Converting non-SMOG to SMOG manifolds

[Steve Antonucci]

Do any members here have any experience with getting non-SMOG exhaust manifolds
drilled & tapped ( properly ) to use with factory SMOG equipment? I have been
hunting the proper dated manifolds for quite some time now. I did find correct
dated non-SMOG manifolds ( which I bought as a backup ). Now, I am faced with
the prospect of getting these manifolds drilled & tapped for the SMOG ports.
I can certainly contact machine shops to do this, but centerline dimensions are
fairly important I would think. Furthermore, the configuration of the actual
ports are also important. I only have one shot at this, so perfection is the
order of the day. To that end, does anyone have a recommendation for a shop
that has done this type of work before? I don't want to learn the hard way.
I would prefer to work with a shop that has this experience under their belt.

Steve

[Mike M.]

Re: Converting non-SMOG to SMOG manifolds

buy the smog tube asemblies, measure the distances between adjacent pipes and drill and tap the manifolds. an average machine shop ought to be able to pull this proceedure off without a screw-up. good luck, mike

[Steve Antonucci]

Re: Converting non-SMOG to SMOG manifolds

Mike,

Isn't there a seat configuration that the tube seat mates up against?
I actually don't have a SMOG manifold to compare against.

Steve

[Rob A.]

Re: Converting non-SMOG to SMOG manifolds

Steve,

The thread for the smog fittings is unusual, not standard thread, so make sure if you go the route you're thinking of, they use the correct tap. I cleaned up some original smog manifolds with a tap, and had to special order it. I can't remember what they call the thread type, but I think it might be called British something or other, definately not regular U.S. type threads.

[Steve Antonucci]

Re: Converting non-SMOG to SMOG manifolds

Thanks for that heads-up Rob. While we are on this subject,
anyone know the thread size?

Steve

[Rob A.]

Re: Converting non-SMOG to SMOG manifolds

Steve,

If the fitting size is the same for samll and big block, I can tell you the size on the tap, I still have it.

[Steve Antonucci]

Re: Converting non-SMOG to SMOG manifolds

Rob,

Dollars to dimes i's the exact same size.

Steve

[Rob A.]

Re: Converting non-SMOG to SMOG manifolds

The tap is marked 1/4 -19 BSPP. If I remember correctly, the machine shop that ordered it for me said that all smog equipment fitting threads, even on American cars were not U.S. measurement threads.

[Craig S.]

Re: Converting non-SMOG to SMOG manifolds

Besides the special size, there is a shoulder in the hole and a small tube that has a flange that seats, so the tree blows air through this tube that protrudes into the exhaust port for enhancing the burn.....Craig

[Paul L.]

Tubes

Here is an example of that tube. I bought a set of five for a now-defunct project on my former 1979.

[Terry F.]

Re: Converting non-SMOG to SMOG manifolds

I have not read the whole thread.....The tubes that I am familiar with have a very special thread type and as you stated, you will probably need to cut the holes with seats at the bottom. I don't think it can be done at reasonable cost or problem. Have you checked Paragon, etc. for the manifolds you need? What about ebay, and the other used parts people. They have to be out there somewhere. Just my thoughts, Terry

[Mark #28455]

try Paragon

When I was looking for repro manifolds, they checked their inventory and had a set of non-smog ones with a good date for my car. They were able to drill and tap them. (I think they are the supplier for the repros also so they should be able to do it right).
Good luck,
Mark

[Steve Antonucci]

Re: Converting non-SMOG to SMOG manifolds

Yes Craig, I believe you are correct. That's why I'd like to find a shop that
has already been successful doing the conversion ( machining ).

Steve

[John H.]

Re: Converting non-SMOG to SMOG manifolds

The GM shop manual includes a caution that identifies these as "straight pipe threads"; having dealt with this before, I can tell you that the threads are identical to 1/2"-20.

[John H.]

Re: Converting non-SMOG to SMOG manifolds

The GM drawing shows the thread size as 1/4"-18 NPSF, which is an American Standard Straight Pipe thread; GM used a .440" tap drill and bored the hole 1/2" deep. The center through hole for the insert tube is 21/64", and there's a slight 120* chamfer at the manifold outer surface of the bored hole.