wr420 kit....how difficult?

I recently killed the top end of my wr400...am thinking about doing that 420 kit. :) Do you have to actually get the cylinder bored, or will the new sleave slide right in? how difficult is it to install one of these kits?


usually to install a sleve, the cylinder has to be bored out quit a bit, The yamaha cylinders come stock with a Nickisil coating which is super hard and is very oil absorbant (super good stuff), I would sugest that since you have to bore the cylinder anyway, you should send it to a company called US Crome to be bored and re nickisiled to fit a 95mm wiseco piston which will make it a 426. I actually had my wr400 bored and re nickisiled to fit a 97mm piston which makes mine a 444cc engine. however, wiseco only makes this piston for the yz/wr 426 engine, the only diference between the 400 and the 426 engine is the wrist pin hole size is larger on the 426 engine. So to use the 97mm piston, you have to pull the crank and install the 426 conecting rod. Probably More work than you want to do , but the horsepower is awesome. you can get the 95mm piston to fit the 400 engine without the conecting rod change, you can even up your compression from 12.5 to 13.5 if you want, if not there is the stock compression 95mm piston available. I dont like sleaves to replace the nickisil coating, because they arent as hard or oil absorbant as the nickisil. Engine life with the sleav will not be as long as the nickisil coating. They can even build up your cylinder walls with the coating to refit the stock piston (cool stuff)

The yamaha cylinders come stock with a Nickisil coating which is super hard and

I didn't realize the cylinder was nickisil. Can anyone else verify that? Reason I am asking is that I am about to install a new piston. The cylinder has no damage or noticable wear, and relatively low miles on it.

Should I do a crosshatch with the hone? :) If it is really nickisil, that probably isn't a good idea. If it isn't nickisil, I would think the crosshatch is necessary to ensure that the rings seat properly.

Please advise! Anyone? Thanks in advance!


if you think about it, the cylinder is obviously made of aluminum, you have to either have a sleve, or some kind of coating to make the inside walls hard, and they certanly do not have a sleve. Not just Yamaha, but kawasaki, honda, suzuki all use the nikisil. I would think you would be ok to hone it up a bit, but you might call US crome and ask just to be sure, you can find there number on the web site i think it is uscrome.com

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