piston clearance 3

[golfpin]

Greetings to all,
Had an old chestnut pop up the other day, my opinion was asked on a 2 liter Golf/polo engine full race 12.5 c/r wet sump what piston to bore clearance would be suggested genuine vw pistons used, any one care to comment [83 mm bore] reason for asking engine broke piston top ring land and ring. Pumped oil out siezed!
Thanks Golfpin

 

[Tmoose]

Is this in search of piston clearance as a cause of the piston breakage?
There's a list several items long for what can break a piston thru the top ring groove, and it starts with looking at the broken pieces as well as the surviving pistons, rings, spark plugs and cylinder head. My list would have piston clearance several items down the list.

What rpm? How wide are the compression rings?
Genuine VW passenger car pistons, or perhaps Genuine VW "race" pistons?

Full details of the original assembly build clearances and dimensions should be available if the effort is to be taken seriously.

An expected starting point for clearance is the nominal bore plus the nominal oversize.
That is, the clearance is typically "built in" to the piston.
That said, the machine shop and engine builder should want to know the desired clearance,in order to control and check it, and "clearance" needs complete description of official measurement point and the diametral clearance specified.
If the description offered describes a "per side" clearance I submit that the source may not be sufficiently familiar with car stuff.

I'm guessing a passenger car piston put into "race" service might require or at least tolerate a few thousandths of an inch ( 0.001s of an inch) or 0.025s of a mm extra (diametral) clearance. Like maybe the passenger car max, plus 0.002" extra diametral clearance.

 

[MikeHalloran]

My first guess would be that your engine builder doesn't know what a ridge reamer is for.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianGar]

If its a 2.0L vw engine its had an overbore from 82.5 to 83. It also started out in life with a compression ratio of 10.5 : 1. Given that .5mm overbore vw pistons can be had - which it appears you have bought, then these would easily be damaged running 12.5:1 cr as these are cast pistons were never intended for that.

Of course, an engine with a static cr of 12.5:1 could have a actual dynamic cr of 2:1 depending on cam spec, but Im sure its up there in terms of increase in relation to static cr.

The piston clearance for an oem 83mm piston measured across skirt at widest point should be .0012 inches, and will still be in spec if its .0030 inches.

Be sure and check that your engine guy bored the block correctly - sometimes a piston that is called 83mm will mean you bore to 83mm and the piston is less than that. Other times the piston will be 83mm, and you have to add the clearance when dialing in the boring head.

I suspect the cast piston could not cope with added cr though.

Brian,

 

[tbuelna]

What makes you think the broken compression ring and piston groove land were related to piston skirt/bore clearance?

As BrianGar noted, cast pistons behave differently than forged pistons under thermal expansion. The shape of the piston skirt (taper, barrel and cam) also has an effect on the operating clearances. A full skirt piston will not have the same fit as a slipper skirt piston does at operating temps. Also, the circumference of the piston around the ring grooves is slightly relieved so that the land faces do not contact the bore surface during operation.

A fractured compression ring and top land can be the result of ring twist due to excessive ring/groove clearance , fatigue due to excessive thermal loads, or fatigue due to excessive pressure loads.

 

[golfpin]

A very good morning to you all,
thank you for the input. The said golf eng was fresh from rebore hence the nominal o/size. Race pistons for golf are available as is anything else ... at a price with the RSA Rand at 10:1 against the US$ and from 15 to 18 against the euro and the Brit pound it is not affordable. The lad in this car is at club level. The std VW piston seems to cope up to 8OOO RPM the races are 8-10 lappers 2.48 Kms.
I have a life long involvement with engine including 2.7 Coventry Climax F1 Repco and Ford DFV and Imsa type V8 so I have had a bit of experience but have been out of it for nearly 12 years, so I thought to ask first? The reason behind the question re clearance was that I thought .004 [thou I am old and still think in Imperial] on a 82 mm bore was a bit excessive and the consequential sequence of failure was:
1 oil past the rings not seated [see comment re oil ]
2 contamination of incoming fuel
3 detonation
4 top ring land broke.
Unfortunately I have not actually seen the parts, the said party too embarrassed perhaps.
It seems on further enquiry that I might have been nearly correct. Race fuel, hopefully 100 octane, is unreliable in terms of quality [octane] and would appear that if the parameters of correct spark heat range, ignition setting, fuel ratio[lean] and fuel octane are not all in sync then trouble is on the cards, These are normal parameters but it would appear that there is less margin for error [std type piston ?]. The other point that I might make is that the bore prep in terms of finish [don,t know how to write mu in scientific/engineering] and then using a fully synthetic oil might have been contributory to the piston failure .
Should in the forum,s opinion, the engine have been broken in on a monograde "dinosaur" oil and then been put onto the synthetic, this I think may be part of the problem. Your comments would be greatly appreciated thanks Golfpin

 

[BrianGar]

To be honest Im pretty sure the piston cracked because of the upped cr. Ive been involved in a lot of Vw engine builds, from home builders all the way to all out race units. I dont think theres any point in looking into anything else - its a known fact that standard pistons will die very quickly if you up the cr. 12.5 : 1 is a huge jump.

