400ex Top End Rebuild Question?

[Wesjay400ex]

My 2005 400ex is in need of a top end rebuild. After talking with friends and reading forums, I want to be sure I do this thing right the first time....

Additionally, can you get away with just doing the Piston / Valves / Cam and not replace the Cam Chain and Guides?

New Parts List:

1) New Wiseco 11:1 Piston
2) Hot Cam Stage 1
3) Port and Polish
(Don't want to bore it out)
4) New OEM Valves
5)New Cam Chain
6) New Cam Chain Guides

I have already installed:
FMF Powerbomb header
FMF PowerCore4 exhaust
DynoJet kit
K&N Power Lid w/ Pre-Charger
K&N Filter

I know it runs well and is breathing fine I just want to be sure that if I do the work I cover all my bases and don't miss something crucial.


Any suggestions / help would be greatly appreciated.

[trailrider894]

Originally posted by Wesjay400ex
My 2005 400ex is in need of a top end rebuild. After talking with friends and reading forums, I want to be sure I do this thing right the first time....

Additionally, can you get away with just doing the Piston / Valves / Cam and not replace the Cam Chain and Guides?

New Parts List:

1) New Wiseco 11:1 Piston
2) Hot Cam Stage 1
3) Port and Polish
(Don't want to bore it out)
4) New OEM Valves
5)New Cam Chain
6) New Cam Chain Guides

I have already installed:
FMF Powerbomb header
FMF PowerCore4 exhaust
DynoJet kit
K&N Power Lid w/ Pre-Charger
K&N Filter

I know it runs well and is breathing fine I just want to be sure that if I do the work I cover all my bases and don't miss something crucial.


Any suggestions / help would be greatly appreciated.


You need to do the cam chain. Get the CRF450R timing chain. It is a perfect fit.

[duroc825]

Yeah I guess you could "get away with it" But it's a cheap and easy switch out that will be better in the long run.
Why the OEM valves though?

[Wesjay400ex]

I know its less than $80 bucks, but what would be the greatest benefit of running Wiseco Valves / Tusk valves? Honda OEM Valve train parts are pretty darn good as is.

I have heard mixed reactions about doing the valve springs since they normally last 2 - 3 builds. So I didnt want to do a full on valve train rebuild.

[xcracer416]

being a 400ex i would recommend the cam chain off a crf. the 400ex generates a lot of heat, that needs to be considered in your build. there is more to it than just putting a piston in. the cylinder needs to be thoroughly checked for out of round, taper, etc... even though it might be running good now doesn't mean any of these specs are towards the max spec on the stock bore. if you installed the piston and didn't check or have them checked it reliability could be shortened.

the stock piston from honda is cast, a wiesco is forged. forged pistons don't swell as much as a cast piston so your piston to cylinder clearance is crucial to ring life. Also check your ring gap and make sure you do your math to get them as close as possible also. the sheet for that is in the piston box

the stg 2 or xr cam would be a sweet cam choice for that build in my opinion.

[Wesjay400ex]

My only concern about a stage 2 cam would be the loss of bottom end. I ride in NJ, a lot of tight trail, woods, fields and minimal track, so I'm not looking for top end speed.

Would the stage 2 effect my bottom-mid that much as compared to a stage 1?

Thank you for info on the cast vs. forged. Much appreciated. I have a good feeling the cylinder is going to have to be sent out since it is the first rebuild on a 5 year old motor. So I will be more than cautious to take the ring gap and tolerances into consideration. That is why I am collecting information and doing this thing right the first time. (That's the hope).

[CJM]

Welcome to the site and excellent another NJ rider. Where you at, Im near six flags in jackson.

Also please THROW out the dynojets, I tried them and they are absolute garbage. Go to www.jetsrus.com and order stock keihin jets and use them-work 100x better than DJ junk.

Anyways this is copied from a post I made a few mins ago and will help:
Lets do a comparison:

1. Stock 400ex, hotcam stage 1
2. 416 11:1, hotcam stage 2, open airbox lid, exhaust.

The above is my friends and mine (#2 is mine). He likes his lowend power, he has no real topend with the cam. yes its there-but it just kinda tapers off to nothing it seems. But even if the compression was higher I dont think a stage 1 would work to well since it was designed for a stock engine (stock bore and basically nothing more than an exhaust), it was designed for trail riders in mind tho and does have good lowend till you hit 4th gear where your topend mostly is.

