Honda ATV Forum banner

400ex top end rebuild..

15K views 56 replies 9 participants last post by  SlammedRanger 
#1 ·
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
5
#28 ·
Yikes! That front stud looks nasty in the pic!

The head slides right over and it appears to be square in person tho.

However, I put the front/rear studs in backwards so I gotta swap those around.

All the parts should be here this Friday, if I do this right it should be burning gas by the weekend!

Quick question, I see a ton of post about porting the head, and a bunch of people saying that I’m gonna lose a ton of power out of the cam and big bore due to a stock head. Is the gains really worth it? I’m no
 
#29 ·
Honestly there are big gains to be made from head porting. But that being said my head isnt ported much. My engine machine shop guy did a few small casting mark touch ups but nothing crazy at all. He flow benched it and said it flows pretty well for not having any real porting done. The cam and 416 kit will wake it up alot reguardless. I even did +1 valves with basically stock porting and noticed a big gain. You will be limited mostly in your stock carb. The 400ex carb dosnt flow well at all. The fcr is 100% the way to go.
 
#31 ·
I do know one thing about intake porting........you 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.
 
#32 ·
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?
 
#33 ·
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 questions.......here 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!
 
#34 ·
Thank you misterclean59. Ive been googling, searching and looking at FB groups and these are all over the place. And then i find post from 10-15 years ago and just want the most up to date answers as possible. Im currently bidding on an aluminum oil tank so hopefully ill get that and scoops to start with.

Guess ill see how she does with all the new parts before i start messing with a rev box and all that.

Anyways, bought some used nerf bars last night. and my uni filter, oil filter and spark plug came in today!

Friday needs to hurry up. Should have my parts from them and my exhaust friday afternoon. My end goal in to hear this sucker fire friday night haha
 
#36 ·
Got the cylinder and piston put back on! But go figure i left the bolts at home lol



Clutch case removed



Chain off



and my parts!




Can someone provide me the torque specs for the clutch basket, the big bolt on the crank and basket as well? my stupid manual i ordered online wont be here until mid next week and was the first thing i ordered!
 
#37 ·
Yeah mister that’s the plan, just run it for now and if I fee I want/need a box I’ll get one later.

Gonna focus on cooling mods and suspension next. I’m sure this will be plenty for awhile.

Plus that the wife just asked for a quad.. so savings time again! ?
 
#38 ·
Clutch Center Staked Nut 80 ft lbs and re stake it of course
The clutch spring bolts there isnt a specific torque. Just snug them done 100s like this with no problem

Primary Drive Gear Nut 65 ft lbs
 
  • Like
Reactions: Shadow91
#39 ·
And just a reminder. As i have forgotten this two dang many times. Before you torque those head stud bolts please put the front timing chain guide in. It gets sandwhiched between the head and jug. I have personally ruined 3 head gaskets this way on various models over the years. Its a friggen bummer believe me lol
 
#40 ·
Well got the top end mostly buttoned up. New HD chain and cam installed.

Anyone willing to save me tho? I went to adjust the valves and two out of 4 caps broke off.. so i need new caps for the valve adjusters.. the 24mm ones..

if someone has 2-4 they are willing to part with and can overnight them to me in a small envolope i would be forever grateful. Just PM me your paypal info.

just awaiting the pilot jet and exhaust otherwise, both if which should be here friday! Shes getting there!
 
#41 ·
A couple things.
About to cooling, with only having the 416 you shouldn’t have a problem with heat. I added to the scoops to my bike and didn’t notice a difference

About the caps. I have a couple off one of my parts bike that I could send you, PM me


Sent from my iPhone using Tapatalk
 
#42 ·
I agree with the heat not being an issue to an extent. But i cant help but notice there are sand tires on this quad. And if hes riding in sand then he will 100% have over heating issues. I only ride a sand quarry sometimes and it get so hot i can feel the heat come up through the seat. Sand is a whole different animal. Its like your riding in an oven.
 
#45 ·
Yeah mixed dunes and trails. Would rather it be over cooled then under haha



Xplicit youre my hero, PM sent.


If only they had liquid cooled on the Honda 400ex. I think you would quite literally have a tank. And no problem bro :)


Sent from my iPhone using Tapatalk
 
#47 ·
Okay, got her all wrapped up today!! Fired right up and is mostly broken in haha

So far all i can say is WOW. What an beast. This thing has no issues pulling the front up lol

But, go figure, my starter soleniod decided to give up, so new started soleniod ordered.. right now ill stick to jumping contacts

Also went back to an OE chain tensioner. Idk, just dont trust myself to set it right and would rather just not worry about it.

Also ordered new front plastics so i can get rid of the dang race cut up front. Shes just about done!!! (for now)







Want to say THANK YOU to everyone that helped!!
 
#50 ·
Done this before^^^^^. The detent that holds the choke in the positions is worn out. There is a tiny little ball in the plastic that drops into the holes in the metal plate with the screw that holds the arm on. Either the ball is gone or the metal plate has a groove in it.
 
#54 ·
Okay. So finally got today to actually ride the quad. Curious tho if it’s already getting too warm. My head is about 300-350, right at the spark plug about 4-450, intake side about 250-300, oil tank about 180 and cooler about the same as the tank. This sound about right? This is using a cheap IR temp gauge
 
#55 ·
Those nunbers are high. Not insanely high. But higher then i would like to see. Although it is winter and temps will be cold so it will be running leaner then when your riding in the summer. It is very very hard to jet properly in the winter. I would suggest waiting till it warms up to your normal ambient air temp to jet it. But again if your riding sand dunes go up a few extra steps on the main and i wouldnt be shocked if you need a 45 pilot jet. Full pipe, cam, 416kit i was running a 45 pilot with mixture around 2 turns out.
 
#57 ·
If you want to PM me your email or phone number i can send you my setup. I used a 4 or 5 inch spal water proof dust proof fan and i mounted it on the engine side of the oil cooler. I used a puller fan so it pulls cool air through the vents in my hood and blows right at the head cooling fins. You can feel air on your hand holding about a foot from each side of the engine. I also wired it on the stock headlight switch since i dont have a headlight anymore.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top