Doc, I started a thread on the forum for this rather than continue in the parts for sale section.
Something odd going on here I think - I have a NOS primary cover and it is stamped 66-7503 so I assume it is the same as what you have (or at least started that way).
The ID of the inspection cover hole is 1.75" - the OD of a pattern brass plug is 1.78" and an original alloy plug (Marked as 27-209) is 1.79".
I have a brass (chromed) plug that is 1 3/8" in my junk box but it falls straight through the hole in my Primary as the widest part of that plug is 1.67".
Is it possible your cover has had surgery at some point? This is a picture of the NOS cover - the weld isn't pretty on this original but might give you a point of comparison.
The second photo is a comparison of the original alloy plug, the pattern brass one and the smaller 1 3/8" chromed brass one.
email (option): rays54@hotmail.com
Hi Doc..The smaller post war inspection plug has a thread of 1 1/2" diameter x 20 TPI...As you are measuring the inside (root) diameter of the thread at 1 7/16" it would appear you have a post war outer cover..You could get a threaded ring machined up and weld that into your existing cover, after opening the hole out, to solve the problem...Ian
email (option): ian@wright52.plus.com
Sorry, that cover is destined for a really sad WM20 I have.
Ian is correct about the size of that pug being post-war but I am a little puzzled by the part number on your outer.
I would have expected it to be the later type with the added extension on the cover as you can see on this one that has been butchered at some stage.
This has the later part number 65-7584:
This one definitely has the smaller inspection hole:
The part number for the matching alloy plug for that type is 65-3364. I don't know enough about the part numbers, when they transitioned etc but you seem to have a hybrid.
Anyway, I'm not sure how much a good cover would set you back these days but given you have too much metal you are on the right side of the equation in that it is easier to remove than add. If you got a machine shop to make up the ring to suit your brass plug it might be an economic alternative?
email (option): rays54@hotmail.com
Hi Ray..There was the problem of post war replacement spares for the WD M20 after girder fork models went out of production..I suspect BSA used the later cover, left off the 'band' round the edge at the production stage and applied the original part number to reduce confusion at Army stores...If that were the case it would be a 'hybrid' cover...Ian
email (option): ian@wright52.plus.com