Do you know if it has the stock piston, or if it's been rebuilt before? I'd go up a bore size with a new piston kit and use a CRF450 timing chain, they're much more heavy duty.

 

I don't know for sure. I know its the stock bore size, and I know it has a brand new clutch, so they have been inside the engine.

So you think I should use a 406 piston? Why is that? Would I have to shave the sleeve down?

Where can I find the Crf450 cam chain? Does it work out of the box, or does it need to be modified?

Also do I just need a piston and rings or do I need an entire top end rebuilt kit?

Thank you very much for the reply.

 

If it's burning oil, there is damage to the cylinder wall. This is why you'll want to bore up to at least the next piston size. A piston kit should include the rings and everything else you need to get going, other than a new head gasket, which you'll also need. The CRF450 timing chain fits with no modifications. You should be able to pick one up at your local dealer.

If you can do all of the work yourself except for the bore and hone which you'll have to pay somone to do, you should still be well under $500, including their labor.

 

Have a look at this kit. It's a 416 kit with higher compression, but will still run on 93 octane pump gas...

HONDA TRX400EX TRX 400EX JE PISTON 2MM OVER 10.8:1:eBay Motors (item 360152658184 end time Jul-03-09 10:30:05 PDT)

Here's your gasket kit...

Cometic Top End Gasket Set | Rocky Mountain ATV/MC

And your timing chain...

02-08 Honda CRF450R CRF450 CRF 450 TIMING CHAIN Cam:eBay Motors (item 330328473837 end time Jul-06-09 07:54:41 PDT)

I'd also suggest taking a good look at the timing chain tensioner while you're in there.

 

If it's burning oil it doesn't have to be a damaged cylinder wall. It could be the valve seals and/or rings. Regardless of what it is, you should at least hone the cylinder. Boring it to the next size should only be done if you want some extra cc's for a little more power, or if there is enough scoring that a hone will not fix.

 

Thank you very much for the links. I'm crunching numbers right now to see if its worth keeping...

 

If it's in decent shape it's definately worth keeping. Besides, you wouldn't want to pawn it off on someone else, anyway.

Put a few hundred bucks in it and you'll still have less than the $2500 you were willing to spend to begin with.

 

Alright, And with a 416 (87mm) piston in it, and a Crf450 cam chain would it be as reliable as stock? Or would the bore cost me reliability?

 

When done right 416's are still known to be very reliable. When you start getting into 426 and 440 and higher compression they can be a little less reliable, mainly because of the added heat.

 

Beaver, about an hour ago I had a friend who is an atv mechanic at one of our local shops (minimum wage, but hey he knows his stuff) take a look at it. Turns out it has both front rotors bent, and the rear caliper was destroyed in what we think must've been a bad landing.
With the engine rebuild, new tires, all new brakes and the few other repairs it needed, its gonna blow my $2500 budget.

So I figured what the heck and called the guy who sold me the bike, explained to him how I had disassembled the bike to the frame and cleaned it, then how I had taken off the rear part of the frame and stropped and repainted it. He said under the circumstances he would take the atv back.So I'm back the square 1

I am out $150 from buying a UM swingarm skid, a Seat Cover, and a outerwear pre-filter. But hey, I saved the parts, and I can use them on the next atv I purchase.

But thank you for the advise, I'm still going to keep looking for another 400ex. Hopefully one that has a good enough engine to atleast last me the summer.