Piston skirt/rod length question/issues

[Yves40]

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.

 

[Yves40]

It would think it was rich. Plugs were more or less black.

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.

 

[LionelHutz]

That really is a terrible looking hone job.

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.

 

[willeng]

I'm surprised about some of the comments on the honing, that is an absolute terrible hone job, in fact it's not a hone job.
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.

 

[Tmoose]

It takes a real rich mixture and probably cold cylinder walls for gas wash to occur, but it may be the best explanation for the scuffing.

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.

 

[patprimmer]

5W20 oil will wash of pretty easy.

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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[patprimmer]

If you have access to a lathe, a cam bearing knocker outerer and replacer is pretty easy to make.

Regards
Pat
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[Lou Scannon]

If you have access to a lathe, a cam bearing knocker outerer and replacer is pretty easy to make.
Er, that's one too many er s, I think!

 

[willeng]

Gday Pat,
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.

 

[patprimmer]

Well the OP was not will this seal up and make really good power, it is why did it lock up after warm up even though it had more than recommended piston to wall.

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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[Tmoose]

There have been successful race engine builders that use shockingly small amounts of lube at assembly. I think Udo Gietl was pretty successful with the BMW motorcycle road racing effort back in the 70s. Back 1978 Udo told Kevin Cameron he advocated this procedure.

"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

 

[MikeHalloran]

Okay, I'll believe that nearly dry assembly could work, on a flat engine with a known reliable oil system.

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

 

[Tmoose]

re:'dry" assembly
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.

 

[patprimmer]

If more than a thin smear of oil is used on the bores it simply rubs off as you turn the engine during assembly.

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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[Yves40]

The bores were oiled and some oil on the pistons. They were not dipped in oil however.

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.

 

[patprimmer]

If you use a lubricant somewhat more viscous and sticky in a gear pump, the lube will stick to the housing and rotors or gears and help seal and thereby pull in oil more effectively to prime easier. I Don't use enough to cause a restriction of oil flow once the engine is primed, just enough to help bridge the clearance.

Regards
Pat
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