XR Rebuild Questions. Suggestions & Opinions

[boogie_4wheel]

I have an XR400 that I'm having an issue with. In July it was starved of oil because a broken link on the cam chain sliced up the chain guides and little pieces of plastic clogged the trash screen on the frame (dry sump). I tore it down and sent it off for a hone/replacement. Machinist said there was barely enough left to bore it (it was a 435), so it was resleeved as a 426. The cylinder along with a Wiseco piston was returned to me, and he also welded and surfaced the rocker arms.

Assembled last weekend. Checked ring gap, put it all together, installed a used cam from my father in law's XR4 (he got a new HRC unit). The cam he gave to me has the auto-decompression assembly on it. Double checked valve adjustment, engine rolled over fine (although a little tight it seemed). The manual decompression lever was not helping any, but I figured it was the auto unit doing its thing.

Start bike, idle for about 30sec then start down the street. Light throttle, 1st, 2nd, 3rd gear and I'm approaching a stop sign (maybe 50yds traveled) so I roll off the throttle, feels like it is starting to sieze, pull clutch and bike stalls. Push it to the house.

Cam lobe and rocker arm face that is used for decompression shows wear. Reinstall valve cover, check valve adjustment (it's a little loose at .015 which is where I set it). Pull plug and bore scope, cannot see any issue. Kick bike and get oil up to the top of the engine so no oil pressure problems.

Pulled head and show some cylinder markings. Doesn't catch fingernail. Piston has wear in same spot, and (coincidence?) top ring lines up with this mark as well. Pull cylinder, pull rings, check ring gap again in case I screwed up.

Wiseco paperwork = .01386" required based on bore
XR Manual = .008-.014" (for stock cast piston)
Actual ring gap= .015"

I noticed that the cylinder is tapered slightly. I was going to check the piston-cylinder gap, but don't know at what part of the cylinder to do this at... The piston is loose at the top and bottom, but mid stroke the piston is tight (just below where the mark is), and it will not fall through the cylinder (I don't even know if it should).

I'm sure I've got the piston in the right direction (arrow forward just like the paperwork says). I'm 99% sure the rings are correct (#'s facing up), and clocked the gaps just as the paperwork said.

What do you guys think? Hone it? Cylinder too tight? Assemble and try again with another cam (I have a stock profile without the auto-decomp installed)?
I've got a message to the machinist, but it's Saturday...

I'm just looking for a opinions on this. This is my first time dealing with a piston/rings/gaps. The second picture is the back of the cylinder (#'s on piston, line up with intake) and I don't know why they are there, they don't line up with anything.

 

[Tonkaman]

Did you check the bottom end for play? Perhaps the crank has a little slop in it?

 

[Gravy]

Yeah check crank end play.

Check your ring gap at as many places as you can on the cylinder making sure the ring is square in the bore (use the piston to make sure it's square).
Any good machinist wouldn't bore a cylinder tapered, and you shouldn't have any ring stop marks on the cylinder after only running up and down the street. That looks like a used cylinder.

 

[boogie_4wheel]

I've come to the conclusion that the cylinder is shaped like an hour glass; it's tighter mid stroke. I checked ring gap in different spots to prove this, and the gap is tighter in the middle. As it started to warm up, the piston didn't fit anymore. Talk about pissed...
Supposed to be a new sleeve and piston, that's what I got charged for anyway.

Thank you guys!

 

[boogie_4wheel]

Follow up...
Spoke with machinist yesterday, and he swears the piston didn't bind when everything was shipped. He said send it back and he'd fix/replace as necessary on his dime.