400ex: Bore to 426

[pickin_aggie08]

I know this topic has been exhausted throughout these forums, but i still have a few questions... From doing a little research, i have decided to go with a 426 instead of the 440, because i dont race, and i dont want to deal with the overheating issue, and blowing head gaskets. As older 400's go, my rings are leaking, and i now have discovered a small oil leak out of the top, so it is due time for a rebuild. I figured while i was in there, i might as well make it a little faster by boring, and im also getting a hotcams stage 2 for it as well. My questions though are, do you need to replace things like the cam chain for a heavy duty one, or just a new stock one (chains stretch!!), and what about the guides, and tensioner? Is it a good idea to replace valves, valve springs??? Im pretty new to all this stuff, Ive never bored an engine before, but have a pretty good idea of what is going on. I know lots of you guys on this forum have done this before, and know what is or isnt going to tear up. Im kinda on a budget, so i dont want to spend $$ on unneeded items, but i dont want my stuff to fall apart the first time i take it out either.... ANY input on this would be greatly appreciated...
Thanks,
Eric

[Sanddraging28]

i have built a few 426's. its a good bike, nice power, and reliable if built right.
a hot cams stage 2 cam is about perfect for a 426. to get the best power out of that cam you need to go to a higher compression piston, at least 12 to 1 compression. but by doing that you need to replace the head studs with a aftermarket set. i would replace the cam chain while you are rebuilding, like you said chains stretch. i would get a cam chain for a CRF 450R. its a lot stronger than a 400ex cam chain, also replace the cam chain tensioner. i don't think you will have to replace the chain guides. never seen any that i had to replace in my motors.

when it comes to the head, you really dont need to do anything unless you really want to. if you are going to have the head ported and polished then a good valve job and possibly +1mm bigger valves would really help the power, but it gets kind of pricey when you start having a bunch of head work done.

[dariusld]

You will learn alot if you read some of the older threads on here.

I usually have a different opinion then most people on this forum, including the previous posting. Hardly anyone goes 12 to 1, 11 to 1 is more practical. Unless you have to, you should go with the smallest bore possable. After 426 you have to buy a new cylinder:( . Why not 406 or 416. When I rebuilt my bike , I just got a stage II cam and a high compression piston and some gaskets. My bike is a 2000 and has been ridden very hard in sand and its still running strong. Haven't had any problem with the cam chain or tensioner. To me ,thats just overkill.

[Sanddraging28]

big difference between practical and scared!!!!!!
if you are looking for power 11 to 1 is just not gonna get it. a stage 2 cam's lift and duration will drop the compression ratio drastically. so with a 11 to 1 piston you are really only getting 10.5 to 1, maybe.

when it comes to bore size, you show me a 406 that makes decent power. its like riding a stock bike. who wants that. come ride in the sand with me and i will show you a bike that runs strong.

and if your bike is a 2000 and you haven't replaced the cam chain or tensioner, then that is just horrible atv maintenance....

leave the advice about building motors to other people.....i just don't think you totally understand it

[dariusld]

Originally posted by Sanddraging28
big difference between practical and scared!!!!!!
if you are looking for power 11 to 1 is just not gonna get it. a stage 2 cam's lift and duration will drop the compression ratio drastically. so with a 11 to 1 piston you are really only getting 10.5 to 1, maybe.

when it comes to bore size, you show me a 406 that makes decent power. its like riding a stock bike. who wants that. come ride in the sand with me and i will show you a bike that runs strong.

and if your bike is a 2000 and you haven't replaced the cam chain or tensioner, then that is just horrible atv maintenance....

leave the advice about building motors to other people.....i just don't think you totally understand it

I don't totally understand building motors, but I can see all your ignorance in your post. For one, did you see where he said he was on a budget?
"when it comes to bore size, you show me a 406 that makes decent power." Talk about not understanding anything.

"and if your bike is a 2000 and you haven't replaced the cam chain or tensioner, then that is just horrible atv maintenance...."
So I should just go around replacing perfectly working parts?

"if you are looking for power 11 to 1 is just not gonna get it"
I bet you 90% of the 416's out there are not running anything higher than 11 to 1. Why didn't you tell the budget minded guy about the expensive race fuel he is going to have to run with that 12 to 1 piston?

[ds268]

Originally posted by Sanddraging28
big difference between practical and scared!!!!!!
if you are looking for power 11 to 1 is just not gonna get it. a stage 2 cam's lift and duration will drop the compression ratio drastically. so with a 11 to 1 piston you are really only getting 10.5 to 1, maybe.

Lift and duration do not affect compression ratio - this is a volumetric measurement of cylinder volume at BDC compressed to the final volume at TDC. Big cams affect cylinder pressure at low rpm because of valve overlap.

