Im thinking of rebuilding my quad and i was wondering how is the 426 and would there have to be any other modifications to the bike for the 426 to work or is it just bore the head out and put the new piston on. Guys let me kno

 

If you are still a stock bore 400EX, then I would suggest go with the 11:1 Wisco piston 416, stage one Hot Cams, and keep the stock carb just rejet.

 

you actually bore the cylinder, not the head. no other mods are manditory but i would consider a cam near manditory if you want a power gain.

 

I recomend the 416(87mm) also. If you go 416 you can then go to 426(88mm) after the 87 is wore out. After 88 you have to re-sleeve, though. I think you can find a piston for around a hundred dollars, and then I would say get a cam also.

A hot cam can be found at 100$ on ebay.

 

I had a 426 (88mm) with 11:1 piston in my ex, I thru a rod and had to rebuild. I put a 440 (89mm) with a 12.5:1 piston and it is alot faster! they make 89.25 and 89.5 pistons if you need to freshen up your cylinder. That with a cam and rejet and pipe, you will be cookin with gas! BTW I recommend putting a racing rod in while you have it apart.

 

yeah, i don't think i woudl go over 11.5:1 with the stock rod. yes, they have failed before that, but it isn't really really common depending on how hard you ride your bike. also with higher compression, you will need race gas. the only problem with replacing the rod is that you need to deal with the bottom end instead of just the top end.

 

yep, the rod and having it installed with a heavy cam chain, piston and sleeve was around $1300.00 and I did alot of the work myself. If your just going to do the top end, you could probably get by for around $400.00 for parts and bore. And like Cal said I am running race gas with the higher comp piston. But it sure is fun to ride.

 

just curious, how does your ex compare to the yfz? i am still a little behind a piped yfz once they hit 4th. until then i am ahead but they seem to pull pretty good in 4th and then it levels off when we hit 5th. i have only raced 2 stock ones and i am a bit quicker on my 426.

 

I just raced in the 250 class at a local track here, there were 2 or 3 yfz's in the class. I came in 2nd, and the guy that won had a yfz, but I was on his tail the whole race. I couldn't have passed him, his bike was to fast coming out of the corners. I have not raced against my yfz, it is setup stock and my ex is set up for flat track. I would say it would be a real close race though.

What all do you have on your bike?

 

Isn't the bore on a 400ex a Nikasil plating, and if you bore aren't you going to have to replate?