And yet I had no oil leak from my transmission and it was totally dry (seriously). There was no sealant in the threads and I did not add any sealant. The tapered thread can seal. They’re just not considered that safe.
All I was saying is that normal straight threads don‘t seal by design because there has to be a small intentional gap.
In the case of the fittings into the transmission, you are correct: they seal perfectly, for the intended use and environment.
Teflon tape, pipe dope and other sealants are intended for use on tapered thread connections.
Unless you have the engineering specification for the fitting on your transmission, you don’t actually know what type of thread you have. For the last week, I’ve been chasing a 1" pipe fitting that fits the outlet of one the three bilge pumps on the boat. The pump manual says 1", but 1"-11.5 tpi NPT definitely does not fit. That leaves me with 1" BSPP or BSPT. It’s extremely difficult to tell the difference between 11.5 tpi and 11 tpi when the threads are less than an inch long. In the end, I’m guessing.
Thread drift.
I have an old hand drawing from David‘s granddad showing some different thread types. He states the whitworth pipe thread is without any play on the pointed edges, well suitable for sealing gas and liquid pipes. The other threads all show open space between the edges.
I see what you did there!
First, there has to be some sort of assembly clearance, or you could never thread the 2 parts together.
The standard engineering tome “Looks good on paper” applies here. Engineering drawings, particularly hand drawn, are prepared in the Land of Oz, where every circle is perfectly round and every corner is 90 degrees. All of the actual construction and fabrication takes place someplace like Kansas, where neither of those conditions are likely to remain true. If I’m writing the piping specification, and I wrote dozens and dozens in my career, all tapered thread fittings, BSPT and NPT alike, get a thread sealant. As a practical matter, about the only place I would have come across BSPT in my career was on one of Sir Bamford’s backhoes, and non-process equipment was not my responsibility.
The drawn Whitworth conforms to DIN 259. The text reads „the Whitworth-pipe thread is without play and seals well. It is intended for gas- and fluid transport of all sorts.“
DIN 259 is very old. So are DIN 11 and 12. Today we have DIN 2999 or EN 10226 for self-sealing Whitworth threads and BS 2779 or ISO 228 for externally sealed threads. I hate standards that are not standards. Especially if they cost money to read.
Yes they get sealant, and that is good, but if the XK engine can seal in pressurized coolant with steel washers and acorn nuts, a tapered Whitworth thread can seal against most things. You‘ll agree that even nicely cut threads will sometimes rub a little here and there. Zero tolerance is of course impossible on a parallel thread but the major leak path at the tip is gone when using Whitworth threads and I think that is the idea here. Not nuclear plants or oil rigs or boats, just piping. Probably gas and steam and water and oil mostly, with some hemp or whatever.
Update: engine started. No leaks…
I figure at least one of two things happened:
The two bolts do not go into the engine so will not leak even if missing. However, they do help locate the gasket and as such are necessary. Good you replaced them and all is working good.
I went through the old books and when the outer Whitworth thread is tapered they say no sealant is required. What was good in 1955 is more than good enough for me!
Congratulations Eric!
