Boyar-Shultz 612 Surface Grinder

[Vinny]

This grinder has a place to attach a vacuum on the wheel's housing. Last nite I finished making the attachment for it and when I went to mount it the bolts wouldn't tighten. They look like they should be 1/4-20 but the hole itself is 0.250" (as measured with a caliper) and the bolt just slides right in. 5/16 is too big. I thought maybe they were stripped but looking in there the threads are nice and sharp. Since this is a US made machine and nothing on it is metric, what are the odds that this is metric? M7 maybe? I can't get the thread gage in the hole, it's too small. One other thing, there are two other threaded holes in it for the wheel's cover that ARE 1/4-20. Would they have used some bastard size so people would have to buy the kit from them?