The Stovebolt Site Forums The Parking Lot Big Bolts Obsolete threads solution

Hope all the big truck lovers are doing well out there. Cup of coffee in hand I'm looking for a good solution to something...

I'm putting air horns and rear helper bags on my '30 Chevy 1.5 ton. I picked up an old brass compression gauge unit (200 PSI, dump valve, 27 pitch threads) that I want to use because of both looks and functionality, utilizing the dump valve for releasing air in the rear bags when desired. The challenge is that I want to separate the unit, using the 200 PSI gauge to monitor system pressure (27 pitch threads to either 1/4 or 3/8" compression) and putting the 27 pitch brass body portion with a 1949 50 PSI gauge that has 1/4" pipe threads.

Murphy's Law strikes again that I'd find something cool with an odd duck thread pitch.... On initial tries I can't seem to find conversion pieces to mate the old threads with the more modern ones. I found that 3/8" male compression threads would go just a couple turns into the brass 27 pitch body. Dimension wise, the 1/4" tapered pipe threads will fit mostly down in the brass opening. I've thought about using epoxy- basically rigging it- but would really like legit conversion pieces to blend the old with the new.

Any ideas how I can have my cake and eat it too?
Thanks

How about a picture or two of the problem area?

Have you looked at McMasters Carr website? Grainger’s? Local machine shops? Just some off the top of my head suggestions. Good luck.🛠

Are you dealing with straight threads or tapered? It would also help if you mentioned a thread diameter. The simplest approach would be to rethread the gauge to an easily-accessible thread. If you don't want to alter the gauge and you have access to a lathe, make your own adapter fitting. Brass threads easily- - - -I love to work with it. Once you get a thread that fits, a few drops of blue Loctite will keep everything assembled and leakproof.
Jerry

Male thread 1/8-27 NPT measures just slightly over 3/8" OD. May not be obsolete threads, could just be tapered pipe threads. If you're not sure of the difference, stop in to visit a local machine shop and just ask.

John

Looking thru my seventh edition of the machinist handbook, copyright 1940.
The only 27 pitch threads I could find were 1/16 and 1/8 tapered pipe and straight pipe threads.

Tapered.
https://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html
Straight pipe.
https://www.engineeringtoolbox.com/american-standard-straight-pipe-threads-d_1520.html

If you have a machinist friend that knows his salt, check with him.
Don

Or you could drill the threads out to fit a brass tube or pipe, then soft solder or silver solder the tube, then adapt to those threads.

1/8 -27 NPT is the thread for most grease zerks if that helps

1/4-28 fine thread tap works without problems for making female threads to match 1/8-27 NPT male fittings.

HUH?? Why not just use a 1/8-27 pipe tap? Those are readily available.

Where is the original poster?

Sorry Mac, I don't agree using a 1/4-28 straight tap to get a 1/8-27 tapered pipe thread to fit.
It may tighten up, but it will leak!
That I can guarantee!

Don

Sorry guys I've been out of pocket a bit. After talking with Fastenal (and finding out they didn't have anything that would work) they gave me a local shop that makes hydraulic lines and the like that has been around for quite a while. I learned quickly that there's a whole lot about fittings that I didn't know in regards to now some things are referred to now as a "No. 4" for example. Learning curve here.

Didn't see anything on Grainger or McMaster Carr sites but a phone call wouldn't hurt to verify. Thread gauge tells me they are 27 and they are straight threads.I got the 3/8 (5/8 diameter) from the fact that a 3/8 compression male fitting would go in about 1turn and stop. The solder idea for some new threads does sounds alright. If I were to come up with a 24 pitch tap and die I do understand that would chew up the threads quite a bit but do you think adding blue locktite would give enough fill/bite to run up to around 140 PSI air without leaking?

I was going to stay out of this when my suggestion was first rejected but since there seems to be a piling on beginning, I am forced to respond:

"HUH?? Why not just use a 1/8-27 pipe tap? Those are readily available."

"Sorry Mac, I don't agree using a 1/4-28 straight tap to get a 1/8-27 tapered pipe thread to fit.
It may tighten up, but it will leak!"

I don't know how many times either of you have tried this or how extensive your collection of threading equipment is. I know that I have tried it many times and I know that I have not looked for one once this suggestion was offered to me at a time when it was needed in a hurry.

I use this method any time I need it and I would not have suggested it if I had reason to believe it was faulty. I have never had it fail me.

Glad the op is getting around to telling the forum what his actual problem is. Maybe now he will get an answer he can use.

I believe that no offense was intended. Those of us that have experience in the machining trade would only be expected to offer the correct solution rather than suggesting to force two different thread pitches together.

John

I most certainly appreciate all the input- and I do mean all from one end of the stick to the other. Apologies if my limited fitting knowledge didn't communicate the problem well. The only "threading up" I've had the occasion to deal with has been picking up a new matching piece and threading it together so this is good experience for me. Thanks again!

My mistake, I should have indeed said 1/16-27 npt, not 1/8. Typical of me to misread and jump to a further erroneous conclusion. My apologies.

Mac, I do similar all the time. Sometimes I don't look before I leap. Sometimes I misspeak. So I may or may not have a suggestion. My training, which I sometimes forget, is to thoroughly understand the problem. In doing this, a person sounds dumb sometimes. These questions are for anyone to answer.

1. In that picture, that thing with a dial is the gauge, what is that other thing with female threads. Is that the piece you are trying to mate to the gauge?
2. I would expect an old gauge to have pipe threads.
3. Does anyone know the exact size and type of the male threads on the gauge and the exact size and type of threads on the mating female part? It seems unverified. So can't continue until those are known and stated for sure.
4. What is that thing sitting on the glass?
5. Why is the desired gauge, a compression gauge instead of a pressure gauge?
6. You say you picked up a gauge, then say "dump valve" in the same sentence. Which part in the picture is a dump valve?

