Piston pin surface finish?

4 replies to this topic
  • Moto-Mike

Posted May 02, 2002 - 04:23 PM


This winter while rebuilding the top-end on my 2000 WR I noticed that the new replacement piston pin had a dull finish on the O.D. instead of the polished almost mirror smooth finish of the one that came with the bike.
I was not that concerned about it until I was talking with a machinist (who works primarily on car engines)about turning down some swingarm bushings and his concern for getting a polished finish to keep the bushing from tearing up the bearings. Of course I did not think of the piston pin while talking with him so I did not get a chance to discuss this specific application with him.
Has anyone replaced their piston pin and noticed this dull finish? Or did Yamaha sell me a defective unpolished part that could cause trouble down the road?

  • Chaindrive

Posted May 02, 2002 - 07:11 PM


Good Question!!
I, too, would be very reluctant to install an unpolished wrist pin. It is, after all, a critical, hard-to-lubricate, extreme load-bearing component. If the surface has been treated (nitrided?), that MAY explain it. But you would be wise to find out definitively.

How 'bout it? Many of you have also done WR top ends...was your wrist pin like Mike's? :) SFO?

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  • Quickrik

Posted May 02, 2002 - 10:10 PM


Yep they all come like that. Its some kinda low friction coating they give it though I don't know what it is. That eventually wears off leaving the polished metal finish. If you look carfully at you old one there should be two 1ish mm gray dulled bands which is the original coating which has not been worn away because it sits in the gap between the piston and the rod.

  • Moto-Mike

Posted May 04, 2002 - 06:25 AM


Thanks for your assistance guys. I have noticed those bands you are referring to on my old pin!
Now I can ride with confidence that I won't have one of those expensive four-stroke seizures. Thanks again; Moto-Mike

  • SFO

Posted May 04, 2002 - 06:35 PM


I galled my oe small end and wrist pin.
I replaced it with a wiseco pin on my 97mm bore which I taper bored the id @7 degrees to pull some heft out of it.
Even after ball honing the small end it galled again.
Can you see if it is a new part#?
Run the pin by a shop with a profilometer and check the rms.

[ May 05, 2002: Message edited by: SFO ]


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