Steam Driver

What is the correct procedure for miking the main bearing bores? I know the area(s) of concern are at 45 degrees, but is only one measurement made in the center of the bore, or a couple of measurements moe towards the edges? Also what about micrometers; there doesn't seem to be a lot of room for manipulating a real inside mike, so are telescoping gages accurate enough?

JCP911S

Bore gage.

irobertson

++ What JCP911S said.
I did mine just off the split line, 90 degrees to the split line, and near each edge of the bore, so 4 measurements in all. That gave me out of round and taper. I also calculated the difference between the two farthest apart measurements, which gave me the max variance for each bearing saddle.
I then measured the bearings installed. Be careful that the bore gauge tips are round & smooth so as not to mar the bearing surfaces. One of my tips got very slightly worn doing the rod big ends that had been honed, and I put a slight scratch in the soft bearing material with it. I swapped the tip right away before going any further.

JCP911S

BTW... before you do any machine work, make sure you can find some oversized main bearing sets. Factory sets are mucho $$$ if you can even find them.