Rotaryracer Reader
5/6/22 7:28 a.m.

Hi all - my brother has a '68 2000 Roadster and is in the process of the deleting the air injection/smog crap and getting the distributor recurved and updated.  Pic of said car here:

He's got everything stripped off and the smog removal kit plugs in the head.  There are a couple of remaining air injection fittings he'd like to remove from the manifold (there are two of the larger one):

Those go into the visible open threaded holes in the manifold shown here:

Anyone happen to know what freakish size and thread pitch that smaller hole is?  I *think* it's fine pitch M10 based on the info I'm getting from afar, but not sure if it's 1.0 or 0.75 thread pitch.  There is a taper to it as well, which might complicate things.  I wouldn't think NPT or BSPT would be used, but dunno.  If someone happens to know the size for all three, better still.  Apparently the HoDepo plumbing guy had that "deer in headlights" look when tried to go shopping in the Vintage Datsun aisle.  :)



stuart in mn
stuart in mn MegaDork
5/6/22 7:59 a.m.

This is from  It sounds like they recommend drilling the holes in the manifold and tapping them for a standard pipe plug, but since he has the fittings removed can't he just take them to the store and match them up with a suitable plug?  Assuming they're metric he may have better luck finding a plug at the auto parts store than at Home Depot.

Question: How do I de-smog a later car?

Answer: First you need to recurve the distributor to the early car specs of 7.5 degrees. There are several ways to accomplish this.

  • 1. Recurve with internal parts for an early distributor-springs, backing plate and weights.
  • 2. Replace existing distributor with a rebuilt early distributor.
  • 3. Replace existing distributor with one of Gary Boone's EI distributors.(My choice)

For installing a new distributor you can go here to read about that!

Next, you will need to remove the air injection tubes and plug the holes in the head and intake manifold. The holes in the head are 10MM X 1.5 Pitch. There was someone making plugs for the head on the list and I believe Ralleye also sells them. The intake fittings are a real bear to remove and you might just want to pull the intake to get them out. You can then retap them for a standard threaded plug. There are actually four things to do to completely desmog the later intake. Plug the two holes in the top where the ait tubes went. Remove and plug the small nipple on the balance tube towards the front. Remove the electrical device with the wires coming out of it on the front side of the balance tube. You can pull it off and use the base as a template to make a solid plate to bolt on where it used to go. 

Reset the timing to 0 degrees TDC, start her or him up and fine tune the timing for optimum performance. You can also static time with points by leaving the ignition key on and moving the dizzy until you get a spark. Tighten the dizzy down and you should be very close. An R16 is 16 degrees BTDC at around 600 RPMs.

It was recently pointed out that you should change out the needles in your carbs to the non-smog type as well. Makes sense to me and never having done a smogged car I missed that tid-bit the first time around. Thanks TR!

That pretty much covers de-smogging a later car.

Rotaryracer Reader
5/6/22 8:05 a.m.

In reply to stuart in mn :

Thanks Stuart!  He swung by Home Depot yesterday and the one that's being a bugger is the smaller one shown.  He just confirmed it sounds like the larger two may be 1/4" NPT (or at least that seemed to work), but was striking out on the smaller one.  He's going to try a dedicated plumbing supply and/or custom hoses/fittings shop today or tomorrow...and if all else fails, that smaller one may get the tube cut off, filled with JB Weld, and sanded smooth till it becomes a plug. 

Steve_Jones Dork
5/6/22 8:29 a.m.

I looked at 3 random ones on BAT. All 3 had the stock fitting modified. One was crimped closed, one had a plastic cap, one had a rubber tube with a screw in it. I'd say you're in for some modification. 

wspohn SuperDork
5/6/22 11:58 a.m.

Take the manifold off the car. 

Clean the inside around the hole with solvent, and apply a good tape to the underside across the hole.

Take a Dremel with a cutting wheel and make two small cuts on the top surface in an X pattern across the hole

Mix up some JB Weld and fill the hole up, making sure to fill the two 'keys' you cut as well (they are probably unnecessary with a threaded hole, but are extra protection for little effort).

When hardened, sand smooth and you are done.

If the colour difference bothers you, you can always paint the outside of the manifold.

I came up with that when I was racing an early TVR. I had come up with a set of factory Weber manifolds (very rare, used on the Le Mans cars but otherwise not generally issued as they only fit the alloy crossflow HRG Derrington heads) that some git had drilled holes for things like brake boosters in - right through the cast in place 'TVR' script.  Had to paint them as I didn't want the rebuilt script to show.  Sory the pic is hard to see.



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