Now mine of course is obviously gonna be faster, but I have plenty of torque-can lift the front up with ease in almost any gear. Would a simple piston and cam do all that without the rest of the mods-Im not sure.

What I can say is I have no issues flying thru some very tight trails with my setup. Not only am I quite happy with the entire thing, but everyone who has ridden with me is impressed. I have no issues at all keeping up with and pulling (depending on rider and setup) alot of 450s b/c I have a ton of torque. I can let off the gas 100% and be rolling and blip it and it pops up.

When I built it I only used high quality components. I have a wiseco piston, cometic gaskets (which are similar in thickness to the thinner XR400 head gasket). So maybe im getting more compression than you would using normal gaskets. I also run 93 octane without a single issue at all.

If you perhaps want to run a lesser compression than perhaps the sparks cam is the one to go with. Its been explained to me that its a 1.5 stage hotcam really the way the duration and lift is-maybe best of both worlds.

Still, what people often fail to understand about these engines is bore size is relative-its compression you want-thats where you make the power. If I could run a 12:1 piston on pump gas Id do it-we cant b/c the heads air cooled and generates to much heat with such a setup. This is why I recommend to everyone 11;1 and run on 93 with the supporting mods.

[Wesjay400ex]

Thanks for the additional information. I wanted to hear first hand experience from other riders before I made the purchases of the parts. Given the minimal price difference from Stage 1 to Stage 2 cam it might not be a bad idea to roll with a stage 2.

I haven't ever had an issue with my Dynojet kit thus far (4 seasons). But i'm always looking for better parts.

I live up in Long Valley (northwest Jersey).

[CJM]

The DJ were ok, but I had a bottom end bog that wouldnt go away period and many others do as well. I switched to keihin jets and it went away.

If you ever want to ride hit me up, Im about an hour south of you and we ride in a very large sand pit with many trails. Usually go on the sundays or saturday if its nicer than sundays forecast.

[Wesjay400ex]

Thanks for the feedback... will look into the Kehinin kit.

If you ever wanna ride up here hit me up. Got 100+ acres and a small MX track.

[xcracer416]

for what it cost i would get a sparks timing key also. you will not be dissappointed

[CJM]

Originally posted by xcracer416
for what it cost i would get a sparks timing key also. you will not be dissappointed

I agree, BUT you cannot run a sparks key on a 11:1 bike-it will overheat. 10:1 is ok with 93 or higher octane tho.

[duroc825]

I went stage 2 Hot Cam and didn't lose any bottom end at all. But the top end sure got a gain! :D

[Wesjay400ex]

Thanks for the feedback on the Stage 2 Cam. Since there is no price difference I will go with a stage 2.

Since I am going to do this right, has anyone dropped teeth on the rear sprocket??? I am thinking of running 15 front / 36 rear instead of the 15 / 38 that is the stock.

[CJM]

That will gain you speed, use this calc and change the rpm to 9100 http://home.earthlink.net/~gellett/400ex.htm

[Wesjay400ex]

That is a fantastic tool.

Thanks for all the feedback. Looks like it will be a 36T rear sprocket than.

[CJM]

Just remember that when you change it you may not like where the power winds up. Ive tried several sizes and finally went back to a stock setup cause it had the best all around drivability. stock is 15/38

39 rear had some nice torque but I felt I was down quite a bit on speed.
14 front was good but 1st and 2nd and some of 3rd was worthless. I was shifting like nuts
16 front was nice all around gain, I just felt it didnt have enough bottom end.

Course mine is also a very worked 416 with a stage 2 hotcam thats designed for mid to higher end power as well.

heres some good info:

* A SMALLER than stock FRONT sprocket will give you more TORQUE.
* A LARGER than stock FRONT sprocket will give you more SPEED.
* A SMALLER than stock REAR sprocket will give you more SPEED.
* A LARGER than stock REAR sprocket will give you more TORQUE.
* By changing 1 tooth on the FRONT sprocket, that is about equivalent
to changing the REAR sprocket 3 teeth.