Yes I am sure about that Coarse Thread , and yes it is a "Set" Screw, most Set Screws are coarse thread, (Fine does exist) this one is a "socket head".
I did not mention the type as I was indicating that a screw was located at the opposite end of the plunger bore as a "stop" for the plunger to rest on. The Set Screw is WO-53035, Bantam No. 62X2 and Ord No. 102379 AKA as a Bolt in some SNL's. I was going by memory as my Trailer Manuals were in another room.
1/4-20 = NC Thread
1/4-28 = NF Thread
The similar casting on the M-100 also has the same coarse thread 3/4" Allen Head Set Screw that acts as a stop for the plunger.
And Uncle Sam's Army trained me to be a Machinist at APG exactly 50 years ago.
Sorry Chuckie :wink: It's very easy to arrive at 1/4-20 as being fine Thread due to the number 20 sounding like a lot of threads.
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