Does anyone have any pointers for mic'ing main bearing and rod journals? My engine has all of the main caps cast as an integral part of the lower block so I can't see how I can reliably use plastigage.


A very accurate way to mic your rod and main journals is to use a telescoping gauge set. They looks something like this set:

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The ends of the "T" are spring loaded and the end at the base of the "T" twists to lock the head in place to set the distance. They work something like this for the main bearings:

  1. Install all of your main bearings in the appropriate saddles while completely dry (without any kind of lube) and into the main bearing carrier.
  2. Install the main bearing carrier into the block along with all main cap bolts.
  3. Torque, in proper sequence, all of the bolts to the torque spec.
  4. Using the appropriate sized telescoping gauge, carefully place the head end into the bore of the bearing, being careful not to scratch the bearing surface.
  5. Since the head is spring loaded and ends rounded, with a little wiggling, it should find the center of the bore. Once found, turn the end of the gauge to lock it in place. Ensure the gauge is square in the bore.
  6. Using the proper mic, measure and record each main bearing diameter.
  7. Using the same mic, measure and record each main journal on your crank.
  8. Subtract the crank journal diameter from the main bearing diameter to get your bearing clearance.

To find the rod bearing clearance, do the same thing, installing the bearing into the rod (a bench vise helps here), measuring it, then the rod journal, then subtracting the two for each to find the bearing clearance.

  • Just to add, this is a preferred method of measuring your bearing clearance, whether you have separate main caps or a continuous one like what you're talking about. It's far and ahead of using Plastigauge. While Plastigauge works, using micrometers is a quantum leap in precision over it. Feb 3 '18 at 17:10

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