Intake heat crossover

[Jim T.]

Clem this is from a post you made many years ago.

if your manifold has the holes on both sides under the carb that supplys heat from the exhaust heat riser crossover i would plug them with the proper sized cup type expansion plugs. this will cut down on the amount of heat the carb see from the exhaust crossover. check to make sure your heat riser valve is not frozen shut. the last and best solution is to block completely the heat riser crossover in the intake manifold.

Where are some commom places that I can purchase the expansion plugs and what size?

[Joe L.]

Re: Intake heat crossover

Clem this is from a post you made many years ago.

if your manifold has the holes on both sides under the carb that supplys heat from the exhaust heat riser crossover i would plug them with the proper sized cup type expansion plugs. this will cut down on the amount of heat the carb see from the exhaust crossover. check to make sure your heat riser valve is not frozen shut. the last and best solution is to block completely the heat riser crossover in the intake manifold.

Where are some commom places that I can purchase the expansion plugs and what size?

Jim----


You can get these plugs in any REAL auto parts store (probably not Kragen, Chief, Pep Boys, etc.). They'll usually be found in the orange, metal Dorman "drawers".

I just don't recall the size, though, since I usually drill and tap for internal hex, 1/4" NPT plugs. I think the expansion plugs are either 7/16" or 1/2". If you have the carb off, just measure the size of the holes and get the appropriate size expansion plugs.

If all else fails and the plugs are 1/2", you can get these from GM under GM #21000867, but they cost about 6 bucks each, GM list.

[Jim T.]

Re: Intake heat crossover

Jim----


You can get these plugs in any REAL auto parts store (probably not Kragen, Chief, Pep Boys, etc.). They'll usually be found in the orange, metal Dorman "drawers".

I just don't recall the size, though, since I usually drill and tap for internal hex, 1/4" NPT plugs. I think the expansion plugs are either 7/16" or 1/2". If you have the carb off, just measure the size of the holes and get the appropriate size expansion plugs.

If all else fails and the plugs are 1/2", you can get these from GM under GM #21000867, but they cost about 6 bucks each, GM list.

Joe, thanks for the reply/information. I will visit my local NAPA store for the plugs. The counter persons at my NAPA I have purchased from before have many years of experience.

[Timothy B.]

Re: Intake heat crossover

Jim, FWIW be careful with these plugs, I had one fall into the hole and had to remove the intake only to find it sitting on the exhaust valve.

Joe is correct that it's better to tap for a small plug. My car likes them out and I don't have any problem with the fuel in the carburetor. Just make sure the gaskets are correct.

[Tom P.]

Re: Intake heat crossover

I have blocked the ex crossover passages on all of my engines by using screw-in plugs with pipe thread and sealant.
Some manifolds only have the crossover running under the plenum in the center, so I use side manifold gaskets that have the crossover ports blocked. Of course, FI base plates are blocked off to begin with.
The ONLY problem I have with any of my engines is the SB400 in the 70 Chevelle conv. I have a 69 Z28 alum manifold (same style as 70-72 LT1 manifolds) on that engine with the crossover passage blocked at the head/manifold interface. Until it gets warmed up, I have to feather the accelerator/clutch a few times to keep it running. But once warmed up, it does great. I'm also running a Q-jet instead of a Holley on that manifold (by way of a Mr. Gasket adapter plate).

[Clem Z.]

Re: Intake heat crossover

you can tap the intake on the engine just use plenty of grease on the tap and keep your shop vacuum running while you tap and you should not get any chips into the exhaust passage

[Domenic T.]

Re: Intake heat crossover

I have blocked the ex crossover passages on all of my engines by using screw-in plugs with pipe thread and sealant.
Some manifolds only have the crossover running under the plenum in the center, so I use side manifold gaskets that have the crossover ports blocked. Of course, FI base plates are blocked off to begin with.
The ONLY problem I have with any of my engines is the SB400 in the 70 Chevelle conv. I have a 69 Z28 alum manifold (same style as 70-72 LT1 manifolds) on that engine with the crossover passage blocked at the head/manifold interface. Until it gets warmed up, I have to feather the accelerator/clutch a few times to keep it running. But once warmed up, it does great. I'm also running a Q-jet instead of a Holley on that manifold (by way of a Mr. Gasket adapter plate).

Tom,
I did the same back in the 70's with the adapter and liked it so much I changed the intake and put an aluminum Q-jet manifold on my 327.

The Q-jet gave me about 4 MPG and a HP boost to boot.

That was back in the days when the guys had their windows filled up with decals to make their car look fast.

Some had 2 or 3 different cam decals and all had the holly decal. The first thing that came out of their mouth was the size holley they were running.

I was not a holly fan, just a practical go fast junkey and the Q-jet did a better job for me along with the MPG.

About 99.9% of the guys that didn't like Q-jet carbs didn't know how to adjust the carb air for the 2 humungus back butterflies.

Holly on the other hand didn't concider the air intake and no matter how the changed the CFM to the carb the tops all looked the same where the Q-jet was WIDE open for air.

DOM