Armen
Serious Thumper
Offline
Half-Witted Wrench-Jockey from Jersey
Posts: 1452
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Top end diagnostics is pretty straightforward, but requires some specialized tools and experience.
I start by looking at all the parts. Look at the rings for wear/break-in patterns. Look for even wear on the ring face, especially the middle ring. If you only see a little bit of wear on the face of the middle ring, prob the motor wasn't broken in properly.
Check out the piston between the ring lands. There should be minimal crud between the top and middle ring lands, and almost non between the middle and oil ring lands.
With a micrometer, measure the piston skirt near the bottom. Take a few measurements.
With a dial bore gauge, measure the cylinder-top, middle, and bottom. Check for out-of-round at each point by making multiple measurements at each depth.
Compare the piston diameter with the cylinder diameter. The manual will have wear specs. Usually the rings can deal with taper in the bore better than they can deal with out-of-round. Obviously, straight and round is the best.
On the race bikes, I make up top and bottom torque plates that can be bolted to using the stock cylinder bolts. With these plates in place, I'll measure the cylinder to get a more accurate read with the cylinder distorted by it's clamping forces.
I like to run bikes with piston to wall at the tight end of the range. This makes for a cooler running, quieter motor. But it also means you are more likely to seize if you don't play nice.
On my R65, the piston-to-wall is .0008". As in, less than a thousandth of an inch. I had to buy the cylinder a few drinks to get the piston in.
Often ring trouble can be traced to improper break-in.
My ritual:
Use a break-in oil in the bottom end. Proper amount.. There aren't too many out there. Dumb, petroleum oil. Honda, Amsoil, and Motul make oils suitable for break in.
Hone the cylinder using a Flex Hone, honing oil, and the proper technique. After honing, thoroughly wash the inside of the cylinder using hot soapy water.
Assemble the rings on the piston dry. Put a dot of oil on the piston skirt.
Resist the urge to slather oil all over.
Once the motor is assembled and the valves adjusted, put a fan in front of the motor. Start it up and hold to 2, 000 RPMs for about 30 seconds. There will prob be some smoke coming out of the exhaust, and it'll decrease quickly.
After 30 seconds at 2K, hop on the bike and ride.
Vary the revs. Go for the mid range, not lugging, or revving the nuts off the bike. Don't hold steady throttle openings, and don't sit in traffic.
By 100 miles, the rings are either broken in or never will be.
The goal here is to use the cross hatch from the honing to scuff the rings in. If you oil up everything, or take it too easy, the rings never have a chance to get up close and personal with the cylinder wall.
YMMV, but this works for me.
NB: the nice thing about using a used barrel for a big bore, is that it has probably distorted all that it ever will, and will stay rounder and straighter than a new one. Again, doing the stone (not the bead honing) honing with a torque plate helps.
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