What is a South Bend 13" Tailstock Ram Thread Specification?

[Grey Rider]

Does anyone happen to know the thread specification for the tailstock ram on a 13-inch? I've got a buggered thread deep in there I'd like to clean up, but can't determine the proper size.

The tailstock leadscrew measures 0.618" major diameter and 8tpi, so I thought it was 5/8-8 Acme LH, which is a standard thread. But a 5/8-8 LH Acme tap has a major diameter of 0.637", and a larger thread thickness. I don't want to send this tap through the ram and end up with a loose fit with the original tailstock leadscrew.

Thank you,
Ryan

 

[iwananew10K]

Chances are that screw is between a 3 and 4G class thread.

Most standard acme taps are 2G if not otherwise specified.

Some boneheaded math in my head says a 2G tap and an unworn 3-4G screw will give about .007ish backlash.

Except in the case of centralizing acmes the tap diameter is irrelelevant, it is all about the pitch diameter, the diameter of the tap will always be over the nominal diameter to give it root clearance.

A wire measurement will tell the tale.