BSPT vs. NPT threads

[Matt Harwood]

I'm re-plumbing the entire fuel system of my 1929 Cadillac and the inlet fitting on the carburetor looks kind of odd. The fitting isn't odd, it's just a 45 degree flare, but the threads that go into the carburetor body are either buggered up or an uncommon type. They look a lot like 1/4 NPT but without the taper. So I bought a fitting with 1/4 BSPT threads that look almost identical to the threads on the original fitting but it doesn't screw in easily. This is a spare carburetor so I haven't done anything to my car yet, but it took some real cranking to get the fitting out so I'm guessing that the threads are buggered, which is why the new fitting doesn't screw in easily. The threads look a little stripped, but not as bad as it appears in the photo below. Does that make sense?

 

Anyway, is it likely that the threads in a 1929 Cadillac carburetor are 1/4 BSPT? I'd like to know for sure before I start trying to screw it into my good carburetor.

 

Original fitting - 1/4 BSPT fitting - 1/4 NPT fitting

[Gary_Ash]

Could be it's a straight thread, not a tapered pipe thread.  Lamp parts are made with 1/4-27 threads for reasons I don't understand.  I recall running into this issue a while back.  It's possible that some carbs had 1/4-27 threads.  Where is Carbking when you need him?  😉 

[joe_padavano]

Matt,

 

BSPT is a different pitch than NPT and the individual threads have a different cross sectional profile as well. The original fitting is more likely NPS (national pipe straight) since it appears to use a seal washer on the underside of the hex. NPS and NPT use the same thread angle, shape, and pitch.

[Matt Harwood]

2 hours ago, joe_padavano said:

Matt,

 

BSPT is a different pitch than NPT and the individual threads have a different cross sectional profile as well. The original fitting is more likely NPS (national pipe straight) since it appears to use a seal washer on the underside of the hex. NPS and NPT use the same thread angle, shape, and pitch.

 

Good point. It does use a copper crush washer. Do you think it is possible to use NPT in a hole tapped for NPS? I don't want to risk breaking the casting, which is incredibly fragile. My parts carburetor threads are probably messed up, although it's really hard to tell.

 

Edited March 19, 2021 by Matt Harwood (see edit history)

[1912Staver]

I definitely would not use NPT in a straight thread casting. The wedging action of the  taper thread as it screws in will place stress on the casting and could very likely lead to a crack.

 

Greg

[joe_padavano]

13 minutes ago, Matt Harwood said:

 

Good point. It does use a copper crush washer. Do you think it is possible to use NPT in a hole tapped for NPS? I don't want to risk breaking the casting, which is incredibly fragile. My parts carburetor threads are probably messed up, although it's really hard to tell.

 

NPT is tapered, NPS is straight. You can probably screw male NPT into female NPS, but not the other way around. The problem is that while it will screw in, it won't seal properly. NPS threads are designed to be structural, not sealing. The crush washer is the seal. An NPT fitting won't have the sealing surface for the crush washer. Plus, since NPT is tapered, it may or may not screw in far enough anyway.

 

[Rusty_OToole]

The original looks like a finer thread than the others. Why can't you reuse it?

[joe_padavano]

29 minutes ago, Rusty_OToole said:

The original looks like a finer thread than the others. Why can't you reuse it?

I'd say these two are exactly the same pitch. Of course, a thread gauge would eliminate all doubt.

NPT and NPS use the same pitch and thread profile, just one is tapered and one is not.

 

 

Edited March 19, 2021 by joe_padavano (see edit history)

[edinmass]

It’s a Cadillac Johnson carburetor.....thus everything about it is sxxt. Let me restate that...........it’s a big POS when new, and you can’t polish a turd. From memory, I have never seen anything quite like that fitting and always ended up using a factory one. Since that car has a gravity flow fuel system, and your running pressure.........you know where this is going. Be careful please! 

[1939_Buick]

Do you have 1/4" SAE 20 tpi?

Thread gauges are good to own

 

Threads

https://trimantec.com/blogs/t/thread-identification-guide

Edited March 19, 2021 by 1939_Buick (see edit history)

[Norman Biggs]

If it is a 1/2" NPT then it only 1/2 threads per inch more.  The almost 2 deg threads are sealing threads.  Straight treads are working threads and usually seal on a washer.

