400ex rebuild time and help

[Stroker81]

Ok guys my buddy is rebuilding his 01 400. He is gonna clean the bore up. What would be better 416 or 426? Gonna get head ported and polished. What should we do about the valves? Are stock valve good or whats the better upgrade? Also on the piston what kind should we get for it, JE, wiesco, or what. We have never done the 4 stroke just 2 stroke all advise welcome. Is +1mm valves good mod for this?
Thanks

[rob_990]

do a 416.the 426 creates a lot more heat.

[F-16Guy]

Do a 406 if you can get away with it. If you have to, do a 416. No sense in cutting away extra metal for nothing.

There have been some isolated problems with intake valves breaking, but I don't think it's all that common. If the valves and seats look good, just have them lapped and call it good.

I have a Wiseco piston in my quad and it has lasted a very long time. As long as you buy a brand name piston like the ones you mentioned and install it correctly (correct ring gap and installation, etc.), you'll be happy.

You should also consider installing a cam, since it will yield the most performance for the buck when paired with the piston. A lot of people on here run HotCams cams because they're cheap and give good performance, but you can also go with an XR400R cam from the bike engine (milder), or a full-race Web Cams cam, which would require valve springs, hardened rockers, and shortened guides. I personally like my HotCams cam, but it boils down to personal preference. The good thing about the HotCams is that they're bolt-in parts, and don't require any special springs, hardened rockers, guides, etc.

One thing I always recommend is having heavy duty head studs professionally installed. If you go to a place like GT Thunder, they will tap the threads for the new studs with a roll form tap, which compresses the metal to form the threads instead of cutting it away, which adds a lot of strength. If you send them the cylinder and new piston, they can bore it and install the studs, and when you get it back, it will be ready to install. Studs aren't necessarily mandatory, but the 400ex is notorious for pulling head studs, which results in a blown head gasket. It's money well spent.

[Honda#4]

With a 416,Stg 2 Cam,11:1 hi comp piston, and a ported head it will definently put a smile on your face. I have what I listed and in upcoming months I plan on getting it dyno'd to see what she's making. For your boring, and port work I recommend Colby@C&D Racing he does great work and his turn around time is pretty quick.

Piston wise there all pretty good, for the cam decide on where you want your power low, mid, high, and a CRF cam chain and HD studs are all good insurence to keep your quad from having problems down the road.

[Sanddraging28]

it really comes down to how much power you want to have. i have both a 416 and a 426 love them both. the 426 has really high compression and runs with my brothers 450r. and with a good launch i will eat him. the 416 is my trail bike. couldn't ask for much better. In my opinion the piston on the market is a JE. i have had real bad luck with wiseco piston, you couldn't give me one now. with either size i would recommend a stage 2 cam, like F-16 said the hot cams are user friendly and don't require any other upgrades to install. when it comes to heat, i have never had a heat problem with any 400ex i have built. i have a 500cc ex now that runs the same temp as a brand new untouched 400ex. if you get your jetting right keep your oil cooler clean you should never have a problem. Porting and polishing is a great idea but this is not a area where you look for the cheapest price. find somebody GOOD and pay him what he asks. a bad port job is worse than a 2 dollar hooker!!!!!!!! i have +1mm valves in both of my little 400's. since you are getting the head P+P it would makes sense to put bigger valves in it at the same time. Look into finding a 450r carb. its one of the best upgrades you can do to a 400ex. one from a 04-05 is the easiest to install. a crf 450r cam chain is a good idea too, but make sure you get a new cam chain tensioner with it.

what it comes down to is it really depends on what kind of riding you do...i build BIG 400's. in a straight line its hard to beat me, but in the woods or on a track i will get stomped by a stocker......some other guys will tell you what works best for them, but when it comes down to buying parts for your bike, make sure it will work for the riding you do, not what the guy in the computer told you you need......trust me......been there

[Stroker81]

Do I need to get a crf tensioner also? What year cam chain and tensioner? Also is the stock clutch good for the 426 kit? Is the +1mm valves worth doing or is it a waste of money. We will be doing trail rides, and dunes.
Thanks

[Sanddraging28]

no you need a new tensioner for a 400ex. i alway tell the parts guy i need a cam chain for a 2004 CRF 450r. the stock clutch will hold for a while but in the long run you will need to upgrade. i would suggest a Hinsin 7 plate kit.
if you are going to have head work done i.e. port and polish i would get the +1mm valves put it with a good valve job and new guides