Hi,
I need to toss this in...
The question was: "How do I know what size bearing to order?"
Good question.
If you know the entire history of the motor, then you'll have some idea of
what to order. In other words, if the motor has never been rebuilt - then
you're maybe 90% sure that ordering the std size will be correct.
If you do not know the history... then your best bet is to pull the crank
and measure. If the motor has been rebuilt, and if the crank was turned -
the machinist would have scribed the crank with the numbers. Maybe
something like M - .010 or something like that. Look for writing or numbers
on the crank webs and on the snout or on the surface the the flywheel bolts
to for these numbers.
So, if you have low pressure and you cannot attribute it to something
obvious, like the screw for the rocker shaft is missing... then you need to
pull the caps and look for something obvious there... if you see scoring or
blueing, you need to have the crank checked out - if there's blueing, then
you may need to toss the crank. Blueing indicates that the bearing ran very
hot and most likely without oil for a while.
As for measuring, if you find a rod journal out of round but the main
journals are still round, you might be able to get away with grinding the
rod journals and leave the mains alone, maybe "polish" them a bit.
So, the answer really depends on what you have there. I'm kind of neurotic
about these things, so I tend to pull the crank and measure. However, I
need to say (again) that these cranks are pretty hard, and I've pulled
several motors apart that have around 100k miles on them and the journals
all measured round and were still the std size... at that point, I just
bought sets of std bearings and installed them.
As for the plastigauge thing, you may still want to do this when you
install everything to make sure that nothing is binding or too loose after
you reassemble. Remember, there are tolerances for everything in your
motor, and they can add up to "very loose" or "very tight" depending on
what you have. Racers sometimes assemble and plastiguage and then play with
the bearings themselves to free up the motor a bit - maybe to get an extra
"thou" of bearing clearance to free things up. Lots of labor, so if you're
paying someone to do the work - here's where your money is going. If you do
the work yourself, then take your time and make lots of notes... it can
actually be a lot of fun.
Good luck!
rml
TR6's
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