[joe_padavano]

2 minutes ago, 1939_Buick said:

Threads

 

https://trimantec.com/blogs/t/thread-identification-guide

 

Unfortunately that site doesn't appear to include NPS, which is not the same as SAE straight.

[Matt Harwood]

16 minutes ago, edinmass said:

It’s a Cadillac Johnson carburetor.....thus everything about it is sxxt. Let me restate that...........it’s a big POS when new, and you can’t polish a turd. From memory, I have never seen anything quite like that fitting and always ended up using a factory one. Since that car has a gravity flow fuel system, and your running pressure.........you know where this is going. Be careful please! 

 

You're not wrong, Ed. It is supposed to be gravity feed and I've acquired a correct vacuum tank that I'll eventually rebuild and make functional. But I just want to get the car back on the road--I haven't driven it in two years and I feel badly. I still need to get it to the spring shop to rebuild the leaf springs and it needs to be operational. Unfortunately, when I went to fire it up recently fuel started spilling out underneath, so I'm reworking the entire fuel system with metal lines. I don't know who did the work or when, but it's pretty hacky with the original lines cut, rubber lines spliced in, wrapped around the frame, a lawnmower filter, and a fuel filter bowl hanging sideways under the dash (yikes!). It all has to go. I can't believe I've been driving it like this as long as I have. So the only things staying are the tank, the pump, the carb, and the regulator. I'm adding a T-fitting and a check valve down in the V of the engine by the carb in anticipation of installing the vacuum tank while keeping the electric pump as a back-up. Don't worry, Ed, it'll be back to gravity feed before long.

 

The large carb fitting appears original, since both my parts carburetor and the one on the car use the same one, although, again, hack work has screwed things up. Dig how they tapped and threaded the fitting to install a 1/8" NPT elbow and barbed fitting for rubber line:

 

 

Like my other fuel systems, I'm planning on using 5/16" tubing and flare fittings throughout and would like to just connect the fuel line to the carburetor using a flare fitting. That original fitting is a 45-degree flare, but it's 5/8" or 3/4" and it's HUGE. I guess I could try to make something work with oversized tubing, but I suspect the original flare nut was for 1/4" or 5/16" tubing and just connected to the giant flare to help it seal. Either way, I need to replace or upgrade that fitting because I don't have one that isn't screwed up. One has buggered up threads (the one I removed from the parts carb) and one has been cut for pipe threads. 

 

I'm going to try to remove the fitting from my good carb and ease a replacement fitting in there. The BSPT fitting looks almost identical to the original threads and when holding them together, the threads align perfectly. I think the parts carb casting has swollen a bit, as pot metal tends to do. Hopefully my good carb doesn't have this same problem! If all that fails, I guess I'll drill it out and tap it for something I know what it is so I can buy the correct fitting and ease it in there. Since it seals with a copper crush washer, I'll use some oil on the threads to hopefully prevent galling.

 

Tomorrow I get started. We'll see how it goes...

[carbking]

7 hours ago, Gary_Ash said:

Could be it's a straight thread, not a tapered pipe thread.  Lamp parts are made with 1/4-27 threads for reasons I don't understand.  I recall running into this issue a while back.  It's possible that some carbs had 1/4-27 threads.  Where is Carbking when you need him?  😉 

 

Carbking doesn't understand the need for all the disturbed pixels

 

I don't have a 1929 8 cylinder carb to look at, but the 1930 V-16 carb fitting is 1/2 x 20 straight thread with a gasket for sealing. A good machinist could make a new fitting in about 15 minutes.

 

Jon.

[edinmass]

1 hour ago, carbking said:

 

Carbking doesn't understand the need for all the disturbed pixels

 

I don't have a 1929 8 cylinder carb to look at, but the 1930 V-16 carb fitting is 1/2 x 20 straight thread with a gasket for sealing. A good machinist could make a new fitting in about 15 minutes.

 

Jon.

