Pretty On The Inside

Wiseco standard compression piston for YZ426F


My venerable race bike is going to a new home. I am lending my 2000 Yamaha YZ426F to my little brother, in the hopes that he will come racing with us. To make sure that he can’t weasel out of it with a mechanical complaint, I helped him put a new top end in it.

The plan was to replace the stock piston with a Wiseco standard compression unit, replace the timing chain, and check the head. All the parts arrived a week or two ago.

We started with a valve inspection. Everything was spot on. That’s great, because it saved us a trip to the dealership to get new shims, and because it means the valves haven’t moved at all since the last time I did the top end.

Then we pulled the head, cylinder, piston, flywheel and stator. John did all the work on cleaning up the gasket surfaces on the cylinder while I threaded the new timing chain onto the crankshaft. We had lunch and a beer, and then took the cylinder to Elli’s Cycles for a quick hone.

Back home, everything went together easily. Assembly is harder than disassembly; you have to clean everything before you reassemble it, many parts need to be lubricated before you install them, and everything needs to be torqued carefully in sequence.

Once it was all together, I filled it with fresh coolant and oil, and let John start it up.

It fired right up and ran at a nice idle, with no funny noises, smells, or leaks. Yay! Another oil change, and it will be time for a break-in ride!


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