Did the standard size for one-eighth NPT threads change at some point in the past half century or so? I find myself with a few fittings that don't seem to agree with one another.
In one group there's a fairly modern vibratory pump made in Italy, stainless steel street tee from Cajon, some brand new fittings bought from Ace hardware and a very small pressure gauge that might be a century old.
In the other (too big) group are a tubing adapter from an icemaker hookup kit, a generic hose barb of unknown origin and a hex plug.
All the male threads are visibly tapered, including those that came with the pump, so it's not likely an NPS-NPT mixup. The mismatch is small, maybe .002" at the big end and .006" at the start. That rather implies a different taper, but AFAIK all pipe threads were tapered 3/4" per foot.
Thanks for reading!
bob prohaska
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There are two series of British pipe taps. The letters R and G are -German- for Rohr (pipe) and Gas.
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To add to the confusion, BPT may mean Briggs Pipe Thread, which is the American predecessor to NPT.
The nominal inch size refers to the original ID of wrought iron pipe. Modern steel pipe has thinner walls and thus a larger ID, though stronger Schedule
80 pipe is close to the original ID. If you need to chuck a pipe fitting in the lathe the threads on a brass nipple will run truer.
EMT conduit has the same ID as Sch 40 pipe and chain link fence tubing the same OD, though its nominal sizes are the actual OD rounded to the nearest fraction. Depending on the gauge of the fence tubing the two may telescope together, I used 1" conduit with a 22 gauge shim to splice a break in my TV antenna mast which is 1-3/8" (1.315") fence top rail, the same size as TV mast tubing but much cheaper and easier to find.
I assembled a cone of pipe reducing bushings up to 2", with the inner and outer NPT sizes labeled, to identify unknown threads. hth, jsw