[Goldhunter_2]

ok I know I should know this but it is just alluding me and normally I just stick a tap in there until I find the right one but this is bigger then the ones I have so I would like to buy the right one to start with.

hole mic out roughly 1.23" at the bottom of the thread or 1.14" at the top of the thread (both inside diameters) so when measuring inside the hole that needs threads chased where do I measure form to get proper size tap orders ?

[Geof]

You mean you want a tap to make a thread about 1.23" large diameter and s1.14" small diameter?

The closest standard inch size to 1.23" is 1.25" with either 7 or 12 tpi depending on whether it is UNC or UNF.

1-1/4"-7 needs a 1-7/64" drill.

1-1/4"-12 needs 1.16" so the closest drills are 1-5/32" or 1-11/64".


Drills and taps these sizes will not come cheap and hand tapping a thread this size would need almost impossible strength; machine tapping would require a very husky machine.

[Goldhunter_2]

yes I am sure the taps this size will cost a few penny's thus the wanting to get the right size with the first one I buy ...lol

this is already a threaded hole in a aluminum engine block, I just need to chase the threads. I almost assume that the 1.23" measurement was supposed to be a 1.25" and I just am not getting all the way down to the base of the threads when I measure. what is confusing me is when i want to measure a internal hole that has threads already do i measure at the base of the threads (farthest out point of hole) or at the top of the thread so I know what size tap to buy?

I am thinking the base but not 100% sure