Determing major diameter

[spinrow]

I have an old Flather metal lathe with missing grease fittings in the two spindle bearing caps. I don't want to remove the caps nor tap them with new threads in place. I determined the ports to be .397" X 24 tpi. I was going to add the "double depth of sharp threads" to this but got into difficulty the last time I tried this approach. I can't cut the threads to fit so can someone help?
I tried a number of grease fittings and cups but could not find one to fit. Thanks Paul

[JCHannum]

To determine the proper size hole to drill for tapping, divide 1 by the number of threads, and subtract from the major diameter.
To find the major diameter, reverse the process: 1/24=0.0417+0.397=0.4387". To make it easy, try 7/16".

[spope14]

Last I recall, the grease fittings were a pipe tap type of fitting. You may want to check some resource such as Machinery's handbook for the specs on this thread. Better be safe than sorry. The tap drill will be included with the tap size. Sounds like a 1/8 pipe tap from my guess. I could look it up, but do not have the resource at this time. If needed, will check for you tomorrow.

[SGW]

I take it the 0.397" is the clear hole diameter. Sharp V double depth for 24tpi is 0.0722", so the o.d. ought to be 0.4692. For standard thread, the double depth is 0.0541", giving an o.d. of 0.4511". Neither of those sounds like anything rational. I'm guessing the 0.397" dimension is somewhat larger than the actual root diameter. I'm with JCHannum -- try 7/16"-24 and see what kind of fit your get.

[JCHannum]

1/8" pipe is 27 threads, 0.405" OD.

[spinrow]

Thanks to all. I made the brass rod diameter .437" threaded it and fit fine. I tapped this piece for a 1/4" 28tpi grease fitting. Thanks again Paul