The part that connects the piston to the crank is the con (connecting) rod,
but what your mechanic probably meant are possibly the babbet bearings on
the "big end" (crank end) (if worn) and the piston pins at the top (if the
pistons are replaced). Worn bearings are usually diagnosed by a loud rapping
or knocking sound under load, coupled with excessive oil consumption.
Doubling the bearing clearance increases oil consumption 5 times. Bearings
don't usually have much effect on compression -- in fact, the extra oil
thrown off by worn bearings may increase the measured compression, kind of
like a wet test. Really bad bearings result in galling of the cranckshaft
journals, which must be built up, machined, and hardened. $$$PITA.
There are two gaskets that need to be replaced when the head and block are
removed: the base gasket (bottom one where the block sits on the crankcase),
and the head gasket (just below the carbs, where the head sits on the
block). However, there are also several seals and o-rings around the
cylinder sleeves and oil passages that must be replaced when things are
separated. The uppermost (rocker cover) gasket is re-usable rubber, and
generaly doesn't need to be replaced unless damaged or showing signs of
leakage.
Unless you suspect worn bearings, the engine does not need to be removed
from the frame to change pistons, rings and gaskets, but removing it does
give you more room to work.
I strongly recommend taking it apart and checking everything BEFORE getting
carried away buying new pistons, etc. If the problem is limited to stuck
rings, you may be able to just replace them. On the other hand, you may find
on measuring the cylinders that a re-bore is required -- which means you
need oversize pistons and rings, not the standard size.
I doubt you will need to replace the valves: possibly lapping them with
grinding compound, or at worst, you might need the seats re-cut, and the
valves re-faced. (Did you ever do a wet compression test?) With the head
off, you can remove the valves and check for pitting or burning. The guides
are pretty durable too, but if they're worn, I would replace them rather
than knurl them on a bike. It's good practice to replace the valve seals and
check the valve spring free-length if you remove the valves. Some shims will
definitely have to be changed if you end up doing any of the above valve
work, but they're routine annual maintenance anyway.
Any machine-shop work (like boring or facing) should be done by someone
experienced with motorcycles. The clearances are sooooo much smaller than
normally found on cars. Everything else, including lapping and honing, can
be done by a "shadetree mechanic" -- including you if you're so inclined.
tafn
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