[ds268]

Originally posted by Sanddraging28
and if your bike is a 2000 and you haven't replaced the cam chain or tensioner, then that is just horrible atv maintenance....


Horrible maintenance? I guess if honda posted this as a requirement in the owners manual. I'm still running my stock chain and tensioner. I've always taken great care of my two wheelers and haven't ever had a problem with a chain or tensioner and don't consider this to be "maintenence" by any means.

[Sanddraging28]

1. with a 12 to 1 piston, race fuel is not required. you can run 93 octane with octane booster..

2. race fuel is not that expensive. i get 112 octane for $6 dollars a gallon.

3. the whole point to rebuilding a motor is to have a reliable motor, why tear into a motor and not replace wear items. cam chains stretch and tensioners wear out. why spend the money on a rebuild and not replace these items......doesnt make sense. have you seen the destruction that stretched or broken cam chain can do to a motor...i have

[Sanddraging28]

ds268,
The more duration the intake valves have the more important it is to have a high(er) compression piston. This is because with lots of duration there is more opportunity for air to escape through the valves as the piston begins its compression stroke. Even on the stock cam, the intake valves are still open when the piston begins its compression stroke. With an aggressive cam some dynamic compression is lost through valves that stay open for a long time. This must be made up for with a higher compression piston. An aggressive cam with a stock compression piston may actually perform worse than the stock cam because too much compression is being lost through the valves being opened so much. I have personally seen this happen--dyno confirmed (although it wasn't a 400EX).

Here's a theoretical example to help in understanding this concept:

-bore: 85mm (stock)
-static compression ratio: 9.1:1 (stock)
-elevation: 1500ft
-intake valve closes: 40deg ABDC (After Bottom Dead Center) (stock at 1mm lift)

The resulting dynamic compression ratio is 8:1. Note that is a smaller figure than the static compression ratio we started with. Again, this is because the actual compression of the compression stroke doesn't really start until the intake valves close.

Now, if we increase the intake valve duration to 55deg ABDC and leave the static compression ratio at 9.1:1, we get a new dynamic compression ratio of 7.4:1! In order to get the dynamic compression ratio back to where we started (8:1) with the increased intake valve duration we would need a piston with almost 10:1 static compression. Just for the heck of it I looked at how 5500 ft of elevation changes things. At 5500ft with the 55deg ABDC spec you'd need a 10.8:1 static compression piston to yeild an 8:1 dynamic compression ratio.

Now, you might be thinking what's the point of adding a cam and a piston if doing both gets you the same dynamic compression ratio you started with?? Good question. The answer is that now you have the same dyanmic compression ratio you started with but also have the cam specs that will allow the engine to breath better. And in the real world, most piston choices are actually going to bump your dynamic compression beyond what it was stock even with the addition of a cam. A 10.5:1 piston with a cam (using the 55deg ABDC spec) would get you a dynamic compression ratio of 8.6:1--0.6 points higher than all stock.

I used a dynamic compression ratio calculator to get those figures. I guessed on certain required values because there's some information I don't have. So, the above numbers are not quantitatively exact. But the relationship between static compression, dynamic compression, and intake valve timing IS accurate.


IMO cam chain replacement is a part of maintenance, its also a very popular upgrade, check the many threads on it.
with the stress that a new high lift cam places on the cam chain, why not replace it with a heavier one. i suggest it to everybody i build a motor for and very very rarely does a customer not want to spend the $30 to upgrade

thanks to GPracer2500 for his addition to this post

[pickin_aggie08]

thanks for all the input, and i had decided on an 11:1 just for the gas issues and all w/ higher compression, but my ? is.. for a 426 i have to get the thing bored .12 or 3mm over right?? And are they just boring the sleeve? or what... im pretty new to all this stuff, and dont really know! this may sound stupid but hey! and for my limited $$ would it be beneficial to go with a 416 instead of a 426?? how much difference in the overall power is that going to make? reliability??

[Sanddraging28]

you will not see a big difference between the 416 and the 426 at 11 to 1 compression. and yes all they are doing is boring the sleeve to your new piston size

[ds268]

This was a great explanation. I was not aware you were specifically talking about the dynamic ratio. I understand where you're coming from so please excuse my ignorance against your knowledge!

[Sanddraging28]

not ignorance, just a misunderstanding. it happens.

[pickin_aggie08]

okay... so tell me if anything seems too off base?? I'm going to go ahead and get a 416 just to be on the safe side, a stage 2 hotcam, a cam chain.... i found a kit on ebay with a wiseco piston and namura gaskets... are the gaskets going to be a problem being namura?? and just to top it off i just talked to a guy who owns a machine shop and he is going to bore my crap for $20!

[Sanddraging28]

sounds like you are on top of it now....the gaskets will be ok. sending you a pm