Originally Posted by ScotcH
Agree with all this, and a great thread topic, except that the inside micrometer is not the right tool for this job. There is no way to get an accurate repeatable reading because you can never get it perfectly level ... and when you're dealing with .001", even a very light angle will make a huge difference.
You also need to measure in various places, and 90º to each other to check the bore roundess. All this should be done by your machine shop however, they better have the pistons in hand to check proper clearence ... any machine shop that says they don't need the pistons is not going doing it correctly.
A dial bore guage is the correct tool, and by sweeping it inside the bore you can find the exact measure, and easily find out-of-round. Also, for measuring bearings (which I assume you will also be doing), you will need something accurate to 0.0005". I got one from Fowler canada for a decent price ... you can go much more expensive, but the Fowler gauge seems repeatable, which is a good sign.
For crank journals you will need an outside micrometer and will be zeroing the bore gauge against it.
I did 4 measurements
A- top inline with block
B-top 90deg opposite
D- bottom 90deg opposite
The inside mic is absolutely perfect for a user with experience, I get consistent readings every time. For novice users the dial bore gauge is the way to go, it's brainless to use.
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