Im stumped

[dxcody]

Ok so i was fixing my buddies 400ex (replaced the timing chain) and i got it all put on and everything and i started it up and it smoked out of the exhaust like you ran through cold water with a hot engine.

I asked him if it always did this and he said yea and i told him that it needed rebuilt. So he asked me to do it and so i tore it all apart this evening.

The piston is horrible, and smelt horrible i might add.. It was burning oil really bad. I told him (its a 426) the only thing to do if the cylinder is thinned out, is to go 440 or got back to stock.. The piston is not in bad shape at all, and the rings dont appear to be. (replacing them both anyways!) But i dont get why it was burning oil so bad? I remembered Ron saying cylinders often get "egg" shaped so i did this method... (idk if it works or not)

I took the top ring off the piston and put it in the top of cylinder, took a feeler guage that fit "snug" in the ring gap, moved the ring down to about middle of the sleeve, it had the same gap as it did in the top, and same for the bottom.

Could the rings just be worn? Like i said i am going to replace them anyways but i just didnt want to put it back together and it start burning oil again..

[CJM]

It might fit snug but it is probably slightly out of round letting oil pass thru the rings. Mine was probably more worn but easily once measured with a bore gauge you could tell.

LEts put it this way, if you assemble the piston and put it in it should be somewhat hard to do. If it just plops in with ease the bores worn out.

[dxcody]

well i went out and checked again, and put the ring at the very bottom of the sleeve where you can still see the hone marks and it measured the same as well.

Just to be safe do you reccomend buying a new sleave?

He doesnt want to deal with the hassel of a 440.

[400exrider707]

Putting the ring in doesn't tell you if it's out of round. You can get the same measurement over and over from the gap but it still doesn't tell you the profile of the cylinder. Bring it to a machine shop and see what they say. Does the ring feel like it still has some "spring" to it, it could just be plumb worn out.

[WwsivicwW]

check your valve seals and valve seats & guides as well.... that will cause oil consumption

[dxcody]

The rings do look pretty worn, they feel extra fragile for some reason. They dont have as much spring to them as the new rings i just oredered for mine. I got to looking at the ring gaps and they arent gapped quite right, they were basically in a stright line..

Replacing the valves and seals!

Thanks for the replies everybody!

[HondaRacing83]

could be carbon build up

[01boneless]

worst advise ever lol

[CJM]

What you really need is a bore gauge and sadly a good one aint cheap.

Did you try putting the piston with the rings on it? When my bore was worn I could EASILY push it in with a piston ring compressor and a tap to the thing. With the new piston, rings and 426 bore it was harder, I had to SMACK it in there.

But really I knew the bore was going and oil was being consumed b/c:
1. Valve seals were new
2. IMPORTANT: piston freeplay when in the cyl on the bike was WAY to much after comparing the two setups. I could move it with ease.
3. New rings garnered the same as the above-so I knew the bore was worn.
4. Ring end gap was to great
5. Despite everything being fine when the bike was assembled I was consuming oil at an alarming rate-but weirdly only smoking a little bit.

IMHO Id buy a new sleeve and if you want the power again do a hi-comp piston std size. Sadly it wont have the same power the extra few cc's gave you (it really isnt much). But starting at the lowest size is a good thing. Had it been a std size now he coulda bored up and not had to do a new sleeve or 440.

[xX-Clutch-Xx]

When he did the 426 punch, did he use a different brand piston? Wiseco , JE , Ross, are all close but the differences could lead to wear or improper seal. otherwise go have the cylinder measured, Thompsosn here just did it free for me, and have the piston mic'd. mau need rings, valve seals, or sleeved. id just look on ebay for a stock cylinder and get a 402 piston and have it bored.