Brian,

 

[Tmoose]

reportedly some (relatively low thermal conductivity) hypereutectic pistons from a well known supplier were able to destroy themselves by causing the top ring to run so hot the ends butted, gripping the cylinder wall real hard when the piston was being summoned hither. Initial fix was extra ring end gap recommendation, followed by slight piston ring relocation or maybe piston head design.

I'm not suggesting that is how these piston (s) failed, but I would not start forming theories until I'd seen some broken parts and surviving parts. For instance, if the ring gap faces are shiny etc it suggests they were butting, and that is a problem unto itself. If only one piston was trashed, and the other 3 look and measure fine, it suggests something. If the piston looks great except for a fatigue crack circumnavigating the piston, well.... If the skirt with the separated dome has multiple cracks leading from added pin oilo holes, the oil ring return grooves/holes, or machining performed in the interest of lightning, it all needs to be known up front.

 

[golfpin]

Thanks once again for the prompt and very lucid, concise replies. Would be grateful if someone would comment on my statement re dinosaur oils and breakin. This was a problem I had once with a re-bore, when the engine was assembled and run with a more modern 15/40 type oil, and bed in and oil consumption became a problem. Switched to a old monograde for about 1000 kms then back to the later type oil and the problem went away.
Tmoose great reply look forward to giving feedback if I get to see the parts.
Thanks again to all.
My thanks to BrianGar I thought to mention on the issue of Genuine pistons one of the genuine [factory] pistons that have been used with success are BMW in a 83mm size they are affordable give a useful increase in capacity and are very easy to fit. These have been used successfully with a 14;1 compression but this is at 6000 ft [Johannesburg] not at the coast where I am retired. This fairly big jump in Atmos. pressure is always a trap for the unwary and the fact that reliable 100 octane fuel is very difficult to get does not help.
Again comments input greatly appreciated.
Golfpin

 

[Tmoose]

Hi Golfpin,
was the ring seating issue with a vehicle in passenger service, and subjected to a version of the popular "baby it" break in procedure?
Any chance the info about the bore finish and cleaning process are available?

I think a version of break-in using multiple very brief periods ( like 5 seconds), of WFO followed by minutes of light throttle can be pretty effective at seating rings and easing down slight piston high spots without much risk of scuffing.

The importance of cylinder finish can not be overstated. Not many new cars fail to several thousand miles without using a quart of oil right off the showroom floor.

I'm not big on the oil > poor ring seal > broken piston theory. And especially until all the broken amd most of the unbroken engine parts are scrutinized with bright lights, magnification and dye pen inspection. Unless of course the owner thinks it is more fun buying parts and building multiple engines than driving, in which case guess away.

 

[golfpin]

G morning Tmoose and others,
Afraid I don,t know what the breakin procedure was but will totter down to his mancave to day some time and find out if I will be allowed to see the wreakage will then get back to you with details. As I indicated unfortunately there are huge egos at play here young aspirant [20 year something engine builder/modern day Al Baartz/keith Duckworth], his opinion, not a potentialy convivial mix as I,m sure you can imagine. Thank you again for all the interest you have shown and the time you have spent.
Golfpin

 

[Tmoose]

So the engine builder has latched on to (presumably not not necessarily) lack-of piston clearance as the one true issue? It would be interesting to hear and see what the documented clearance was during the first build. And where on the piston and how it was measured.
Are we talking skirt clearance, and not top land clearance

I'm thinking skirt clearance issues that might actually cause a piston to scuff, seize and break would not seem capable of getting much of a grip on the piston near the top ring groove.

At this point is there anything beyond the verbal claims to confirm a piston even broke?
the Grand Jury would not admit this in court.

 

[golfpin]

Good day All,
Thanks for the input, sadly as Tmoose alluded the truth may never be known I have a feeling that the engine will not last that much longer.It was rebuilt new crank and the damaged rod resized and used again! no more comment on that. A very qualified friend of mine was at the next race meeeting and while the car ran well in the first heat between heats the said genius/engine builder apparently advanced the ignition and my friend who was watching, said he could hear the engine rattling [over advanced] when under WOT coming out of a corner! the car finished low in the field. In my home language there is a maxim that goes like this " daar is geen pille vir domheid" there are no pills for stupidity. As I mentioned in a previous post quality race fuel is increasingly difficult to find, so at sea level and 12.5 C/R the line between an engine lasting and surviving is thin, BTW the car is on carbs and I don,t think any knock sensors are fitted. Thanks again to all who contributed.
Golfpin.