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Piston skirt/rod length question/issues

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Yves40

Yves40

Automotive
Nov 20, 2010
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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.
 
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Oiling issue, or as you say, too much rock. At what rpm were you breaking in? Some manufacturers have included cylinder wall oiling jets in the connecting rods to mitigate scuffing at low rpm (i.e. when the oil being thrown from the crankshaft is inadequate for this purpose.
With the short stroke and long rods, I don't see why the pistons can't have decently long skirts.
 
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What oil pump and pressure and viscosity?

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
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
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Can't remember rod side clearance exactly but was something like 15 thou.

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.
 
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PS : couldn't measure oil pressure, since body was not on the frame and no gauges. Oil was coming on top after a short while.

Could be oiling issue as stated. Have difficulty in accessing this.
 
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Ok, and i shouldn't forget. The block was obviously not plate honed as i had asked which is clearly seen by the markings on the top of the bore. Might have caused some additional blowby.
 
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Can you post pictures of all the pistons and bores?
Do the heads and head gaskets have "steam holes?"
is this engine for the StingRay?
 
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Could be the angle, but the hone marks look like the stone was not going up and down fast enough, or was rotating too fast, and may not have had enough oil sprayed on it.


Mike Halloran
Pembroke Pines, FL, USA
 
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Cylinderbores were cleaned with tolueen solvent and white paper towels untill no discoloration on the towels anymore.

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.
 
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Do you have close ups of the rings' faces?

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.

 
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Not much on the rings faces. Will try to make pictures.

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.
 
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It looks like a simple glaze breaker bore job, and a refresh or overhaul, coolant jackets are a mess. Most likely not using a hone plate. Finish looks okay.
Loss of oil film. So that area has the stated clearance?
 
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Coolant passages are a mess, but they were not like that in the beginning. I drained the water out of the engine after she was run, but due to the siamesed bores, some remained on top of the cylinders and cause some rust.

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.
 
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The hone job is pretty rough, but I have seen worse run OK.

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
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
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