400ex Complete Rebuild

[Metzroth]

I was just looking for some advice on how to rebuild my 400ex / what to do and what parts to use. I don't know what caused it yet, but my valves were ticking horrible and finally after a day of riding i lost all compression and the quad wouldnt even start, I'm assuming I bent the valves because I could put my hand over the exhaust and feel it suck in and blow out. I also checked and none of the valves dropped so thats not what happened either. Also, I have a few stripped threads in my head (for the cylinder head cover bolts) so can anyone tell me what heli-coil kit i need to buy and where the best place to buy it is? Second, should i have a 3 angle valve job/oversized valves/milled etc.? Not alot of people know of it, but I am having high velocity porting done by a friend of a friend for $100 which is an amazing price. I don't know if i should use oversized valves with high velocity porting so I will check with the guy who is doing it first. Also, what size overbore should I go with and what compression. I want power and reliabilty. I am willing to run 50/50 premium unleaded/race gas if i have to. I dont want to have to worry about overheating or even stressing the motor as far as heat goes, I loved the reliabilty of my quad for the first 2 years and just recently last year it started having problems. What bearings/bushings need replaced when doing a frame off rebuild? I don't have a whole lot of money to spend but I want to do it right so i dont need to rip it apart again anytime soon. I am buying a stainless bolt kit from fastenerguy on ebay. my swingarm bolt was trashed and I would like suggestions on what one to buy? (not the most expensive one hopefully) Should I run a HD cam chain/ aftermarket cam chain tensioner? (one of these possibly the cause of the bent valves) I cant afford to do suspension yet, but most if it can be done without really tearing the quad apart too much. I also need new plastics and I want to have my header ceramic coated. I hate the look headers get from thermal deterioration so i need to fix it. Also, my R4 silencer needs repacked how hard is that? I've never done it. It looks really straight forward but maybe im wrong. Just drill out the rivets slide in the packing and replace the rivets? So many questions, so many things im going to have to worry about. Any suggestions are welcome. Thanks in advance. Also, if you have any of the parts I will need and you can give me a decent deal on them or you can do any of the machining/other work I will need done for a good price let me know. Thanks!

[zrpilot]

Wow,

You have a lot of questions. I'll address your engine.

The first thing to do is to disassemble the motor and assess the damage. If the head had bent valves, there may be more damage (like piston damage). Once the engine is apart, depending on damage, you have two routes. Used engine or re-build the one you have.

If rebuilding the one you have, I'd use Colby at C&D and over bore to a 416.
Also, if running 50/50 premium and race gas is what you want to do, then it gets more expensive. With a high compression motor like that (12:1) you are probably going to want heavy duty head studs for reliability. Next you will want a cam. The Hot Cams stage 2 would be a nice cam to run. Lastly, as far as the head goes. Most people do a 3 angle valve job. Over sized valves = $$$ and give you slight improvement over stock valves.

Building a motor is like making a cake, all the RIGHT ingredients come together to produce a great tasting cake. $$$ spent on big valves, porting / polishing, milled heads, ETC without a bigger carb and a full exhaust system will not yield desired results. These types of mods may actually hurt performance unless they are all done together!

For a reasonable amount of money you can build a 400EX to make HP numbers in the mid-low 30s. For a whole lot of money, you can make a 400EX make HP numbers in the low 40s... is it worth it???? Your call...

Read some more about others experience/builds on the exriders boards. Ask questions. Keep in mind your riding style and needs. Start saving some $$$. Tear your 400EX apart to see what your working with. And lastly make a build sheet that outlines the build. You'll know what to do....

good luck

[Antifaith]

I don't know anything about engine work so i'll leave that for others. But i did notice that you were considering buying the stainless kit from fastnerguy on ebay. Just wanted to let you know that the whole kit has allen heads. I wasn't aware of that when i ordered it and i wouldn't have if i had known.

It's still a great kit, but I've had so many experiences with the allen design, i can't stand them.

[Metzroth]

Wow, kinda glad to know that about the bolt kit. I think i will send him a message and ask him if he has a standard head bolt kit. If not, I will probably still buy it. Me and a friend are going to start ripping it apart this weekend. I will get back to you guys and let you know the cond.

[Metzroth]

Well, It's been awhile but im finally ready to order all the parts come tuesday when C&D opens again. I'm going with a 416 JE 10.8:1 piston. I'm getting stainless steel valves and replacing my valves seals. The cause of this mess was my cam chain tensioner, and that is what posting about. I was wondering if there is an aftermarket cam chain tensioner that does not rely on springs. I don't mind checking the tensioner after every few rides and i dont mind tightening it. Anyways, I know these kinds of cam chain tensioners are made, i just don't know where to find them, and if they are made for the 400. Let me know! - Mike

[Metzroth]

Ordered all my parts, everything is here. The only thing I haven't ordered is a bolt kit. The cyliner is at the machine shop ready to go. I bought a new head from 300EXtreme and it should hopefully be here this week. Colby said I would be running somewhere around 11:1 compression with the thin gasket kit and a 10.8:1 piston. I also ordered some stuff I didn't mention. My muffler had no packing, so I ordered that. I ordere Kibblewhite valves and lightweight valvesprings/retainers. An HD cam chain, and a new tensioner. I'm having a 3 angle (technically 4 angle) valve job done on my head and then high velocity porting on top of that. I think I am going to start assembling the first quad this weekend. I will assemble the 416 when i get my frame back from PC. Thanks for all of your help.

[RiderTRX400ex]

good choice on all your parts.. this will defenitely make some decent gains over stock.

I have never seen an aftermarket cam chain tensioner for a 400ex, my buddy had one go on him and it made horrible ticking noises, but we swapped in a new one and good to go.

good luck with your build!