I'm trying to identify the thread on an old automobile oil pressure sender. I measured it with a caliper at about 9.47mm in diameter. It appears to be straight, not tapered. I have a thread gauge that says Whitworth on one side and Metrisch on the other and the only one that fits says 20G. But when I look all these details up on a thread guide, I don't see anything that matches. Am I doing something wrong?
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Help identifying thread
- Thread starter Hobby2409
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If it's for a British car then it's probably British standard parallel pipe thread (BSPP) and probably either 1/8" or 1/4" size
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What is the vehicle or engine you are working on ?
On pipe threads, it refers to old pipes that don't exist anymore. It's nothing close to the thread size, rather what USED TO BE the hole in the middle of the pipe. Even that's not the same as nominal anymore....
The close ones I can think of off hand- 1/8 British parallel pipe is about right diameter, it would have 28 threads per inch. 1/8 US straight pipe is a whisker fatter than you measured, 27 threads per inch. 3/8 Whitworth would be 16 TPI. 3/8 British fine bolt thread would be 20 threads per inch. Was the 20G something about that? I don't recognize that. 3/8 US fine and coarse would be 16 and 24 tpi.
What sort of car/engine/sender are you working on? Who built it and when might help get you on the right system.
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The way I heard it, the size referred to the minimum hole in the center after a certain amount of crud built up/ years in service.
So one could always count on having at least that much flow. Silly plumber's tales. I have no idea if that's even remotely true
So one could always count on having at least that much flow. Silly plumber's tales. I have no idea if that's even remotely true
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Travers Tool Supply sells 3/8-20 taps. https://www.travers.com/catalogsearch/result/?q=special+taps
So, odds are it won't be "metrisch", but what's on the whitworth side for thread pitches? (That's just cool. German?)
I'd speculate that you're looking "most likely" at either a British standard fine (BSF) 3/8-20, or a BSPP, which would be 28 threads per inch. Any of your thread gauges match those two pitches?
Pipe threads are an OLD standard. Originally, they were named after the inside diameter of the pipe. Materials and manufacturing processes got much better, and required a LOT less material to build a pipe of the same capacity and durability. Taking metal off of the outside would mean designing and implementing a new thread sizing system which would not be backwards compatable, and would surely be outgrown again in the near future. So they shaved metal off of the inside of the pipe, and left the outside the same, so new and old threaded fittings would work correctly. And once again (it was early, but it wasn't the first....), American standardization got bollixed up, confusing, and generally meaningless because... Well, you know.... Why would a one inch pipe be remotely close to in inch in any dimension..... But hey.... Grandad didn't have to see his grand kids learn anything new, so that's a win, right?
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As much as I love machining, I’m not a car guy nor an engine guy (automotive work is third on my list below plumbing and painting in my pantheon of hated activities).
I thought: What is an “oil pressure sender”, how does it work, and what does it look like?
It never I occurred to me until this moment that the oil pressure gauge in my cars needed some sort of sensor…
Google has been slightly helpful, but I’m still a little unclear.
I thought: What is an “oil pressure sender”, how does it work, and what does it look like?
It never I occurred to me until this moment that the oil pressure gauge in my cars needed some sort of sensor…
Google has been slightly helpful, but I’m still a little unclear.