I'm looking to make a fixed cross slide indicator for my LeBlonde Regal (can't afford a DRO just yet), and want something better than the mag base dial indicator and rod I use now that has to be aligned and set each time.

I found a nice setup I am going to try and emulate, and rather than drill and tap new holes in the side of my cross slide I want to use some of the ones that previous owner(s) had put in there.

Problem, grease / crap filled holes and no idea of what size / TPI they are.

I ended up cleaning the holes as much as possible and checking the minor ID, making a estimated guess, and then using some different taps I had on hand to see which fit, just going finger strength as to not chew into a different thread pitch if I got it wrong.  Finally figured out I had a 1/4-20 and a 5/16-18 hole available.

But, I was at a serious loss as to what I could have done to determine what they were tapped with any other way, I couldn't get a thread pitch gauge in there to figure out the TPI.  What other options would i have had to determine what those holes were tapped?

Thanks!

Try a bunch of different bolts?

ETA:  Or this...

Nut and Bolt Thread Checker (Complete SAE/Inch and Metric Set)

https://www.amazon.com/Bolt-Thread-Checker-Complete-Metric/dp/B06ZYD4SS8/ref=sr_1_9?ie=UTF8&qid=1542731316&sr=8-9&keywords=thread+gauge

That might have worked better, but expensive way to figure it out. :)

Thanks!

True. But if you do a lot of that kind of stuff.............. Or go to your local Ace Hardware and buy one of each from the bolt drawer. Likely come out the same cost wise.

There was a few other sets that were cheaper, but less inclusive.  There was one for $28 that probably had all of the common sizes.

Brilliant!  I like that idea. :)

Thread pitch determines tap drill sizing, I'd have started there.  #3 is pretty close to #7 but I feel like you should be able to tell with basic practices and good drill to use as gages.

Awesome idea!  Personally, I would've narrowed it down and bought some different sized screws.