Welch Plug Dorman To Plug 66 427 390 HP Intake Manifold Exhaust Cold Enrichment Heat

[Tom D.]

Found several good discussions about plugging the exhaust holes at the top (under carburetor) of a Corvette cast iron intake manifold. Harder to find the correct size of either a pipe plug, or a "welch plug" to use in this application.

Should I look for a 7/16" welch plug? A 7/16 inch bolt drops freely into the two exhaust holes. Do I have to drill and tap and install one inch plug?

The manifold is on the engine and the engine is in the car. It is a 427 390 HP 1967. The carb is the original Holley 3811.

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[Peter M.]

Re: Welch Plug Dorman To Plug 66 427 390 HP Intake Manifold Exhaust Cold Enrichment H

Tom,
It has been a while since I did mine. I think I used 1/4" pipe tap and plug. The manifold was off when I did it.
Kind regards.

[Joe L.]

Re: Welch Plug Dorman To Plug 66 427 390 HP Intake Manifold Exhaust Cold Enrichment H

Found several good discussions about plugging the exhaust holes at the top (under carburetor) of a Corvette cast iron intake manifold. Harder to find the correct size of either a pipe plug, or a "welch plug" to use in this application.

Should I look for a 7/16" welch plug? A 7/16 inch bolt drops freely into the two exhaust holes. Do I have to drill and tap and install one inch plug?

The manifold is on the engine and the engine is in the car. It is a 427 390 HP 1967. The carb is the original Holley 3811.

[ATTACH=CONFIG]100660[/ATTACH][ATTACH=CONFIG]100661[/ATTACH]

Tom------


I think a 7/16" cup-shaped expansion plug will work. However, as I recall, the holes can be tapped with a 1/4" NPT tap without the need to even drill them out first. Then, install 1/4" NPT internal hex drive pipe plugs.

[Tom D.]

Re: Welch Plug Dorman To Plug 66 427 390 HP Intake Manifold Exhaust Cold Enrichment H

Today, I installed two Dorman 555-003 7/16" expansion plugs (in 7/16" holes, for the application I mentioned above). They are a tight fit.

I was reluctant to tap the holes, as the manifold is installed. A long time Michigan chapter member says he's done it on the engine. He didn't seem worried about the metal shavings. Says there is no route to a valve, unless...

Joe: By chance do you mean a 1/2" NPT tap?

[Joe L.]

Re: Welch Plug Dorman To Plug 66 427 390 HP Intake Manifold Exhaust Cold Enrichment H

Today, I installed two Dorman 555-003 7/16" expansion plugs (in 7/16" holes, for the application I mentioned above). They are a tight fit.

I was reluctant to tap the holes, as the manifold is installed. A long time Michigan chapter member says he's done it on the engine. He didn't seem worried about the metal shavings. Says there is no route to a valve, unless...

Joe: By chance do you mean a 1/2" NPT tap?

Tom------


No, I really mean a 1/4" NPT tap. For NPT threads, the size is not the same as OD. The 1/4" NPT taps will cut light threads in the manifold holes. This is PERFECT for the installation of the 1/4" NPT plugs. 3/8" NPT can also be used but that requires further drilling out of the holes.

There is no real danger of any chips from the tapping getting into the engine in a harmful way. For one thing, the tap will cut very light threads and very little in the way of chips will be generated, especially if you coat the tap with grease before tapping. For another thing, the holes only go through to the exhaust crossover in the manifold. The cross-over only goes to the exhaust system. So, even if any chips did get through they would only get into the exhaust system where they would do no harm.