Piston skirt/rod length question/issues
Piston skirt/rod length question/issues
(OP)
Hi,
I have some questions about an engine that started to seize its pistons during break in. The subject is a small block chevy, 400 ci siamesed bore block with a 3.250" stroke crank and some long 6.250" rods with 2618 pistons of CH 1.125".
They pistons have suffered from slight scuffing after only being run for 20' untill the engine warmed up. The engine showed 20" vacuum initially but this went down to 15" when warmed up.
The is slight scuffing on the inside walls of the cylinderbores on both banks and the pistons show scuffing on this side (opposite to the chamber) right below the oil rings.
At first i thought there was insufficient piston to wall clearance however, there was 0.0055" clearance vs the .004" that was suggested by the piston manufacturer.
Seen the large bore, long rod combo i would like to hear opinions if and what the causes could have been. I would like to rebuild this combo (although with another block) but i would like to be sure that piston stability has nothing to do with the problem. Main reason is that due to the nature of the beast the pistons are short and have rock in the bores.
My intent is to have the block kept to standard clearances as suggested by JE Pistons.
Thanks for any input.
I have some questions about an engine that started to seize its pistons during break in. The subject is a small block chevy, 400 ci siamesed bore block with a 3.250" stroke crank and some long 6.250" rods with 2618 pistons of CH 1.125".
They pistons have suffered from slight scuffing after only being run for 20' untill the engine warmed up. The engine showed 20" vacuum initially but this went down to 15" when warmed up.
The is slight scuffing on the inside walls of the cylinderbores on both banks and the pistons show scuffing on this side (opposite to the chamber) right below the oil rings.
At first i thought there was insufficient piston to wall clearance however, there was 0.0055" clearance vs the .004" that was suggested by the piston manufacturer.
Seen the large bore, long rod combo i would like to hear opinions if and what the causes could have been. I would like to rebuild this combo (although with another block) but i would like to be sure that piston stability has nothing to do with the problem. Main reason is that due to the nature of the beast the pistons are short and have rock in the bores.
My intent is to have the block kept to standard clearances as suggested by JE Pistons.
Thanks for any input.





RE: Piston skirt/rod length question/issues
With the short stroke and long rods, I don't see why the pistons can't have decently long skirts.
RE: Piston skirt/rod length question/issues
What coolant temperature?
What compression ratio and what fuel and timing?
What do the plugs look like? Rich, lean or OK?
Is it on a dyno, in a car or in a boat?
The pistons should be stable enough.
Regards
Pat
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RE: Piston skirt/rod length question/issues
Regards
Pat
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RE: Piston skirt/rod length question/issues
Was running richt. 10° initial timing, rpm varied between 1500-3000 rpm. Run in on a bare frame, no load, oil cooler and radiator/fan attached.
CR is a solid 10/1 static, DCR close to 8/1.
Thermostat was 160 and had just opened when engine was shut off.
As for piston skirt design. I asked myself the same question, it just how these off the shelf pistons come. Of course they are pretty light this way.
RE: Piston skirt/rod length question/issues
Could be oiling issue as stated. Have difficulty in accessing this.
RE: Piston skirt/rod length question/issues
RE: Piston skirt/rod length question/issues
Do the heads and head gaskets have "steam holes?"
is this engine for the StingRay?
RE: Piston skirt/rod length question/issues
RE: Piston skirt/rod length question/issues
http://i20
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Oil : castrol synthetic, 5w20
RE: Piston skirt/rod length question/issues
RE: Piston skirt/rod length question/issues
Mike Halloran
Pembroke Pines, FL, USA
RE: Piston skirt/rod length question/issues
Describe your cylinder cleaning process.
RE: Piston skirt/rod length question/issues
not2fast
RE: Piston skirt/rod length question/issues
I had a lot of problems with the guy that did this engine. The first block came in and had rust pits in the bores. I complained and after a lot of discussion i treatened to withdraw my visa payment. He then sent me this block. I took all the oil plugs out and put brushes through them. A lot of gunk came out. I cleaned it as best as i could with the tools i had untill nothing came out, blew it further out with solvent and compressed air and assembled. There was some dirt left somewhere in the system as was obvious by the marks on the bearings. What troubles me is the scuffing.
RE: Piston skirt/rod length question/issues
I feel better about soap and water cleaning, with a check of final cleanliness using white rags or paper towels, but with motor oil as the solvent.
How did you determine piston to bore clearance? What is the finished bore size, roundness, and taper? (Without deck plates, I know).
