todd j Posted October 23, 2023 Share Posted October 23, 2023 I have a restored instrument cluster from a 1933 Plymouth that I am going to use in the build of my 1930 Model A highboy roadster. I am having trouble with the correct fitting for the oil pressure gauge. It is female threads and appears to be 1/8" NPT, but that fitting will not thread in. It will go in maybe 1/2 of a thread and then seems to bind. Threads are clean on both fitting and gauge. Is this a different size other than 1/8" NPT? Thanks for your reply Link to comment Share on other sites More sharing options...
ply33 Posted October 23, 2023 Share Posted October 23, 2023 It has been a long, long time since I had mine apart and my memory is failing. With that in mind, are you sure it isn't either a compression fitting or a flare fitting. The hard line from the dash unit through the firewall on mine got plugged up and I had to make a new one and I vaguely remember it being a tube nut and flare for the connection. Link to comment Share on other sites More sharing options...
Fordy Posted October 23, 2023 Share Posted October 23, 2023 try a 3/8 24tpi - that was used on a lot of brake tube nuts - its possibly what you have there. It may also be BSP as it is 1 thread different to NPT but unlikely for an American car. Steve Link to comment Share on other sites More sharing options...
Bloo Posted October 23, 2023 Share Posted October 23, 2023 (edited) If it isn't double flare, it is probably "threaded sleeve" and probably 1/8". That was the most common setup on American oil pressure gauges up unitl the mid 50s at least. probably longer. Look down inside of it, If there is not an obvious inverted seat at the bottom of the threads for a reverse flare, and instead it has either a taper or a radius down there, it is probably threaded sleeve. https://blackhawksupply.com/collections/plumbing-brass-fittings-double-compression If that's what it is, and if you are in North America, you'll probably have to use copper tubing (not that there's anything wrong with that). 1/8" bundy tubing seems to be non-existent outside of the UK in 2023. Edited October 23, 2023 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
DFeeney Posted October 23, 2023 Share Posted October 23, 2023 Hello Bloo, Years ago I helped a fellow MoPar owner with this same problem. I found my notes and you might want to look into BlackHawk Supply 3/16 fitting 6100x3. Sorry I don't remember more. Link to comment Share on other sites More sharing options...
todd j Posted October 24, 2023 Author Share Posted October 24, 2023 Here's a picture of back of gauge. Sorry it doesn't show much detail. Putting a tape measure across the opening shows 3/8" Thanks for the help Link to comment Share on other sites More sharing options...
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