtly obvious if the car is run on a rolling road with power monitored as the engine speed increases, and there may be a sudden unexpected dip in the power output right in middle of the sweet spot on the power curve. Using some modern technology, an O2 sensor in the exhaust may also disclose the lean running condition.
The solution then is to install vibration isolating flex mounts between the intake manifold and the carburetors. Once this was resolved, Twin Cam enthusiasts have enjoyed many years (even decades) of high speed motoring enjoyment with a generally reliable engine. If this problem could have been known by the factory it might have been fixed during production, and the MGA Twin Cam model may have survived for a much longer production life. The Twin Cam engine might also have followed on to subsequent MG models. Can you say MGB Twin Cam without a flutter? There was at one time a Twin Cam engine MGB prototype. These pages then are dedicated to the trials, tribulations, and final solutions to the Twin Cam carburetor vibration problem.

"Has anybody here ever encountered an out-of-round crank pin?"

Yes, but generally from wear presumably from absolutely dismal maintenance. Many manufacturers permitted a wear limit of 0.001" out of round.

Crank grinding errors generally were not ovality, or oversized journal diameters, but taper, crappy fillets, heat "checks" (cracks !) from loaded up grinding wheels and bad technique, excessive journal width (rod side clearance or crank end play).

The bearing manufacturers spec the heck out of journal ovality and rod big end ovality, and warn specifically against combining components each with maximum ovality.
Probably for fear of creating some of reduced or overpowered hydrodynamic effect.

The big end bore of conrods are usually honed round/cylindrical. But the bearing shells often have a tapered wall thickness to improve the operating hydrodynamic oil film properties. The crank pins on high-performance engines are often ground with a slight barrel profile to reduce the effect of edge loading in the bearings due to pin bending.

To :gruntguru: the rebuild with Arias pistons is what I did recently.
The failure history of the Original engines is interesting:
All the engines I took apart (about 10) with the Original H/C pistons showed seizure marks on the sides.
When it became apparent at the factory,after the American G.I.'s broke their engines on the autobahns in Germany, they simply gave more clearance in the bores,causing another problem;oil consumption.It was only when they used the later, L/C piston with GREATER ovality that the problem was solved. All this happened in the period that Healey took over the factory and I am certain that the Healey guys did their best to torpede the MGA twin cam.
I had several Original H/C pistons reground to the later spec ovality with perfect results.The set in the broken engine was one of them.
A second problem were the heavy conrods. In the Le Mans races several engines succombed to rod bolt breakages.
A third problem was and still is that the distributor drive gears are positioned on a half speed shaft replacing the original camshaft in the block.This half speed shaft needs to be shimmed perfectly to avoid axial movement of the shaft. I came to the conclusion that this movement is the cause of sometimes very erratic ignition,hence detonation problems.On my cars I shim to a practically nil clearance.The ignition point will move only a few degrees on the stroboscope.
The race engines I built with Arias lighter pistons and 7" rods will rev freely to over 8000 and are reliable.Only trouble is head gaskets when running with high compression,12:1(now using Cometic with copper rings inset)
Dirk