Crank rebuilding
Crank rebuilding is generally $50. This includes pressing apart the crank,
installing new pin, bearing and con rod, and truing to within 0.0005"
Truing work done between precision centers, using precision dial test
indicators for accuracy. $25 extra charge if crank is rusted & needs
bead blasting. Note: while the lobes can be ugly, the stubs must be clean
and undamaged at seal-mating surfaces and bearing-mating surfaces. Tapers
must be clean, and keyways sound. Threads should be good. Center holes on
crank stub ends must be perfect. If somebody used a hammer on the crank
stubs and distorted the ends, it's nearly impossible to true the crank.
I'll polish cranks with damaged center holes and you can use them for a
paper weight.
Why true a crank?
The main bearing on a crank often outlasts other engine bearings and bushings,
making it possible to keep old cranks in service a long time, and often allowing
them to remain un-rebuilt during an engine overhaul. That's fine if the big end
bearing shows no play and axial play on the connecting rod is within tolerance.
The trouble comes when the crank is removed and reinstalled, which in vertically
split engines generally requires some method of force to pull the crankshafts
into their supporting bearings. Unless this is meticulously performed, it is
very easy to throw the crank halves out of alignment, causing the crank shafts
to run eccentrically, which stresses the supporting bearings and introduces
engine vibration.
Any time a crank is removed, it should be
tested for trueness before reinstalling. While the process can be done on
v-blocks, using dial indicators, it is best performed in a lathe between lathe
centers, using dial test indicators. The crank can be brought back into parallel
and concentric alignment.
How true is true?
A re-trued crank should generally measure less than 8 ten-thousands away from
perfection.