I agree. That gauge sure looks like a 1/8 NPT thread to me, but hard to tell for sure from the photo. Have you checked with a caliper to confirm it's straight? That would be unusual.
The taper is pretty slight, only 1 in 16 on the diameter.

NPT thread dimensions

[on edit]On further study of the picture, it may be straight threads and the item on the left is the mating fitting for the dump valve for a compression test gauge. What are the threads on the other end of that fitting? Probably more standard than the gauge. If you can adapt to the opposite end of the fitting, You could drill the check valve out of it and plug the dump valve. Then it'd just be a dumb coupling/adapter.

[on edit again]If the gauge thread is indeed 1/8-27 straight pipe thread, then here's an item that would adapt from 1/8 NPSM to 1/8 NPT. It's a swivel adapter, but would work. 1/8 NPSM (National Pipe Straight Mechanical) and 1/8 NPTF National Pipe Taper Fuel) are the same OD and pitch, except for the taper.

The fitting with the dump valve is brass, it would be very easy to drill, tap, modify as needed, can you show us the other end of the dump valve fitting?

What is in the picture is a very old engine cylinder compression gauge tool- taken somewhat apart. (***This may be part of the breakdown on my part because I'm not real knowledgeable when it comes to fittings and proper terminology-) I mentioned above that the threads are 3/8". the inside hole diameter is 1/8. I got the 3/8" threads part (it measure.565 or 5/8 diameter) from the fact that a standard 3/8 compression male fitting would go in about 1turn and stop. Threads are straight as verified with my calipers with a 27 pitch according the thread gauge. The small round piece sitting on the gauge face is a brass "spacer"(?) with a gasket on top that sits down into the brass body with female threads- then the gauge screws down on top of that gasket making the seal. What is not seen in the picture is the "other end" of the main brass body- which (was) a tapered rubber piece that pressed against the spark plug hole. That rubber piece was pressed on a barbed connection that affixed to the main body somehow.

The reason I'm utilizing this piece is because it's really old and really cool. I want to see the overall system pressure so am using the 200 PSI gauge itself for that. The "Compressometer" name is simply a name (yet another cool factor to me) and will register the approx 120 PSI pressure of the system.

Since I am putting helper bags on the rear and want to have a separate gauge for that. I also need to release the air pressure there (most of it) when not hauling a load so for that I get to use the main brass body portion with the release- we've always generically called them dump valves. I'll never run over 50 PSI for the bags and for that will use another old gauge (1949 R. Perlick Brass Co) which has 1/4" tapered pipe threads (approx 1/2" diameter at the narrow end). That is where the modification takes place- the Perlick gauge with 1/4" male pipe to the original cylinder compression gauge body with straight 27 pitch threads.AND the straight 27 pitch threaded 200 PSI gauge going to a 3/8" compression fitting for the overall system.

Mr. klhansen I think you've started me in the right direction on first of all taking out the check valve at the end of the main body threads to simply drill and then re-thread to a standard thread choice. And also for the link to the buyfittingsonline website. Thanks for that.

Sorry to lead you boys down the rocky trail but once again I appreciate the help and chance to learn more new things to me all the time.

Sounds like something the Welder-Up guys in Las Vegas would do. You got this, carry on.🛠

Since you're sacrificing the compression tester anyway, why not use the female thread that fits the gauge, cut off the barbed fitting with the rubber cone, and silver solder a more conventional fitting onto the end? Sometimes the glaringly obvious solution to the problem gets ignored by engineering types who make the perfect solution the enemy of the "good" one. My younger brother with all the letters behind his name does that all the time!
Jerry

You are so right and boy have I done that before. One of my favorite quotes is from when the Three Stooges were starting on one of their escapades. Mo was standing back watching and said, "you know that was my idea but now I don't think so much of it."

You are indeed correct in the simple solution. The plan is to modify the barbed end pretty much like you said to tie in to the new system. Where I am making it more difficult (challanging? Ha) on myself is that I'd like to use the 200 PSI gauge on the over system so I can read 120 PSI on the dial when pressured up. (without buying another gauge). The older 50 PSI works perfect for the bags portion. Which comes back full circle back to me sometimes being my own worst enemy. I can typically find a way, I just sometimes take the hard route getting there!

Justhorsenaround- you are pretty close to the mark! Thanks for the encouragement!

When I was paying the rent and feeding my family by fixing 18 wheelers, they had many yards of 1/8" OD plastic tubing carrying full air system pressure for transmission shifting and dash gauges. It would be simple to adapt your system pressure gauge down to the 1/8" tubing compression fittings those systems used.
Jerry

Put the "Compression" gauge on the Fireplace Mantle and buy another old gauge that has pipe threads and save a lot of headaches and ruining of a gauge.
Ebay is full of old pressure gauges. This is a rat hole you are going down for no reason. You have a special gauge with special threads/fittings and it's for compression testing. I believe the straight threads are for easy assembly and disassembly for a mechanic. Not made for permanent connection for how that gauge was used, like pipe threads would be. Seems like the same type threads/quick disconnect on A/C gauge/hose sets. (hand tighten most of the way). It's the wrong gauge for your deal in my opinion.

Drill this, drill that, tap, cut off, buy a tap, go to a shop, find a machinist.......all kinds of good ideas for someone in a real bind. You are not in a bind.......yet.

If this is fun stuff to modify and tinker with, your type of drill, tap, weld project, go for it.