400 rebuild time....

Opened up my engine today, time for some new engine parts. Just a smidge of wear on one side of the piston, bore is good with original hatch marks showing, bit of play on the big end of the con rod, looks like the piston has been barely kissing the exhaust valves too (that was close).

So, I'm going to stick with stock bore, new factory piston/rings, crank bearings, rod, time chain, and valve train parts. My questions regard the following:

Cyl. bore prep: My mechanic here say's he does not want to ball hone it... but wants to do "something" by hand, I don't know exactly what. Anyone have any good recommendations as to how to treat a stock bore for new rings/piston? Seems to me a real quick light swish with a well oiled ball hone would be the ticket, but I hear a scotch brite works good too (I don't want to fight with my mechanic)... anyone have a recommendation as to exactly which scotchbrite to use and what specifically is the prescribed technique?

I have access to a new 426 rod and new 400 rod, is there a way I can use the 426 rod with the stock 400 piston? Can I re-machine the hole in the 426 piston for the bigger sized pin (sounds like a bad idea to me)? Oh... *sigh*, and the geniuses here don't have piston pin keepers to sell with their new pistons... with this old style circlip can I re-use them or is that complete blasphemy (I'm an unrepentant blasphemer BTW)?

Top end... valve parts are not easily had here (south america), and they all I'm lacking for proper rebuild. What, lacking obvious signs of valve train problems, is ESSENTIAL to replace? The valve keepers (two-part cotters) are most crucial right? What about the (disk-looking) "retainers"? Are they prone to wear or breaking? Springs are cheap, just not available here without fedxing them from the US, but do they really break much on a stock 400? And about the "valve seats"... let me get this straight... these seats are easily replaceable by a machine shop? Nice idea, wish I could get them here.

Likely dumb question is (drum roll please), is there any way to replace the valve keepers, springs, seals, etc (and add a hot cam) at in a couple weeks without removing the head again???? I know it can be done on cars with a proper tool, how bout bikes?

Lastly, any other typical trouble spots on this machine that I should look for? How bout the oil pump? Oh, the woodruff key too huh.

I'd prefer to do this all a once (and let someone else get their hands dirty) but maybe it's best just do the piston/rings/bearings now, put the head together with what new parts I can get now and then do that part correctly myself in a couple of weeks when all the right parts come.

thanks y'all,

Mark Sheldon

how many KM did you have on the bike? why did you strip it down? was it getting noisy?

Yes, it was getting noisy, I was hearing the piston touching the valves for one thing, I did not know what the noise was till now. Why... do you think it's unusual for a bike that is 7 years old and basically untouched to need some attention?


Oh, and have no idea how many Km is on the bike, have only owned it a year and the odometer is useless. Honestly though, for it's age... not many.

In would worry more about the valve springs than the keepers and retainers...they are the most highly loaded parts. To use a 426 piston, you need to use the 426 rod. I believe you can swap it as a direct substitution when you have your crank rebuilt. What about a Wiseco 13.5:1 piston kit instead of stock? Big boost in performance.

Absolutely DO NOT reuse circlips...even if you have to wait 3 weeks for new ones to clear customs. It's asking for trouble!

Valve sats are not "easily" replacable, though they are easliy recut if a machine shop has the right equipment. Most automotive shops don't have the right tools. You want someone with a "Serdi" machine.

I would be somewhat leary of a hand job on the bore. That's just asking to get it out of round.

As far as getting valve train parts, I believe the TT store will ship internationally. Yes, Customs is kind of a pain, but should take no more than 3 or 4 days, tops. You might want to call or e-mail them to discuss specifics.

I don't think the stainless valves on the 400s wear as much as the Ti valves on the 426's. Measure the diameter of the stems, look for wear on the faces, make sure the stems aren't bent, and lap 'em in good.

Thanks Birdy...

Mark Sheldon...

Why are the valves contacting the piston is the question I would be asking myself. Is it valve floating from weak springs at high rpm or did the timing chain skip? At any rate, your valves are probally bent and need replaced. A close inspection of your retainers and keepers should reveal if there is damage/wear to them.

You can replace the valves and seat them in using valve grinding compund and a lapping tool(few buks), then do a leak test with solvent or gasoline.

I believe you have a nickel plated cylinder(Nikasil?) and it is a very hard metal, and if not showing any signs of wear you can clean it and install your new piston and rings. Dont try and hone/glazebreak/or cross hatch it.

i've finally found a 426 barrel, so its rebuild time for me too now. only been looking for about six months.

going to upgrade to a 426 and go back to wr timing, so as to reduce the 'savageness' and hopefully prolong its life.

from all i have learned here the 426 small end is a lot stronger in the 426 rod/piston than the 400 and 26 extra cc's cant hurt to keep that front wheel flying......

good luck and keep us informed of your progress.

funny though, my big end had a little play in it but the small end was tight.



Are you sure about the play?

conrod can tilt at the top and move left to right at bottom.

It shouldnt move up down.

had an issue on ktm, conrod gone bad, but no play could be felt.

It was minor bearing surface defect, only to be seen when dismantled, but bike had quite strange noises coming out of cases when running.

but than again, it was ktm

no, actually im not sure, thats what the man told me... personaly I could not feel any movement at all on any end, except the side to side you speak of.


Well, there is an option to check play in big end, but crank has to be removed...

grab the conrod in one hand, with crank hanging on the lower side.

Hit the conrod on the small end with your palm and there shouldnt be any noise, it should be compact.

my big end bearing had very little play in it, but i noticed that when I spun the crank by hand, the bearing would bind up after spinning freely for a few seconds (so, it would feel normal for a few rotations, then all of a sudden, it would bind completely)...I would check that things are spinning freely before ruling it out...

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