my 03 400ex needs new rings and i was wondering what size overbore should i go with? i looked at a wide variety and they all have different compression rations?, what do they mean? I went to my local Honda dealer and they told me that it would cost about 800 to do it. almost 450$ of the cost would be labor. i talked to the machinist and he can bore it for 65$. would it be possible for me to tackle the removal and replacement of the new piston myself and if so would i need any special tools? sorry for my noobness. any help would be greatly appreciated.

 

It would be a big undertaking. Not the place you want to start if you haven't done much mechanical work to these things before. It'll be a complete tear down.

Bore wise, go with the first step up you can get away with. I believe it would be a +.010 giving you 416cc. Compression ratio is.....(im not the best at describing these things..) um, how tightly the fuel/air mixture is compressed inside the engine. A higher compression engine will provide more power (in general) but, more stress on internals.

I've heard of 400ex having head gasket issues with bigger bore, higher compression builds but, that just the word around.

 

I think you can do it. The main thing is to buy the piston first and take it to the guy who is going to bore it. That way he can get it exact. But you don't have to tear the whole thing down. Just take the jug off and take it to him with the piston. You don't have to get into the bottom end at all (unless you drop the timimg chain in there.) Just be careful of that and it's not a hard job.

 

the first upgrade I gave my 400ex was the .40 over big bore kit, hot cam, edlebrock carb and white bros exhaust- think I had about $650 into the entire project- that included the cylinder work at a local machine shop... I did the rebuild myself, but I'm no stranger to the top end of an atv / dirtbike.
I'd say if you're the kind of person who pays attention to detail, can work slow- can read and understand repair manuals, and have a decent assortment of hand tools- you could tackle the rebuild- otherwise, let a pro do it...

.... and like mentioned above- don't drop that timing chain in the case!

 

Doh! budedm is right. If you don't have major damage there's no reason to get into the case. For just the piston it's not that big of a deal. But, if you just need new rings take the jug to the engine guy and have him check it. If it's in tolerance just get new rings. No need for a piston change.

 

I remember hearing something about that back then- but can't remember the details.

 

The only problems I've heard with head gaskets is when you go to a 440 big bore jug and a high 12:1 or higher piston in the 400ex.

Get a service manual and do the job yourself, really not that hard, just need to do things correctly.

Like bud says, get the piston first (first overbore .o10) and take it into the shop. The machinist will mic the piston and bore the jug to it.

With piston, wrist pin and all gaskets you would have around $125.00 - $150.00 inparts , look on eBay, great place to buy parts.

Like I said, a service manual IS A MUST!

 

^ x2. 416 with a 10:5 compression with stock head studs. Or for $100 plus shipping gt thunder will install hd head studs. Then you can run 11:1 compression, with a good head gasket it will not blow. Over 11:1 compression you will need to use race gas.
Add a stage 2 hot cam and rejet and a 11:1 416 will be able to run with a stock 450r all day long. Compression=power.

 

bore it to a 416. a lotta people ik with 440 kits they leak oil out the head gasket

 

With the 440 you really want to run hd head studs and a second or oversized oil cooler with a fan. Jetting is also important so you do not run lean and hot. Race gas will also keep temps down. All nuts and bolts get torqued in increments and in a star pattern.