How can one repair A.I.R manifold tube threads?

[Greg L.]

Well since I couldn't find a tap to clean out the A.I.R holes in my manifolds I thought I'd just try them with a new manifold. To my suprise they screwed in all the way too, but keep turning and turning and turning!!!

Here I thought I was going to have to clean out the threads when in fact there are hardly any left to clean out. Only one held tight so now I need to repair the other seven. Any ideas on how to do this?

I can just imagine how hard it will be to find a heli-coil of this type when I couldn't even find a tap. Any help on this one is really appreciated!

Thanks again.

[Rob A.]

Re: How can one repair A.I.R manifold tube threads

Greg,

I was able to find a tap to clean the threads in a set of '66 small block AIR manifolds. It is a special tap meant to clean all threads without bottoming out first. If you're still interested in going that way, I can give you the specs off it.

[Joe L.]

Re: How can one repair A.I.R manifold tube threads

Greg-----

It's not clear to me which threads you're referring to----you mention that you tried "...them with a new manifold". This implies that you are referring to the threads on the manifold tubing nuts. If so, there's no way to fix them other than grinding off the flare, removing the nuts, replacing them with new nuts, and re-flaring. This can be done if you're careful. However, the tubes will be slightly shortened.

As far as the threads in the manifold go, I don't know of any way to repair them. It MIGHT be possible to install 7/16-20 heli-coils. However, this would be quite difficult to do properly.

[Greg L.]

Re: How can one repair A.I.R manifold tube threads

Your second thought was right Joe...it's the threads in the exhaust manifold that are bad. The manifold tree is a new one from Bill Hodel. FYI, the pitch is 18 and not 20 so that is why I'm thinking that finding heli coils will be difficult.

If the heli-coil option doesn't work I was thinking that maybe I could build the threaded area with brass and then re-tap the holes... What's your thoughts on that?

Rob, I just might need the specs on that tap..or better yet, where to get one from!

Thanks guys.

[Terry F.]

Re: How can one repair A.I.R manifold tube threads

That is a tough fix. I would look at it and consider drilling them all out and making a bushing and press fitting it. I would then tap the bushing. Make bushing out of some type of exotic material. Machinest would know what to make it out of. Something will some stainless in it. Just my 2 cents. Terry

[Joe L.]

Re: How can one repair A.I.R manifold tube threads

Greg------

I was mis-informed regarding the 7/16-20 thread working for these AIR tube fittings. The thread is an odd-ball and I don't think that any Heli-Coil is available in that size. I don't even know where to get a tap for this size thread. As I recall, it's some sort of British, non-metric thread design. I can't imagine why it was selected for this application when a standard tubing nut would have been so much simpler.

[Joe L.]

Re: How can one repair A.I.R manifold tube threads

Greg------

A little more delving into the outer limits of my memory and checking back on some old notes, I believe that the thread for these fittings is one of two---either "British Standard Pipe Parallel" (BSPP) or "National Pipe Straight" (NPS). If it's the former, then I believe the thread size is 1/4-19. If it's the latter, then I believe the thread size is 1/4-18. I don't know which one it is, though.

I believe that taps of these sizes are available from industrial suppliers like Grainger or McMaster-Carr.

[Greg L.]

Re: How can one repair A.I.R manifold tube threads

Thanks Joe...I'll try those suppliers for the correct tap. I do believe however that the pitch is 18 and I was told that it is 1/4" so I hope that this is a "common" tap even if it is a special order.

I know that there is a NPS 1/4-18 because that is what the local guys tried to sell me. If there is a BSPP 1/4-18 then I might be in luck. What do you think of my idea of building the threaded areas with brass and then re-tapping the holes? Maybe as Terry suggested I should braze an insert and then drill and tap that...

I'm almost in over my head on this one but with repro manifolds running $380 or so US a piece my only option is to try and repair my originals.

[Tracy C.]

Re: How can one repair A.I.R manifold tube threads

Greg,

You can braze the holes full, redrill and retap (when you find one). I've done this to repair stripped threads on the bottom of exhaust manifold with success.

Good Luck
tc

[Rob A.]

Re: How can one repair A.I.R manifold tube threads

Joe,

You're right about the odd-ball thread size. I special ordered the correct tap from a machine shop. I have used it with good results.It is marked: 1/4-19 BSPP 55 MOD WHIT 32685R#

[Clem Z.]

repairing threads

the best way to fix this is a bore out the orignal threaded holes and press or shrink fit in sleeves with the threads already in them. you will need to check what has the same expansion rate as the cast iron manifolds to make sure the inserts do not come loose from the heat

[Joe L.]

Re: How can one repair A.I.R manifold tube threads

Greg-----

How do you know that the threads are 1/4-18 and not 1/4-19? It's very difficult to "count threads" and discern a small difference like this. It may well be that it is NPS 1/4-18, but I'm curious as to how you came to this conclusion.

If you are able to purchase an NPS 1/4-18 locally, did you try this to see if it's the correct thread? You can match the threads on the tubing nuts to the threads on the tap. Or, better yet, use the tap to cut threads in a piece of scrap steel and see how the tubing nuts fit.

The BSPP thread is 1/4-19 and not 1/4-18. So, if it turns out to be BSPP thread, then it has to be 1/4-19.

I think that your brazing idea will work. The difficulty will be in drilling the proper hole size prior to tapping. The thread depth in these fittings is quite short. If you repair it this way, you may need to have a machine shop do it with a milling machine.

Please post what the thread turns out to be after you finally determine it.

[Clem Z.]

yes you will need a machine shop

to do the work because alignment of the holes will be a problem. you had better get the center to center dimentions before if you braze these hole because after that all bets are off. you will need to bolt the manifolds to a 90 degree fixture to hold them in the milling machine. that is why i suggested press or shrink fit inserts or even brazed in inserts if you think the press fit will not hold. i think in the end unless you can do this yourself the cost of new ones is not that far out of line with all the machine work needed to do this correct

[Joe L.]

Re: yes you will need a machine shop

clem-----

Yes, I agree. Regardless of how this repair is done (brazing and tapping, drilling out with inserts, etc.), it's really going to be a machine shop type of operation. Depending upon how many fittings need to be repaired, the cost of the machine shop work could equal or exceed the cost of a replacement manifold. If there is just one bad fitting, the cost of repair might be less than purchasing new. If it's all 4, I think the repair cost for the machine work will exceed the cost of a new manifold.

Machine work like this is not cheap. There is quite a bit of set-up time involved.

[Clem Z.]

as a graduate tool and die maker among

my other endeavors machine shop work is not cheap even if you can find a shop that would want to do it.