[Fot] Con Rod Bearing clearance

[Scott Janzen]

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.