Honda ATV Forum banner

400ex top end rebuild..

13437 Views 56 Replies 9 Participants Last post by  SlammedRanger
Okay, so about a week ago at the dunes and noticed i had oil leaking from the head gasket. Finally got around tonight to tear down the inspect the engine and measure the bore. PO told me it had a stage 2 hotcam and a 440 kit but as i just found out, he was lying out his rear end. After measuring the bore, its an 85mm piston and looks to be a stock (very worn) camshaft. All in all, exactly what i was hoping for. Was also extremely surprised with how well the bike ran and how much power it had. When i removed the crank plug to rotate the piston, with the spark plug still in, i was able to move the piston with nearly just the weight of the ratchet.. I assume there should be considerably more compression on these..

Anyways, I want to do at the very least a top end rebuild. Thinking about doing a 416 kit and stage 1 cam. But from the looks of my head i need a new one.. center bearing has some pretty good grooves in it and i dont think there is much that can be done to save it (correct me if im wrong)

I want the best balance of power and reliability. So just off my head what i need is:

Wiseco 11:1 piston
Hotcams stage 1 cam
GT Thunder head studs
Cometic gasket set

Anything else I should worry about right now? What about valves and springs?

Anyways, few pictures of the parts..

First off the cam

The head

and the piston/cylinder
See less See more
1 - 4 of 57 Posts
I do know one thing about intake can go too big. Air velocity is important for mixing and with a cam with a lot of overlap at high rpm's. Generally straighten the intake tract with minor increases to the diameter and a satin finish. The exhaust really can't be too big (generally). Straighten out the path and mirror finish.
  • Like
Reactions: 1
Okay. I might grab the dermal and just smooth the cast marks. Don’t think I’ll port it really. I don’t have the skills nor the cash flow, haha

Let’s talk about cooling mods real quick.
Different cooler, Different tank, Air scopes, etc.

Just don’t wanna overheat this after I’m done.

Rev boxes worth it with the cam and big bore?
There are others that have much experience with your particular model on this site......I am not one of them. Again I have much general knowledge about your is some of it in context to your questions. Anything you can do to help cooling an air cooled engine is a good mod. Increased horsepower equals "exponential" heat increase. Rev boxes work but they increase the heat and stress the valve train. Hopefully others here can consider the upgrades you have done and give you some specific advice. Good luck!
  • Like
Reactions: 1
Why don't you just run it without the rev box first? You can always add one later. Usually a rev box doesn't work well unless you have the motor built to rev. Building one to rev takes much money.
Try the richest main jet that still runs good.....this will keep the heat down quite a bit a may help your mid range torque. It may take a few revs off the top tho.
1 - 4 of 57 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.