Thread Spec: Equipment vs Pipe

Consult with your piping engineer. They will know how to properly connect these two incompatible thread sizes, or know who to call for advice.

Most piping engineers are good at jury rigging: Jury rig - Wikipedia, the free encyclopedia

Duct tape, or teflon tape can be used to effect a seal.

Maybe a welder?

Maybe an A-dapter kit, like Johnny Cash used on that Cadillac transmission?

JOHNNY CASH LYRICS - One Piece At A Time - A-Z Lyrics "So we drilled it out so that it would fit , and with a little bit of help with an A-daptor kit"

Hey, I'm just joshin'.

Well, I sure hope that your G 2-1/2 isn't 75.191 mm OD, or you're in deep trouble.

On the male threads wrap some thread/string on the threads like is done with steam piping (old school). Then use LOCTITE Thread Sealant with PTFE, #30534 for the pipe dope. Assemble with a 18" pipe wrench. It won't leak with this combination.

If you ever want to take it apart it will require a 24" or 36" pipe wrench to get enough power to break the seal.

If all else fails use a 2-part epoxy for thread pipe dope. That will certainly not leak. When you want to take it apart heat it with a torch and it will come right apart. Some times you don't even need the torch if your pipe wrench is long enough. Don't use cheaters, they are dangerous! Use a longer wrench.

Good Luck, Old Salt

Taking it apart with a 36" pipe wrench,..... does the pipe have a chance, or will it crush? Or cross that bridge when it comes.

Just reread it, and saw SCH XS

With an o.d. of around 3" (2-1/2" nom.) the 36" shouldn't be a problem with crushing. I wouldn't normally use a 36", probably a 24" wrench to start with. The 36" would provide 1-1/2 times the torque to the joint than the 24".

As with anything like this, situational awareness is extremely important. For something as simple as taking apart a pipe joint, the less skilled simply pays attention to the force, or the more force, needed. The skilled pays attention to many more things and doesn't crush pipe.

Have a pipefitter journeyman do the work and not an apprentice. Either that or an engineer with his callused hands dirty from cutting oil and pipe joint compound!

Good Luck, Old Salt

Book knowledge is good, but with all of it I was forced to do it is the stuff I have forgotten most. The stuff that has really helped me the most and forgotten the least of is the common sense and "old school" ways of doing thinks that the helpful "old school" mechanics, engineers, operators, laborers, drivers, and other names that have been so informative by sparing the time to enlighten me. They were all nice people after they got to know me and I got to know them. Always there when I needed them and willing to go the extra mile for or with me. Very few of them asked for anything in return except for me to respect them, be fair, say hello and honestly ask how they were doing.

God bless the "old school" teachers and students!

Good Luck, Old Salt

Absolutely.

And a good point for codemaster on the air piping, although I would use it for some things like mating BSPP and NPT fittings where the additional bulk seems to help seal the marginal threads just past the contact point.

Another thing is to adjust the die (as per Tornado's post). With my Rigid 535 I can set it quite oversize, so that it cuts a parallel thread for the first part without overextending the thread. This seems to work a bit as BSPP anyways is a more consistent fitting than NPT.

The 535 will do a good job of it, and many other modified threads. Unfortunately I sold mine years ago. Was great when I needed it, at that time I really did, but become cumbersome when not needed for extended periods.

Another way I accomplish a good mixed joint is to use the 00-R or 12-R, these are non adjustable, dies but leave a thread or two uncut at the end of the pipe. This looks like a thread or more to go in the die. This provides an oversize male with a shorter engagement of the threads but oversized at the imperfect threads which can be tightened onto. When used with the 700 hand power head this isn't difficult on up to 2" pipe.

The OP does not mention what pressures he is operating at but if it is low enough I would forget all this mismatched thread stuff. Put a straight thread nipple in the filter, a NPT pipe to connect it to and use a Fernco or Skinner Coupling Clamp to hold the two pipes together. This is how many compressor manufacturers hold their filters onto compressors. They have the added advantage that they transfer very little vibration from the compressor. If the filter has to be changes loosen the clamp, take it to a convenient place to work and swap elements. This is much faster and therefore cheaper than doing it in place.

This Keeps It Simple, cheaper and easier to do therefore it gets done when it should.

Good Luck, Old Salt

I heard somewhere that hemp/dope is OK for water, but not gas, for that you need PTFE tape or proprietary sealant. On gas hemp/dope leaks slightly, unlikely to be enough to cause flammable mixture, but a smell problem. As the OP is on air, either should be OK, but I thought I'd mention it, to see if anybody can confirm or deny.

"shee...it baby, I aint gonna make rope no mo!!"

Discover X-pando™ pipe joint compound. (That's an endorsement, but without any financial involvement.)

Some pipe dies can be set a bit over- or under-sized, and thereby compensate for slight mismatches.

Your filter has threads under the British pipe standard. Which are generally used on Metric pipe. Not pipe under the ASTM standard. You maybe able to find an adapter. Other then that correct one or the other.

G2½ is parallel female. You can use a parallel or taper male fitting with parallel female, but parallel seals against a flange which you don't have, so taper male with thread sealant is required.

According to my data minimum OD of male fitting = 2.969" = 75.413mm. Basic gauge dia is your 75.189mm = 2.96". So ASTM A53 Gr. B, sch XS, NPD 2 1/2" (OD=73mm) is too small OD. JIS STPG NPD 65A (OD=76mm) should be OK, but check the wall thickness. My data gives minimum OD of female fitting, equivalent to thickness about 4.5mm. Oddly, it doesn't give maximum ID for male fitting, but on same basis this would be about 75.189 - 2*4.5 = 66.189mm. Thread depth = 2mm so 4.5mm wall should be OK, you might get away with a little less.

Parallel threads and straight threads can be done sometimes but need the utmost of care in doing it. With a male straight and a female taper there is only one or possibly two threads of engagement at the inner end of the pipe thread. This can cause the joint to wobble slightly and not have a good seal. Likewise, a male taper and a female straight there is only one or two threads of engagement at the outer end of the joint. Same wobble problem and not enough threads of engagement for a good seal. Most straight threads have an "O" ring around the male threads at the back with a hex nut machined into the fitting to provide a seal, just as a spark plug seals into the head of an engine.

The nominal length of thread engagement for a 2" taper thread is 3/4" and 15/16" for a 2-1/2" thread.

Hope this helps.

Good Luck, Old Salt

I'd agree with that. My gen says parallel/parallel is sealed by flange faces with washer, but I don't see why it wouldn't work with thread sealant. Might depend on the details of the female part on the air filter - how many threads, and whether it opens out behind (so nothing to tighten against). Though he could make the pipe thread the right length and screw it in as far as it will go.

If he wants to use parallel male with washer or O-ring he has to weld on a flange (making sure it's nice and square to the pipe!) and needs a smooth surface on the filter. If it's rough or has casting ridges it won't work.

Recommend Expando. Google it. It is a powder you mix to a paste and apply liberally to the male threads and it expands. The compound expands and fills irregular spaces in the threaded joint. I recommend you install a union behind this incompatible joint so you don't have to break the joint apart for maintenance etc. Once you make up the Expando joint it is difficult to break apart but I guarantee it will not leak. As a service pipefitter earlier in my career, I kept a can of this on the truck for those pain in the butt joints that just would not seal....until I applied Expando.