I recently assembled a GT-6 engine, and checked the clearances several
ways. It is time consuming, and requires the use of micrometers,
etc. If you do not have these, find someone who does or invest in
some. By the time I got done paying the machinist, I was halfway to a
single micrometer purchase.
The goal is to get all of the clearances approximately equal. This
can usually be accomplished by mixing and matching the bearings, as
they are usually not perfectly uniform in thickness. If you are
really going for perfection, it may help to have a second set of
bearings from which to select.
measure all the rod and main crank journals. They should be mic'd at
four locations, being "front" and "rear" and again at 90 degrees to
the first location. This will determine if they are round or egg
shaped, and if they were tapered by the polishing belt. Hopefully,
this will confirm that you have a good machinist! I found on my
cranks that they were almost exactly round (one was 0.0002 out), but
that one main had about 4/10,000 of taper from the leading edge to the
trailing, caused by the belt presumably being pushed harder against
the front edge.
Measure all the bearing shells with a micrometer. You will need a
ball tip on the mic because of the concave surface. These should be
measured on each side of the oil channel or at two locations on the
rod bearings. I found, depending on manufacturer, either great
consistency or up to 5/10,000 variance from one to another, and
2/10,000 from one side to another. Now you see how you can adjust the
clearances.
Measure the bores of the con rods and of the main bearing bores. For
this, you need an inside mic or other bore- measuring device, and the
caps should be torqued to the correct spec
do the math, subtracting the bearing thicknesses and crankpin diameter
from the bore diameter. I used an excel spreadsheet so I could
compare and re-shuffle the bearing measurements.
Using one set of bearings, I was able to get variation down to about
2/10,000 on the mains and 4/10,000 on the rods
I then confirmed all this with plastigage on assembly and found the
same results in terms of tolerance, though the absolute clearance
dimension varied by a few 10,000s.
The other way to do this is with an inside bore gage, assembling the
bearings in the bores and torquing them down. You still need to move
them around to get the best combinations.
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