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banda

Don't do what I did - stupid wristpin clip trick

10 posts in this topic

I decided to put a new piston in my 2000 YZ426. I ordered a Wiseco kit from another web store, a new timing chain, and a gasket set. Once I had the old OEM piston off, I found that the wristpin and retainer rings sent with my Wiseco kit were wrong, too large in diameter. No way the wristpin was going to fit in the con-rod or the piston. The retainer rings were likewise too large.

"Dang", I thought, "No way I'll get to ride this weekend... but what if I used the old OEM wristpin and the OEM clips?"

Bad idea. The dimensions aren't right. Neither clip went in easily, probably thicker than the grooves in the Wiseco piston for the retainer rings, and the second one was harder than the first, probably because the OEM and Wiseco wristpins aren't equal in length...) I was a little uneasy, but I thought "...it might be fine..."

Back together the bike started right up, and ran crisply. I felt a little less ill at ease. Then in the first hour of operation, the motor wheezed and seized up. When I looked inside, I found a cracked piston with one wristpin boss sheared off, its piston skirt poking through a shattered cylinder into the water jacket, and a bent con-rod.

I have another cylinder I can get replated, but clearly I need a new crank, piston, ring set, wrist pin, clips, and gasket set. Easy $600 bill and weeks out of service all because I chanced it on a couple of $1 parts that weren't right. Feel free to pile on and tell me how stupid I am, or point out what else needs replaced because of my folly. I can take it. Perhaps one day someone else looking at a piston kit shipped with the wrong parts will pause for a moment because they read about this one dumbass who trashed his YZ because he wasn't patient enough to call and get the right parts. One can hope.

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I think it's cool you posted up. We all do dumb things...most cringe, then laugh it off and make a note not to do it again.

Good stuff! I learn more from experimenting than I do by following the rules anyway.

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Don't beat yourself up too bad... we've all done stupid stuff. Kudos to you for posting, and good luck with the rebuild.

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LOL @ Zach Gilbert.

The head and valves are fine. The cylinder involved in the fiasco is trash.

I have a spare cylinder that is slightly out of round. I discovered that 2 years ago during a previous rebuild, and bought the newer cylinder. I kept the old one. I have decided to send it to a plater for repair - that would be about half the cost of buying a new cylinder.

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as a mechanic, i've seen alot of these kinds of things, but you are the first I've seen admit it. kudos.

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Followup:

I found a pretty good deal at Ron Ayers on a new cylinder, and went that route. It took me about 20-25 hours of careful work, but I have rebuilt this bike using all OEM components:

  • MY 2000 piston, rings, wristpin, wristpin clips, timing chain, counterbalance shaft bearing, cylinder
  • MY 2002 crank

When I bought the bike it had a Wiseco high comp piston in it. I had previously swapped that for a spare piston included with the bike that I supposed was stock OEM. The third piston, the Wiseco std compression unit was invloved in the disaster. When I received the OEM piston (the fourth), I found that it was different from the second piston that I thought was stock... maybe it was stock and Yamaha superceded it? That was a surprise. Or maybe it too was an aftermarket unit. When I got it back together, it was noticeably easier to kick over.

I had a few other little things that needed fixing - the O-ring at the coolant pipe from the head was shot, and it leaked like crazy. I went to Ace Hardware and bought a variety of O-rings to try to resolve it. I tried one slightly larger in diameter than the hard old one, thinking that squeezing it into the groove in the head was better than stretching it onto the pipe. Wrong. It was a slightly thicker o-ring of slightly tighter diameter that worked.

The bike runs well now. I'm still breaking it in gently. It's not quite as snappy as before, and the idle fueling is a little off. this makes me suspect (along with the easier kick) that the pistons I had in it before were all high comp. going back to stock is probably the best thing for rideability and reliability - I plan to hand this bike over to my little brother, who hasn't got as much race experience as I do.

You can believe that I quadruple checked the wristpin clips this time around.

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