Bought a blown up 400ex

[matholland]

I paid 500 bucks for a really nice condition 2002 400ex with clean title signed over to me. It was a buddy of mine, and he let his friend borrow it, but there wasn't much oil in it. Well it blew up. When I went to go look at it, the top end had been pulled, and so had the clutch side case.

Crank is good, tranny is good, it didn't lock up. Fourwheeler is in really good condition and I got plans for it.

I don't know alot about getting deep into them. Here's the deal. I have a 07 450r as well, thats what I ride. I came across this, and just couldn't pass it up. I am going to keep this 400ex as a secondary four wheeler. I would like it to be somewhat close to the 450r in power. But, doing it as cheap as possible.

The jug is good, hasn't been cracked, however its going to need to be bored. The honda shop will bore the cylinder for me for 50 bucks they said. I would like to make it a 440. If I go on rockymountainatv, and get the big bore weisco piston replacement, is that all I need? Or can I do it like that? Do I have to get the actual 440 kit or what? I am confused on that part.

For it to run with the 450r, does it need a stage 3 cam on top of the 440?

I think that should wrap up my questions for right now.

Thanks@!

[F-16Guy]

I don't think you want to get into resleeving for a 440 (89mm) piston. That will be a lot more than $50, and it opens the door to a lot of problems if not done correctly. Get an 87mm (416cc) piston, a stage 1 or 2 HotCams cam, and an OEM XR400R top end gasket set. Since it's been hot, you may also want to do heavy duty head studs so you don't end up with a pulled stud and a blown head gasket. It's a known problem with the 400ex. I personally wouldn't build a 400ex without doing studs, but some people here say they haven't had any problems. You can have it done at GT Thunder; they sell and install HD studs. If it were me, I'd send them the cylinder and new piston and have them do the studs and bore and hone it. When you get it back, it'll be ready to assemble.

[bigbad400]

yeah guy you cant bore them that far out, youll have to get a kit with a new sleeve, have your stock one pressed out and have your new one pressed in, with a 440 and a stage 3 cam youll eat your 450 if its not all done out too! mines mostly stock and i keep up with 450s all day hell i keep up with a ds650... unless you ride were you NEED the power you prolly wont have any problems keeping up with a 416 or a 426 at 11 or 12:1 (12:1 means race fuel or 104 octane will be needed) and even just a stg 2 cam. a nice port and polish wouldnt hurt and a 450r carb would be the whip. but either way you go your gonna need a pipe and a 450r carb, more compression and more displacement means you need more air and more flow in the exhaust, it doesnt take much for a 400ex to keep up with a 450 unless your flat out racing. for what your talking it sounds like its gonna be more economical to do a smaller bore and spend the money on a good pipe and carb, a cdi box will let her rev too and i felt a big difference in how the snap off the line was being able to grab slightly higher rpms. good luck, i love building up quads and dirtbikes... i had a blaster that could have kept up with your 450!!!! had a 240 kit running on 20% nitrous and a 36mm kehin racing carb off a honda 250r i ate up stock banshees with it!!! if you got the time and money to do it you can make a 300 ex keep up with your 450, i saw a suzuki rm125 at silver lake dunes drag race a fully modded single seat sand rail with a 1300 huyabusa motor and 33inch paddles on fox shocks and the 125 smoked him!! i dont know what that thing had in it but it was a fast little bike. its not necasarily better to go big sometimes little things make more power. the new thunderbirds came out with something like a 470hp 3.4 liter v8........ small bore but lots of power, my neibors got a 68' pontiac tempest with a 350 that dont make that power.

[rideahonda]

go with a 416 if you want cheap performance

a well built 416 with cam jets and exhaust will hang with 450s all day