 

Besides Joe P.......where are you going to find a good machinest today? 🤔
 

Does Carbking agree that using a D box from a septic system is a better choice of carburetor than the Cadillac Johnson used new on the 1929 Cadillac? Certainly it would leak less fuel, be easier to adjust, and provide a much better stoichiometric fuel mixture.

Edited March 20, 2021 by edinmass (see edit history)

[JV Puleo]

It's an NPS thread as has already been pointed out. If I were faced with this problem I'd make an adapter that screwed into the carb and solder the desired flare fitting to it. It could be made in as one piece but in order to get a perfect seal the angle on the flare fitting has to be perfect and, for my part, I'd rather buy one I knew was perfect than muck around trying to make one. The thread is not hard to cut...but I'd have to do it with a relief at the base because my lathe won't thread up to a stop. I'm make the adapter with a large enough hex to fit a fiber washer between the fitting and the carb body. You could easily use one of the original fittings and attach the flared fitting...look at the McMaster fittings intended to be soldered on to tubes.

 

Edited March 20, 2021 by JV Puleo (see edit history)

[carbking]

Shucks Ed - one wouldn't need a really good machinist, even I could make one; and I am not a machinist, just someone who has been forced to learn to use a lathe and milling machine.

 

And Johnson will definitely never be in my list of top quality carburetors.

 

Jon

[8E45E]

Brennan carries lots of adapter fittings to NPT:  Conversion Adapters – Brennan Industries (brennaninc.com)

 

Craig

[edinmass]

14 minutes ago, carbking said:

Shucks Ed - one wouldn't need a really good machinist, even I could make one; and I am not a machinist, just someone who has been forced to learn to use a lathe and milling machine.

 

And Johnson will definitely never be in my list of top quality carburetors.

 

Jon

 

It’s not even considered a carburetor on my list.............

[Frank DuVal]

10 hours ago, joe_padavano said:

BSPT is a different pitch than NPT

 

Except 1/2" and 3/4", which have the same TPI (threads per inch), 14. The other sizes are different through 2".

[joe_padavano]

4 hours ago, Frank DuVal said:

 

Except 1/2" and 3/4", which have the same TPI (threads per inch), 14. The other sizes are different through 2".

 

But BSPT still uses a 55 deg thread angle vs. 60 deg for NPT and NPS.

 

[Matt Harwood]

It IS 1/2-20!

 

So all I need is a 1/2-20 to 1/4 NPT reducer bushing or a 1/2-20 to 5/16 flare adapter. I'm sure both of those exist, so that should solve my problem. I bet I could even drill and tap a 1/2-20 bolt for a 1/8 NPT fitting. Nice!

[joe_padavano]

Typically used in hydraulics. Yes, these adapters are common. You can swap the o-ring for a crush washer.

 

 

[carbking]

Joe - you take the fun out of it!

 

Why buy one off the shelf when you can have the enjoyment (and subsequent boasting) of making one yourself?

 

Jon.

[JV Puleo]

I don't see where this answers the question. If I read his post correctly, Matt has a 1/4-18 NPS thread and wants to go to a 1/4" or 5/16 flare. I suspect that is a pretty unusual adapter.

[joe_padavano]

9 minutes ago, JV Puleo said:

I don't see where this answers the question. If I read his post correctly, Matt has a 1/4-18 NPS thread and wants to go to a 1/4" or 5/16 flare. I suspect that is a pretty unusual adapter.

 

Scroll up and read Matt's most recent post. And once you have an adapter from his current 1/2-20 straight thread to any NPT, the rest is easy.

 

 

Edited March 20, 2021 by joe_padavano (see edit history)

[joe_padavano]

37 minutes ago, carbking said:

Joe - you take the fun out of it!

 

Why buy one off the shelf when you can have the enjoyment (and subsequent boasting) of making one yourself?

 

Jon.

 

What can I say, Jon? I'm lazy. 😉

 

And I hate to say it, but I'm rapidly coming to the painful realization that I have too many projects and not enough time. Also, the "free" home-brewed solution ALWAYS takes longer and costs more than just buying the right part first.

 

No, I never take that advice myself.

