Cylinder scratches after fresh rebuild HELP!

[scott7556]

I rebuilt my trusty 400 last winter, and being a machinist, I did everything by the book. I double checked all work that was done by anyone other than myself. Problem is that after I put everything back together, it would start smoking after 15 mins of hard riding, but just when the engine would "rev-down" from high rpms, never while on the throttle. Especially during "engine braking".

It bugged me that it was burning oil so I tore it apart. I notice that the ring end gaps had re-staggered themselves, but not one right on the top of another. I also noticed that the cylinder had slight score marks, some traveling the whole length of the bore/piston travel. Nothing you can catch a fingernail on, but still visible and noticed.

I went with a 85.5 10:1 Wiseco, I replaced my valve seals, I had a leakdown/vacuum check done just on the head, my 400 is very mild, nothing radical at all. I lubed everything well during assembly, and I rode easy for the first 5 hours, nothing WFO.

Does anyone have any input on what I might have doen wrong, or would some of you try new rings? Throw some ideas at me fellas, I just don't want to bore to the next size for no reason and have the dang thing do this all over! Can send pics of the cylinder if you want them!

[Mikeman1]

Was the clearance correct for sure when the boring and honing was finished?

[scott7556]

I measured ring end gap, wall to piston clearance, and everything else I could think of! I not only used the very best bore gages, but also a Brown & Sharpe CMM ($60,000). I wanted to do the best job possible...

[scott7556]

The other thing I forgot to mention is that even though I am to believe that the oil is coming from past the rings, the piston show sno sign of burning oil past the top ring. The piston dome is however very "sooty", along with the head. But I just can't believe that my brand new valve seals would be letting that much oil past, so I am almost positive that the oil is coming from the bottom up.

[rcatvrider]

Do you maintain your air filter well? I have heard of k&n's passing dirt if not properly and frequently maintained.

[sc400ex_rider]

what kind of valve seals did you use? i dont trust the $24.95 everything kits. i have read on here to only use the honda seals and replace them everytime you get a chance and have it apart. if you used honda seals then there is a chance the valve guides are worn. also when you bore or hone a cylinder you should always wash the cylinder with hot soapy water to get the grit and filings out.