426 vs stock size high comp piston

[buster024]

In previous threads I had a respone that I could use a stock size high compression JE piston as opposed to a re-bore since my bike is so new. Am I still gaining the same power gains by not re-boring? Do I have to re-bore if I want a true 426 or can I get the full gain by just adding the piston (and hotcam, port/polish).

Do I have to add a sleave to avoid overheating if I don't re-bore?

I've got this on several forums so any help would be great....Not a motorhead, so if this is retarded I apologize.

[Chino886]

I responded in the 400EX section....;)

[jsmith2232]

you will still get gains but not as much as with an overbore, there is no replacement for displacement and no you dont need a different sleeve.

[F-16Guy]

All things being equal, any time you add displacement you will add power. In this case, how much is debatable. If you go all the way up to a 426cc, you will probably feel a little difference since you are increasing the piston diameter by 3mm, but 99% of the power increase will come from the compression ratio increase and the cam. The problem with going to a 426cc with a quad that new is do you really want to go directly to the last overbore size possible with the stock sleeve? If you need to rebuild again, you will then need to re-sleeve, and that will add to the cost and complexity of the rebuild. All that for a very slight gain in performance. I would probably go with a 406cc, which is a 1mm overbore. The minimal bore job will give you a nice, clean surface for the new piston and rings to mate to, and you will have plenty of cylinder wall left over for the future. I would also recommend getting oversized head studs installed (GT Thunder does a great job, and can bore the cylinder at the same time). With the higher compression ratio and higher operating temps, there is a tendancy for the aluminum around the studs to weaken and allow the smaller stock studs to pull out (usually the forward one by the exhaust). It will cost about $100 for the studs with parts and labor, but it will save you lots of headaches down the road. As far as porting and polishing goes, the 400ex head flows pretty good. If you keep the displacement under 440cc's, you should probably save some money and just get a clean-up port to remove the casting flaws and have the exhaust ports polished (don't polish the intake ports!). Good luck with the build!