Edited March 20, 2021 by joe_padavano (see edit history)

[Matt Harwood]

Well, given that it's Saturday and I'm the impatient sort, I may try making my own fitting by drilling and tapping a 1/2-20 bolt. That should be easy enough and should permanently solve the problem without looking as hacky as the old work.

[Bhigdog]

9 minutes ago, joe_padavano said:

 

What can I say, Jon? I'm lazy. 😉

 

And I hate to say it, but I'm rapidly coming to the painful realization that I have too many projects and not enough time. Also, the "free" home-brewed solution ALWAYS takes longer and costs more than just buying the right part first.

 

No, I never take that advice myself.

 

But what's the use of having a full machine shop, lots of raw stock, and an understanding wife if you just go out and buy what you need?......Bob

[carbking]

Joe - I resemble the "lazy" comment! But sometimes "lazy" just won't suffice.

 

Learned about special fuel fittings 40 years or so ago when we started doing cosmetic as well as mechanical multiple carburetor restorations. It is amazing just how many weird fuel fittings G.M. found to use on the multi-carb set-ups. And of course, the fitting is either right, or it isn't right; close does not count.

 

We paid a tool and die maker some serious dollars to make us tooling so we could fabricate these fittings, even those with female inverted flare.

 

Since we no longer do these, eventually I will offer the tooling to others.

 

Jon.

[carbking]

3 minutes ago, Matt Harwood said:

Well, given that it's Saturday and I'm the impatient sort, I may try making my own fitting by drilling and tapping a 1/2-20 bolt. That should be easy enough and should permanently solve the problem without looking as hacky as the old work.

Matt - if you cannot find a brass bolt, maybe you could paint it brass-colored?

 

Jon.

[joe_padavano]

30 minutes ago, Bhigdog said:

 

But what's the use of having a full machine shop, lots of raw stock, and an understanding wife if you just go out and buy what you need?......Bob

 

That's never stopped my from acquiring more tools. 😁

[joe_padavano]

30 minutes ago, carbking said:

Joe - I resemble the "lazy" comment! But sometimes "lazy" just won't suffice.

 

And case in point:

 

I recently acquired a 1984 GMC crewcab dually to replace my 99 Chevy that has 300K miles on it. This is supposed to be just a truck, not another project. Of course, I just bought a clutch pedal assembly to swap out the TH400 for a manual trans, ya know, because... 😉

[JV Puleo]

1 hour ago, joe_padavano said:

 

Scroll up and read Matt's most recent post. And once you have an adapter from his current 1/2-20 straight thread to any NPT, the rest is easy.

 

I missed that...1/2-20 certainly makes it easier. I should have known better because I once made a 1/2-20 Banjo bolt for my 1927 Cadillac carb.

[Matt Harwood]

I bought the correct 1/2-20 to 5/16" flare fitting, which I'll have on Monday. Spent the weekend re-plumbing the rest of the fuel system which is full of hack work. I never looked very closely at it because I paid a reputable shop a lot of money to sort out the fuel system when I first bought it 11 years ago and assumed they did a good job. But now that I'm looking at everything, I'm just appalled by their workmanship. Original copper fuel lines just cut and rubber hose jammed on with a hose clamp, those rubber lines wrapping around the frame (fuel lines are inside the frame but they mounted the electric fuel pump outside the frame), Home Depot fittings and hardware, none of the lines secured, just a wrap of tape on part of the old metal fuel line, and a pretty serious kink in the line where they bent it around the clutch pedal. The fuel pump was always noisy as hell and I found out why--the mounting hardware is all stripped so the pump was just kind of hanging from the bracket and buzzing around loose. I'm so disheartened by the workmanship of this shop and with hindsight, I feel lucky that nothing ever went wrong and burned the car down. That'll teach me to carefully examine any shop's work even if I trust them.

I ran new 5/16" copper/nickel fuel lines front to rear, routing them through the frame to connect to the fuel pump and back inside where I re-used the original mounting clips. Went from about ten joints to four and zero rubber line so it should last forever. I didn't take any photos just because I wanted to get it done--the car has been inop with a major fuel leak so I'm doing this job on the floor, not on the lift. Man, I quickly realized how spoiled I am with a lift because this was a real pain! 

