Reading a piston

73 Mike

73 Mike

I'll drive it someday
Joined
Mar 30, 2008
Messages
713
Location
Boston, MA
As some of you may remember, I had a minor mishap with a nice stroker engine that I was running.

Brokencylinderwall.jpg

The piston in that cylinder is now in a display jar in my office. The single largest piece is probably 1/24th of the total piston so there isn't much to give a good indication of what went wrong.

I now have a replacement block at the machine shop and I'm starting to get the other components ready. Though they look good, I don't plan to reuse the piston pins but needed them out to remove the rods, which I will be reusing. I knew that the piston in this cylinder had a small crack where the retaining clip was attached, but when I gently nudged the pin with a hammer, this is what I saw:

Missingpistonchunk.jpg

Missingpistonchunksideview.jpg


Since it doesn't look like the clip came out and then the piston was damaged, does the fracture at the base of the piston give any hints about what went wrong? I always attributed the piston failure to detonation. Now I'm not so sure as this piston looks like new on the crown. No visible cracks either other than the on shown.

I'm very concerned about making the same mistake twice.
 
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Full floating rods? I suggest either the pin locks hammered out or the pin clearance was wrong. Either the pin bore OR the pin end clearance, maybe both.
 
TimAT said:
Full floating rods? I suggest either the pin locks hammered out or the pin clearance was wrong. Either the pin bore OR the pin end clearance, maybe both.
Click to expand...

They were full floating. Pins and clips came with the pistons from the manufacturer. I certainly didn't check clearance between the pin and the clip if that's what you mean. I don't knw that I even saw a spec for this.
 
Just did a quick search-- I found these numbers for pin clearances

Pin to piston .0006 to .0008

Pin to rod (full floating) .0008 to .0010

Pin end play 0.00 to .005

Chevy recommended .072 Spiroloc retainers way back there. they said they were more resistant to getting beat out of the groove, but the pistons had to be machined to fit those retainers..
 
TimAT said:
Pin end play 0.00 to .005
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That's a pretty wide range and I imagine I was well within it. The fit was quite snug which was why I was concerned about the notion of end play.

I don't know which clips are which, but these were the spring wire, almost circular clips that were bent so that you could grab and install them with needle nose pliers.

While I do intend to go with Spiralocs on this build, I have no evidence that any of the clips came off. The only ones missing were those on the shattered piston and the one on this piston. Pretty hard to tell if it was cause or effect.
 
Mine were delivered with spirolox. I don't remember there was any spec for clearance on them. They were a bitch to install, I don't think they can drop out that easily. Can't say much about the circlips.

Not all pictures were that clear.

What side was damaged most : workside or the other ?
 
Belgian1979vette said:
Not all pictures were that clear.

What side was damaged most : workside or the other ?
Click to expand...

Definately not the best pics. Very hard to get a good, close, focused shot while not getting blinding reflection from the flash.

I'm not sure I totally understand your question but I'll try to answer. The only damage on this piston is to this side. The crown looks good enough to use again as does the pin retainer on the other side of the piston. I don't have a pic of the other side but this is a pic of the crown when I pulled the heads off.

Crapincylinder2.jpg

The bits of piston are from #5 on the other side. I had piston shrapnell pretty much everywhere.
 
Just another random thought- any nicks or sharp edges are good places for a stress riser and a crack to develop from. If there was a nick in the pin hole, even on the inside edge of the pin boss, that could be why that 2nd one has a chunk out of it.
 
Is that picture the same piston as the one with the broken off underside ?

If it is, how many miles did the engine have ? I think it is kind of strange there is no black on top of the piston. Could be that the soot was being washed away by coolingwater or excess of fuel. This could mean that this piston was trying to compress water/fuel instead of air/fuel mixture....I don't think I have to explain what happens when a piston tries to compress a fluid...

I have seen this kind of thing happen before with a water leak or an injector pissing (leaking) fuel. In this case it was a diesel engine in which the injector was really leaking fuel into the cilinder. The top of that piston was completly washed clean, just like yours...

Just a thought.
 
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Sorry for the delay in responding. I was out of town.

The piston with the broken off underside is the same as the top pictured. Less than 100 miles on the engine (no odometer so hard to be exact). I watch the oil pretty carefully during break in so I don't think that coolant leakage caused the original failure as there was no sign of coolant or raised oil level immediately prior. When the piston exploded, the cylinder was ruptured and gallons of coolant flowed through the intake manifold into all cylinders. Hydraulic lock certainly stopped the engine but I would have expected cracks on the crown or upper side of the pin flange if this was the cause of cracking on this piston.

You are right though. The pistons were very clean, especially compared to the heads. No injectors (Q-jet) and no sign of liquid fuel going though. I think it may have been running lean if anything but no O2 or AF data to support this. Just temperature and smell of exhaust.
 
The pistons i saw after a liquid block didn't have cracks in the piston crown either, so I don't think these kind of cracks can exclude a conclusion. All pistons that I saw with a liquid block had bent rods however. But all in all I think it depends on the quality of the rods.
If you use for instance very strong steel rods. I can image the rod breaking the pistond with the piston wrist pin and entering the block like it did with your applications.

Anyway, seen the fact that you use a Q-jet, i would look for an indication of a head gasket failure or damage to the chamber.
 
No obvious indication of headgasket failure, though with coolant everywhere, it was a little hard to tell.

I did have good connecting rods. Scat 4340 forged. The rod with the broken piston was bent in a S-shape but has no indications of cracking. I've joked before that I should have just hammered it straight and re-used it. The pistons, however, were hypereutectic.

Interesting observation regarding the pistons that you observed. I wouldn't have expected that.
 
73 Mike said:
No obvious indication of headgasket failure, though with coolant everywhere, it was a little hard to tell.

I did have good connecting rods. Scat 4340 forged. The rod with the broken piston was bent in a S-shape but has no indications of cracking. I've joked before that I should have just hammered it straight and re-used it. The pistons, however, were hypereutectic.

Interesting observation regarding the pistons that you observed. I wouldn't have expected that.
Click to expand...

I dunno to hammer it, but put it in a press and straighten it out....

:clap::crutches:
 
mrvette said:
I dunno to hammer it, but put it in a press and straighten it out....

:clap::crutches:
Click to expand...

I would, but I've already received a weight matched replacement from Scat (great service by the way) and made a desk trophy out of it. Great conversation piece.
 
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