How complete was the cooling system, regarding fan and thermostat, what was it filled with? How was it filled for the break-in run?
Still haven't heard if the heads have steam holes to match the stock 400 block setup, or how that all was handled.
RE: Piston skirt/rod length question/issues
P-t-W clearances were measured with a mic (bore gauge).
0.01-0.02 mm taper, same out of round
System was complete with functioning shroud, fan and clutch. Filled with regular water untill water came out of the thermostat housing which is the highest point. Overflow tank was attached.
Steamholes were drilled in the heads. Since block was original the block also had them.
RE: Piston skirt/rod length question/issues
Loss of oil film. So that area has the stated clearance?
RE: Piston skirt/rod length question/issues
I agree on the glaze breaker thing. I hope that it is what i think it is : loss of oil film due to the bad honing pattern and blow by.
RE: Piston skirt/rod length question/issues
The scuffing on the piston skirts looks like contamination due to poor cleaning prior to assembly. Maybe some dirty crevice you did not find with your hand clean up of a poor initial job.
With CPs and a short stroke big bore configuration and custom length rods, I guess this is supposed to be an at least half serious race engine.
Frankly, that hone job is nowhere near serious race std.
0-20 is a VERY light oil for an American V* with 0.005" piston to bore. If it was also rich, you may very well have washed down the bores.
Do you know anyone who can do a decent job of plateau honing with ahead plate.
Make sure the cam bearings are knocked out before any further work and are not replaced until everything is spotless. The cam bearing gallery on a SBC is a very elusive hiding spot for grit which gradually flushes out soon after start up.
At 0.001" piston to bore over the piston manufacturers recommendation I would be using at least 0w30 and probably 5w40.
I would never start a new engine worth anything without some sort of oil pressure gauge rigged up. I generally use an old industrial grade gauge.
Regards
Pat
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RE: Piston skirt/rod length question/issues
This is a semi-race engine.
As is evident i was not to happy with the crappy work. I tried to clean things out as good as i could, but i didn't remove the cam bearings since i didn't have the proper tool to do the job.
RE: Piston skirt/rod length question/issues
Picture 2238 shows something that almost looks like a crack about 3/4 of the way down that bore.
In hindsite, you should have just stopped payment, returned the crap you were given and gone somewhere else after receiving the initial junk from that supplier.
RE: Piston skirt/rod length question/issues
The only worse hone job I have seen was two elephants mating at the Zoo!
The engine is not going to make power with a finish like that--full stop!!
Correct ring seal is everything!
Many may disagree but forget the Synthetic oils for that engine especially if your not using a roller camshaft.
An easy way to check your skirt clearance is to remove the rings from the pistons & insert the pistons in the correct bores upside down around the correct way & slide the pistons down the bores with a correct size feeler gauge on the skirt.
Yes, I know that's what dial bore gauges are for but I have seen many different readings from person to person using them.
This should always be done as a test proceedure before fitting any pistons, slide the piston on the rod with the feeler gauge all the way to the bottom of the bore & you will feel any tight spots etc.
It's easy & good insurance for any build & believe me you can tell the slightest tight spot with this easy method.
The rod ratio is fine with the stroke & the rod length you mention, it's not what I would call the ideal rod ratio but that's my view only.
You can re hone that block & knurl the piston skirts to give the correct clearance, this is a quick fix method though. Ideally you should start over & make whoever did that job pay for it!
It looks like that block should be crack tested & chemically cleaned.
RE: Piston skirt/rod length question/issues
Now I'm more concerned the ring scuffing I think I saw in the pictures might be real, so re-using the rings is probably not a good idea.
RE: Piston skirt/rod length question/issues
10 deg advance also souds a bit retarded, but just a bit.
With such light oil, if it has wide clearances, or a worn oil pump or a small oil pump,it might be getting oil starvation at the end bearings and big ends and not throwing enough onto the boles.
Certainly that hone won't hold much oil.
Correct ring seal is important for maximum power, but an engine can run without seizing with a pretty crappy ring seal.
I have seen engines where salt water was left in the bores and they where locked solid, become free with kero and auto transmission fluid and rocked gently back and forward with a long breaker bar until they gradually freed up start and run. On disassemble deep scores where in the bores and the cross hatch was well and truly gone, but it ran well enough to get home.
Regards
Pat
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RE: Piston skirt/rod length question/issues
Regards
Pat
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RE: Piston skirt/rod length question/issues
Er, that's one too many er s, I think!