 

I'll have that last fitting tomorrow and can connect the carburetor to the regulator and test for leaks. Looking forward to driving my old friend again and not worrying about fuel leaks ever again.

 

 

Edited March 22, 2021 by Matt Harwood (see edit history)

[ply33]

34 minutes ago, Matt Harwood said:

. . . Spent the weekend re-plumbing the rest of the fuel system which is full of hack work. I never looked very closely at it because I paid a reputable shop a lot of money to sort out the fuel system when I first bought it 11 years ago and assumed they did a good job. But now that I'm looking at everything, I'm just appalled by their workmanship. Original copper fuel lines just cut and rubber hose jammed on with a hose clamp, those rubber lines wrapping around the frame (fuel lines are inside the frame but they mounted the electric fuel pump outside the frame), Home Depot fittings and hardware, none of the lines secured, just a wrap of tape on part of the old metal fuel line, and a pretty serious kink in the line where they bent it around the clutch pedal. The fuel pump was always noisy as hell and I found out why--the mounting hardware is all stripped so the pump was just kind of hanging from the bracket and buzzing around loose. . .

I am puzzled about why these types of workmanship issues took 11 years to be discovered. . .

 

I guess I am compulsive: On the occasions I don’t do the work myself I end up spending almost as much time as it would take to fix something just to examine the work I paid for.

[Matt Harwood]

7 minutes ago, ply33 said:

I am puzzled about why these types of workmanship issues took 11 years to be discovered. . .

 

I guess I am compulsive: On the occasions I don’t do the work myself I end up spending almost as much time as it would take to fix something just to examine the work I paid for.

 

I bought this car from an estate when I was working with my former business partner. He commissioned the repairs expecting that we would be selling the car, I simply bought it instead. I assumed the work was done correctly and never really looked at it, I was just happy to have the car and was just starting out. I didn't do the work myself and just drove the car. Lesson learned.

 

I'm embarrassed that I didn't notice it, but I trusted the shop that did the work and just assumed that it was good. None of it was particularly visible, as most of the fuel system is hidden by the exhaust system--all I really could see was the fuel pump on the outside of the frame rail with rubber lines attached to it. I wasn't thrilled about the rubber lines but figured that they knew what they were doing and it would be OK. The car always ran and never let us down and was bulletproof on the road, so as long as it was operable I didn't think about it too much. It was only when one of the rubber hoses cracked and started leaking that I decided to do the work and found all the other issues. 

 

I'm not the same person I was 11 years ago. I was just happy to finally have a Full Classic of my own and was eager to get on the road. Today it wouldn't happen as I'm especially picky, but this car worked so well I never questioned it.

 

Don't judge me. I'm 100% certain I'm not the only guy running around with a hacked-up fuel system. My efforts on my other cars prove this is the last time it'll happen.

[ply33]

14 hours ago, Matt Harwood said:

. . . I'm not the same person I was 11 years ago. I was just happy to finally have a Full Classic of my own and was eager to get on the road. Today it wouldn't happen as I'm especially picky, but this car worked so well I never questioned it.

 

Don't judge me. I'm 100% certain I'm not the only guy running around with a hacked-up fuel system. My efforts on my other cars prove this is the last time it'll happen.

 

I am sorry I came across too strongly. It was not my intent to judge you or be harsh.

 

From your posts, you seem to me to be a very competent and thorough person when it comes to older cars, so it was just a bit surprising that this got past you.

[edinmass]

Matt........after all these years, first few things I check on any car........age of tires and tubes, battery disconnect, and run the entire fuel line looking for bad rubber hose........and you find it 95 percent of the time. I have seen top shops do poor fuel pump installs because it often get given to thr new kid in the shop, and doesn’t get inspected by a top guy. Been there, done that.

Edited March 22, 2021 by edinmass (see edit history)

[Matt Harwood]

Should I use Teflon paste or anti-seize on the threads of the fitting going into the carburetor? Not so much to seal them (I'm using a copper crush washer) but to prevent galling and to protect it from seizing up in the future.