Tech Tip: Ad-Hoc Precision I.D. Measurement

I recently found myself doing an output shaft replacement to a gear case that had been subjected to violent happenings and had a crack welded up. Removing the bearing carrier proved nearly impossible, and it had to be driven out with wedges and a sledge hammer with the case gently warmed to 60 degrees C with a heat gun. Uh-Oh.

On re-installing the outer conical roller bearing, the case bound up. Uh-Oh indeed. This spurred suspicions that welding heat and / or prior violence had rendered the bore out-of-round, but I had no inside micrometer on hand to confirm this, and this went down deep in the boonies where tools and equipment was a long way away. What do?

1: Obtain a screw slightly longer than the bore to be measured.
2: Take the head off the screw with a hack saw.
3: Install a cap nut on the screw along with a lock nut.
4: File down the cut end of the screw until it comfortably fits in the bore to be measured.
5: Round off the cut with a file, finish with 600 grit sand paper and a battery drill.
6: Use improvised tool in the same manner as a traditional I.D. micrometer.

Using this method I was able to ascertain that the bore was 0.15mm out of round at the outer end, warping to 0.25mm in the other direction at the inner end. Scrap gear case confirmed.

I’ve never heard of this method before, it just sort of came to me while pulling my hair out, so figured I’d share.


That’s one way to do it. I prefer make up a set of inside calipers. I’ve found it can be difficult to ensure the nut is seating properly in the space to be measured and that can throw your measurements off slightly. The homemade calipers can be witness marked to prevent this. Also, I can never find a cap nut when I need one.

Good job using your head and coming up with a solution with limited resources. I love it.

I’m not a machinist but curious what thread pitch the screw was you had on hand?

How is that done? Sounds extremely useful.

Mine was a very fiddly tool to use, since you have to set the counter nut when the tool is deep in the bore. I found that you can do the final adjustment by applying a bit of torque to the nut, stretching the tool up to ~0.2mm. I don’t think there’s much scope for mis-measuring, since you can easily double-check as many times as you like once the nut is tight. Indeed, the most time consuming part of the job was finding that damn cap nut :stuck_out_tongue:

Thank you @WiperExtraordinaire :blush: The screw was M6x1.0; A finer pitch would have made it much easier to use, but I don’t have many M6x0.75 cap nuts laying around…

The rest of us use a telescoping gauge for that job.

You cut out two pieces of thin material with a bit of a hook shape facing out and drill a hole in the center, insert a bolt, tighten it until you can barely move it, insert it and position it, scribe your witness mark then tighten your bolt the rest of the way.

Really depends on what you have laying around on what is easier to make but I’ve always found the homemade calipers easier to use.

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So do I, when one is at hand. We call them “inside micrometers” in Norwegian. My bad for messing up the jargon, this ain’t my first language.

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No bad at all. I wouldn’t even attempt a reply in Norwegian!

When I read inside caliper I see this:

Or the latest version which I have never used:

The homemade bolt is great for shade tree mechanicing but if accuracy is important then the right tools should be available and used. But, some of the greatest MacGyvering I’ve ever seen and greatly admire can be found on the Russian YouTube videos.

having built aerospace stuff for years I’m mostly lost without browne&sharp, mititoyo, starrett, etc. but the ordeal you went thru was clever… another dog trick would be to spray some layout die in there and see where the tight spots are.

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As you might guess from the suspiciously round numbers, the measurements were made with vernier calipers. That would have been quite useless for filling out a build sheet or anything like that, but I was shooting for a comparative measurement to confirm my suspicion. Lucky for me that it was bad enough to nail down with agricultural methods. 1/4 mm can pretty much be seen with a measuring tape.

Rotating the gauge in the bore accomplishes that as well, but a dye print gives a much more precise indication of the geometry.

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Good that you added this for us deckies trying to follow along.
This was what I was puzzled about.