RE: Piston skirt/rod length question/issues
I have to agree that the old IC engine will want to fire & run no matter what condition the internals are in, but you have to admit for a new build for a semi race engine this is really horrid work.
It does look like the hone hasn't had much vertical movement at all in the honing proceedure.
I would like to know how straight those bores actually are or are they bellied out in all sorts of places causing excess rocking & jamming.
The other thing is how much oil was used on the pistons etc on assembly.
To get scuffing like that in 20 minutes warm up time sort of hints that a lack of lubrication along with a gravel type texture on the bores & maybe bellied out bores hasn't helped at all.
I plunge the piston assembly into a container of clean oil & then insert them into the block immediately, lots of oil.
This allows for the engine to be fired & run at around 2000rpm immediately to run the camshaft etc in & to get a good ring seat.
Once that's done it's WOT fully loaded work from there so oil is very important.
RE: Piston skirt/rod length question/issues
I agree the piston to wall might vary considerably depending on what spot in those bores it was measured.
I still think it was a lubrication/contamination problem.
I have never found a need to dip pistons in oil, I simply use Clevite assembly lube, place a thin smear on the bores and the skirts and work it well into the ring groves. I prime the oil pump as well as I can on the particular engine, and add a little assembly lube to the ump to aid in prime.
Regards
Pat
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RE: Piston skirt/rod length question/issues
"The pistons, rings and cylinder bores are solvent cleaned and assembled dry save
for a drop of oil on each piston skirt.The engine is started and run at
half red-line for nearly a minute. Upon teardown, the rings will be
seen to have seated very nicely"
http://www.omnilex.com/public/bmw78/kotb.pdf
RE: Piston skirt/rod length question/issues
On a V8, where too-short pump drive shafts and other oil pump problems are not uncommon, I'd like to have enough assembly lube present to allow a little time for verifying that there actually is oil pressure and flow before opening the throttle.
Mike Halloran
Pembroke Pines, FL, USA
RE: Piston skirt/rod length question/issues
I was not advocating it. At all. I mentioned it only after finding a link to the article I <thought< I remembered. I just like to try to keep all the 'right" answers in sight, just in case. Despite Udo's success, I'd fear for rings assembled his way, too. Not sure if his assembly techniques evolved after 1978.
I use Spectro assembly lube or GM EOS on all kinds of stuff. I'm concerned about scuffing or seizure or at least unnecessary amounts of wear until high spots get buffed down to plateaus. For whatever reason I'm not very concerned about oil causing difficulties 'breaking in" rings. I'm Far more concerned with honing swarf ravaging new rings, and ring grooves too.
RE: Piston skirt/rod length question/issues
I think it is important to have plenty of lube in the ring groves and at least enough so a small bead forms all the way round at the top and bottom of ring travel. Any more is simply scraped of and deposited in the sump, no harm there, but also in the chamber which is certainly not desirable on a high compression race engine.
I certainly wipe the top of the pistons clean before installing the head.
I certainly use proper assembly lube rather than plain oil because it is sticky and stays in place between assembly and start up. I also hope a good brand has the correct supplements to aid in avoiding scuffing, although I have never seen data to that effect. I simply trusted Clevites reputation.
On any engine where you can drive bthe oil pump externally, I pup up oil pressure with a drill just prior to starting.
On any engine with an external accessible gallery plug high in the system, I prime the pump, filter and galleries.
If I can't pump up pressure externally, I at least pump up pressure with the plugs out.
I never ever fill an oil pump with grease or assembly lubebto assist in prime, although wit a difficult case I might fill the pump only with gear oil. I do squirt a bit of assembly lube into the pump and turn it over a few times by hand then wire off excess from the pump ports to help the rotors seal on the body nd prime quickly.
Regards
Pat
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RE: Piston skirt/rod length question/issues
I agree on all comments about the hone job, just not sure about how much contributed to the problem.
As for priming. I had a tool to prime the oil pump, but it didn't have the collar to pressurize the galleys. I rotated it with the started for 10-20 secs to get oil around before firing. Maybe that caused it, however i would have assumed that the front cylinder would have had the worst scuffing if that was it.
Then there is also to consider that the scuffing took place on both sides towards the lifters.
I do not intend to reuse the engine,as I would need to have it honed round again and clearance would get to large.
I already have an option on a bowtie that will bore to 4.125 and start over with fresh pistons. Would not want to make the same mistake or have the same problem again.
And Pat : i didn't quite get what you were saying about me not wanting a seal or something like that. Please explain.
RE: Piston skirt/rod length question/issues
Regards
Pat
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