Just measured connecting rod bearing clearance with plastigage and found that the clearance is beyond the "allowable .022-.069mm (.00086-.0027inch)". How do I find out which type of rod bearings I need to get back within the allowable range? I understand that going to a machine shop to have the crankshaft machined properly is the correct way to go. But it is only my first car and I just want to put an end to the rod knock it has.enter image description here

  • Do you have a micrometer? May 22, 2017 at 22:32
  • no. I do not own one May 23, 2017 at 2:33
  • Are you measuring the old bearings with the plastigage? And which engine are you working on? This makes a difference, because the tolerances you state seem really tight. May 23, 2017 at 11:46
  • i am using new standard size bearings on the 3.8l v6 engine May 23, 2017 at 16:16

2 Answers 2


I gave this answer here : Oversize rod bearings question

A decent machine shop should machine the crank pins and provide the correct bearings as part of the job.

  • so I would maybe need .002" undersize bearings? May 23, 2017 at 18:13

Did you measure the journal in the car? If so, not good enough. It can be tapered and/or out of round.

The bearing is likely a symptom not the cause. A new bearing will be a short term cure at best. And the crank situation will just get worse. TomO

  • yes, I just want a short term fix. How would I proceed? Without taking the crank to a machine shop. I dont know how much they (machine shop) would charge me, or where the closest one to me is. I just want to know how to find out what size bearings would put me in the allowable range of clearance. May 22, 2017 at 21:46
  • We used to drop the pan on a race car to remove a rod cap and half of the bearing. If positioned at maybe 30 degrees off BDC you can rotate the crank to BDC and mike it –best if done in several positions. If replacing bearing inserts the upper insert can also be rotated –note if locking tabs are involved. This operation is usually the second semester after mastering self-brain surgery. TomO
    – TomO
    May 22, 2017 at 22:08
  • @TomO possibly another idea would be to use a dial gauge with the base bolted into one of the bearing cap holes and rotate the crank slowly.
    – Solar Mike
    May 23, 2017 at 5:34

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