Wrist pin circlip failure

[JP911]

I just helped a buddy tear down the top end of his 3.0 spec 911 race motor. The motor was down on power (chassis dyno showed about 10-15 HP shy of where the motor should be) and was blowing smoke. The motor had a top end 2 years ago, with new pistons (CP forged copies of Euro SC CIS pistons; approx 9.5:1 comp CIS). All was looking normal until we got to P/C #6. There was oil pooled in front of the piston; we thought broken rings, but that wasn't it. When we pulled off the cylinder and checked out the piston, we found that the wrist pin slid out easily in the direction of #5. A quick look at the cylinder showed heavy scoring that matched the direction that the rod had floated.

Further investigation and we found that the wrist pin circlip was still in place. The wrist pin had mauled the inside of the piston, but the opposing circlip was still in place. The rod and rod bearing looked fine. We inspected the other pistons and cylinders, but all looked good.

Have any of you seen this before? What would cause this? Any insights much appreciated.

[roblav]

What kind of circlip are they? Post a picture. I've dealt with these on JE and CP pistons in other race engines besides Porsche. You may have the wrong type of circlip.

[Steve@Rennsport]

Originally Posted by JP911

I just helped a buddy tear down the top end of his 3.0 spec 911 race motor. The motor was down on power (chassis dyno showed about 10-15 HP shy of where the motor should be) and was blowing smoke. The motor had a top end 2 years ago, with new pistons (CP forged copies of Euro SC CIS pistons; approx 9.5:1 comp CIS). All was looking normal until we got to P/C #6. There was oil pooled in front of the piston; we though broken rings, but that wasn't it. When we pulled off the cylinder and checked out the piston, we found that the wrist pin slide out easily in the direction of #5. A quick look at the cylinder showed heavy scoring in that matched the direction that the rod had floated.

Further investigation and we found that the wrist pin circlip was still in place. The wrist pin had mauled the inside of the piston, but the opposing circlip was still in place. The rod and rod bearing looked fine. We inspected the other pistons and cylinders, but all looked good.

Have any of you seen this before? What would cause this? Any insights much appreciated.

Call me on Monday.

[JP911]

Originally Posted by roblav

What kind of circlip are they? Post a picture. I've dealt with these on JE and CP pistons in other race engines besides Porsche. You may have the wrong type of circlip.

I didn't know there were different types of circlips. Can you educate me on this?

[HawgRyder]

In my opinion...SpiraLocks are the way to go.
But...they are a total ***** to install or remove.
They look like a closely coiled spring (flattened) and when they are sort of "screwed" into place...they are there to stay.
The other type of wrist pine retainers we used was Teflon buttons...they just slide into place at the ends of the wrist pin...no locks...no special tools...the one end of tehm butts up against the pin...the outer end rides on the cylinder wall.
We used these in drag engines all the time...makes for a quick change of pistons when necessary.
I have seen the Teflon in an engine with over 300,000 miles on it...and they looked like new...surprised me...but I figured there was very little side thrust involved.
Bob

[roblav]

I agree with the HawgRyder. SpiralLocks are what I use. Teflon buttons sounds like a neat idea though.

[JP911]

Thanks for the ideas!

[Walt Fricke]

JP - your post is a bit unclear. One circlip was in place, but the other one was MIA? Have you found the missing one? Or did it come out unnoticed in an intervening oil change?

Hard to imagine the pin causing damage if both clips are still in place.

Not hard to imagine somehow failing to install one clip, or not getting it seated.

Teflon buttons aren't going to work on race or late style pistons with the shortened wrist pins where the piston does not reach out to the OD in that area.

[HawgRyder]

Good eyes Walt...yup...on the new typr pistons where viewing from the bottom of the piston...there is not a continuous circle of piston wall...the buttons will not work.
Unless...you machine a block to fit into the cavity of the void and let it have an insert into the hollow of the pin to keep it in place.
I would not try this myself...due to the weight of the block and the possibility of having it fail (allowing a big piece of Teflon to float around in the sump).
The result would be a failure of that cylinder because of the pin moving sideways.
Always a pleasure exercising the brain for me...keep the problems coming so I don't give in to Alzheimers....LOL
Bob

[LJ851]

Measure the rod journal for and rod big end for taper. Also check that the rod ends are parallel to each other. Any angle on the reciprocating parts makes life very hard on wrist pin circlips.

[JP911]

Both circlips still in place.

[Walt Fricke]

JP - could you take some pictures?

I get the part that a wrist pin which can move sideways will score the cylinder wall.

None of the rest makes much sense. How can the wrist pin maul the inside of a piston? There is not enough space for a wrist pin to move out to the cylinder wall and let the connecting rod end loose. Besides, if that happened you'd most likely have had blown up engine syndrome, not what this engine had (fortunately).

Thinking cap on: If the pin moved way over to one side, that put such force on it that it mangled its hole in the piston on that side? While still attached to the rod, though, so disaster did not strike?

The only way I could see that a wrist pin could move sideways more than the usual few thousandths of clearance is that either the wire used in that particular clip was of way too small a diameter (and thus it did not intrude on the pin bore of the cylinder). Or that these aftermarket pistons had a manufacturing defect, and the clip's groove was cut too deep, allowing the clip, when expanced, to clear the pin. Or both, I suppose.

You say the clip is in place. Were you able to push the pin past it? If so, got to be a defect, as this would not be the case with any of the other 11 clips?

[roblav]

Ahhh... sounds to me like the circlip is too small for the circlip grove in the piston (or VV).

[JP911]

Originally Posted by Walt Fricke

JP - could you take some pictures?

I get the part that a wrist pin which can move sideways will score the cylinder wall.

None of the rest makes much sense. How can the wrist pin maul the inside of a piston? There is not enough space for a wrist pin to move out to the cylinder wall and let the connecting rod end loose. Besides, if that happened you'd most likely have had blown up engine syndrome, not what this engine had (fortunately).

Thinking cap on: If the pin moved way over to one side, that put such force on it that it mangled its hole in the piston on that side? While still attached to the rod, though, so disaster did not strike?

The only way I could see that a wrist pin could move sideways more than the usual few thousandths of clearance is that either the wire used in that particular clip was of way too small a diameter (and thus it did not intrude on the pin bore of the cylinder). Or that these aftermarket pistons had a manufacturing defect, and the clip's groove was cut too deep, allowing the clip, when expanced, to clear the pin. Or both, I suppose.

You say the clip is in place. Were you able to push the pin past it? If so, got to be a defect, as this would not be the case with any of the other 11 clips?

Sorry, I don't have access to the piston and the cylinder at the moment. Will try to post some pics.

Your thinking cap is spot on. We took some measurements and found that the wrist pin could float all the way to the cylinder wall and still remain engaged with the rod. The gouging inside the wrist pin bore looks like it was caused when the pin "wobbled" once it had moved as far as it could go.

The wrist pin easily slides in and out of the bore with the circlip still in place. However, it will not do the same thing with the circlip on the other side of the piston. Manufacturing defect? Bad circlip? Not clear at this point. We left the circlip in place so that his builder could inspect it as it sits.

[Porshaah]

Based on the information provided it would appear the cylinder in question experienced hydraulic lock (i.e. evidenced by the oil pooling) and in turn hammering the piston wrist pin bore.

[JP911]

Originally Posted by Porshaah

Based on the information provided it would appear the cylinder in question experienced hydraulic lock (i.e. evidenced by the oil pooling) and in turn hammering the piston wrist pin bore.

Could be, except that it appears that the grooves created by the wrist pin are the source of the oil in the combustion chamber. Would hydraulic lock not have had more serious consequences, like a broken rod?