Head Scratcher

[Grande Dog]

Howdy All,
Looking for some ideas here. I have a customer who ordered a Big Bore 044 Kit in June and has been running it since. He said he was starting to to hear a little rattle so he pulled it down and found the piston was a little loose on the rod. He pulled the pin and saw the wear, and sent it to me to look at. The center of the pin is pristine and measures at .472". The ends of the pin measure about .461". The pin bosses on the piston are worn also but, not as bad as the pin. Any ideas?
Regards
Gregg

 

[thomas1]

Improper heat treatment?

 

[Grande Dog]

Howdy,
That's the first thing that came to mind for me also but, I just couldn't figure why piston pin bearing didn't leave any wear on the center of the pin.
Regards
Gregg

 

[galde]

The wear on the pin and on the piston suggests that the rod locked to the piston pin and the pivoting took place between the pin and the piston.

 

[SkippyKtm]

Looks like the wrist pin bearing is not doing its job, and the piston bosses are taking the wear.

 

[SkippyKtm]

The wear on the pin and on the piston suggests that the rod locked to the piston pin and the pivoting took place between the pin and the piston.

Drat! You beat me by a minute, I was still typing, gonna have to take a speed typing course, LOL.:tongue2:

 

[HARRY BARKER]

The wear on the pin and on the piston suggests that the rod locked to the piston pin and the pivoting took place between the pin and the piston.

yup.

 

[galde]

Is the wear on the pin ends even all the way around the pin, or is the wear concentrated in a smaller area? It would be useful to check the fit of the pin inside the rod with the actual rod bearing in place. The wear pattern on the pin ends might reveal whether the pin properly fitted the rod bearing but was sloppy in the piston bosses--- or the pin wasnt turning in the rod bearing and wore in the piston bosses......

 

[thomas1]

If the heat treatment were proper wouldn't the pin just beat the holes in the piston larger, if the bearing was bad?

Merely trying to learn, not argue with the bearing theory.

 

[galde]

Under some circumstances a soft material wearing against a harder material will result in greater wear in the harder material than in the softer material.

 

[J.W Younger]

The pin should not have been turning with a roller bearing in the small end of the rod, but I'm sure you know this. It don't look like the pin is at fault but the other race for the rollers may be the problem.

 

[lfnh]

How does the pin center of 0.472 GD measured compare to OEM 044 pin diameter ?
just askin ..

 

[one.man.band]

questions?

play in big end of rod?
signs of heat on wristpin ends?
sign of detonation/pre-ignition on piston?

piston/bearing could be taking brunt of the 'hit' if motor is pre-igniting. piston would be moving down ......when crank and rod are stopping at TDC and going other way. ring lands won't last long if this is case.

 

[Grande Dog]

Howdy All,
Thanks for all the responses. It was late friday when I posted this, and I was in hurry. One thing I didn't mention is the this guy has 2 saws doing the same thing.
To answer a few questions:
The .472" is within a couple ten thousandths of being the original size of 12mm.
The ends of the pins are darkened but, not blued.
I don't have the saws, so I'm not sure on the big end play.
No sign of pre detonation. The carbon patterns are towards the dark, and cool side of the spectrum.
The skirts still have some of the coating on them so, I know we're talking pretty low mileage.
It wasn't very scientific but, I put a calipers on the the worn area and turned it. The worn areas for intents and purposes is still round.

I've replaced these for the customer so, I'm not deciding whether to warranty but, just trying to find out if there's a manufacturing issue.

Thanks for the help.

Regards
Gregg

 

[parrisw]

I'd want to take a look at the bearing and rod as well, before those saws go back together.

 

[Scooterbum]

I'd check the wrist pin bore in the piston being true. If it's off it'll put more stress on the pin ends then the center.

 

[abikerboy]

Any chance that maybe he didn't prelube the bearing when he assembled the piston and rod?