Thermoquad to square-bore adapter mods

[sweatybetty]

After getting tired of chasing issue with a Holley 600, Ive decided to try a thermoquad. But my intake is a square-bore and I had no success finding a spread bore locally. After a bit of research I bought an Edelbrock 2696 that was supposed to be the one to use. Nope. It is specifically made to run a square carb on a spread bore manifold. After a bit of deep thought and a few cold man-pops, here is what I did to make it work. All in all, I spent about 90 minutes and $5.25 for the hardware. First 4 pics show the adapter and hardware.
The first problem is the mounting holes are backwards for what I need to do. The inner (marked with arrows) are needed to bolt the carb to the adapter. The outer holes are needed to bolt the adapter to the manifold.
The hardware is
4- 5/16 x 1-1/4" allen head bolts
4- 5/16 x 1-1/2" hex head bolts
4- 8-32 x 1/4" set screws

 

[sweatybetty]

The hex head bolts dont fit well in a round hole. So after a couple minutes rounding the corners they now fit snug. These are the ones that hold the carb down to the adapter.

 

[RustyRatRod]

As you are finding out, that's the wrong adapter for what you are wanting to do.

 

[sweatybetty]

So how will these work if they spin while trying to tighten the nuts? Ask no more. Simply drill and tap a small hole for the set screws. I also ended up grinding a slightly large flat spot on the bolt head for the set screw to snug up to. Problem solved.

 

[RustyRatRod]

That's not the right adapter that you need. This is.

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[sweatybetty]

The last issue is the bolts that hold the adapter to the manifold. This is fairly a straight forward fix. I drilled out the threaded holes in the adapter with a 21/64" bit so the bolt will fit, then drilled down with a 31/64" just enough for the bolt head to sit below the adapter surface.
Mission accomplished.

 

[RustyRatRod]

Wait till you open the secondaries. You'll see.

 

[lemondana]

You do have a wrong adapter. You have one for a Quadrajet. There is a difference in adapters.
Look up the the thread- Square - bore vs. Spread - bore? by Bill Crowell on Feb. 3 2021. I explained the differences and posted a bunch of pictures of the same. You need Edelbrock 2691 for a TQ, I know, the picture is identical to the 2692 Q-Jet model and then the one you have is the Q-Jet also. You will understand when you see the comparison pictures, noting the area in the rear between the two bolt holes.

 

[LO23M8B]

Adapters like this just isn't going to perform correctly. If you really want the Thermoquad experience just buy a correct manifold. If you are on a tight budget buy a used stock cast iron one, they work great.

 

[Bewy]

On your intake. I would open up [ remove metal, widen the opening ] the carb pad where the secondaries are. Leave 1/4" of metal to seal the base gasket. This will allow you to remove metal from the mating area of the adapter, smoothing out the severe angle of the secondary openings. Use 2 turns for the air valve spring adjustment.

 

[gzig5]

So how will these work if they spin while trying to tighten the nuts? Ask no more. Simply drill and tap a small hole for the set screws. I also ended up grinding a slightly large flat spot on the bolt head for the set screw to snug up to. Problem solved.

View attachment 1716471215

View attachment 1716471216

View attachment 1716471217

View attachment 1716471218

A quick way to accomplish the bolt not turning in the hole would have been to grind off five of the six corners on the bolt head and then mill/drill/grind/carve a spot on the hole for that last corner to drop into. The bolt is now keyed in to the hole and can't turn while tightening, but just drops in and out like a normal bolt. No extra hardware required and faster than drilling/tapping for a set screw.

Good job accomplishing what you were trying to do and for sharing. I don't know that if the plate will work as intended, but looks like you can at least mount the carb now.

 

[lemondana]

Your Quadrajet adapter can still leak on the back edge when used with the TQ.. Go back and look at my pictures.

 

[rumblefish360]

That’s not the adapter to use.
Grab the open spacer.