heat slot revisited

[William F.]

Don't go away just yet even though this has been discussed before. I want to use drive in plugs or tap and use NPT pipe plugs to block my '67 300 hp hot slot holes while intake manifold is on engine. Problem is, can't find a Dorman drive in plug or pipe plug of correct size. The 1/4"threaded pipe plugs(this size pipe plug is actually 1/2" or slightly larger in size) requires a tap too large to use without drilling and drive in plugs are too small or too large.Has anyone had this problem and found a solution? Does anyone have a special gasket or right size plugs(my hole is minimally smaller than 1/2") that I could buy?
Thanks,
Bill Ford

[Gene M.]

Re: heat slot revisited

Grease a tap, 1/2 fine thread and use a set screw with loc tight. Don't use a pipe thread, it's too big.

[Jim T.]

Re: heat slot revisited

William I just did this to my 68's 327/350 intake. I used left over parts from a master cylinder rebuild kit. The parts are two brass seat parts that are not used in my rebuild of my 68's brake master cylinder. Each seat has a small bolt that screws into the center of the seat. The brass seat was a firm fit in the hole and I used a length of stiff wire and secured each end of the wire by wrapping the wire around the bolts just as a safety measure to secure the seats from going anywhere. My intake was still installed on the engine. I bought the small plugs and was leary of them and came up with this procedure.

[Gerard F.]

Re: heat slot revisited

Bill,

I used 7/16" freeze plugs on my 67/327/300, and drove them in with a 7/16 bolt:



As I remember, the NAPA guy had a hard time finding that size plug, and I got his last 2 in stock.

Test the fit before you drive them in. I know someone on this TDB who drove them into the exhaust manifold, and that's a problem.
Also use some high temperature sealant to hold them in place.

[Timothy B.]

Re: heat slot revisited

Bill,

I think Jerry's suggestion is the best with the manifold on the car. I installed cup plugs and had no problem.

Issues arose when I removed them troubleshooting carburetor issues and one got side ways and fell into the manifold. Long story short I removed the intake manifold to find it sitting on #3 exhaust valve which I removed with a magnet.

While the intake was off, I taped the hot slot holes for 1/4 pipe threads and installed short plugs. Worked great but not necessary if the intake is on the car. Be careful to drive the cup plugs straight and tape the four plenum holes so no accident. IT WENT WHERE ...

[William F.]

Re: heat slot revisited

Jerry's would be easiest solution but like he said, 7/16" plugs are hard to find. Does anyone ACTUALLYhave a couple or know EXACTLY where I could get 2?
Would appreciate the help.

[Timothy B.]

Re: heat slot revisited

Bill, try a machine shop you may find them there.

[Clem Z.]

Re: heat slot revisited

7/16" diameter is the correct drill size for a 1/4" NTP tap. just grease the tap good and have someone hold a shop vac near the area to catch any stray pieces.

[Lawrence M.]

Re: heat slot revisited

William, I recently did this on my 1969 350/350 with the manifold on the engineand used the plugs as Gearld did. My local NAPA has a machine shop and they had the 7/16 plugs. I drove them in with a punch that just fit inside the plug. Very easy to do.

Larry

[Timothy B.]

Re: heat slot revisited

Here is a picture of how I did my 67 manifold after the ordeal..Also, a pic of the 63 with the cup plugs..

https://www.forums.ncrs.org/showthread.php?t=71175

[William F.]

Re: heat slot revisited

End of my hot slot saga: The 2 holes in the hot slot must vary among manifolds; a 7/16"plug would fall through my holes and a 1/2'' pipe plug would require too large a tap. I wound up installing short 1/2"fine thread set screws after carefully using a 1/2"-20 fine thread tap, keeping the tap greased and cleaning out chips and putting a small magnet down hole to remove any metal that fell through in spite of grease on tap. So far, so good. I'm back running.Thanks for ideas.

[Al R.]

Re: heat slot revisited

what is the purpose of closing off the holes? I have a 67 300HP and to my knowledge I've never had a problem of any kind. Just wondering? TIA

[Timothy B.]

Re: heat slot revisited

Al,

The holes were installed to prevent icing around the primary throttle blades at warm up on cool humid days. The heat will not hurt anything as long as the gasket/shield arrangement is kept in place.

On the other side of the argument, with todays gasoline, the boiling point is lower so keeping the aluminum carburetor cooler helps to prevent perculation. It really makes a difference if you run the engine and feel the float bowl temperature.

Keep in mind aluminum conducts heat to atmosphere fast and feels hot when touching so you can imagine how it cooks the gasoline in the bowls after shut down on a warm day.. On my 67, I never noticed a difference or had a problem with the holes open but there is a huge difference in the temperature of the float bowls when the engine is running.

[Joe L.]

Re: heat slot revisited

Al,

The holes were installed to prevent icing around the primary throttle blades at warm up on cool humid days. The heat will not hurt anything as long as the gasket/shield arrangement is kept in place.

On the other side of the argument, with todays gasoline, the boiling point is lower so keeping the aluminum carburetor cooler helps to prevent perculation. It really makes a difference if you run the engine and feel the float bowl temperature.

Keep in mind aluminum conducts heat to atmosphere fast and feels hot when touching so you can imagine how it cooks the gasoline in the bowls after shut down on a warm day.. On my 67, I never noticed a difference or had a problem with the holes open but there is a huge difference in the temperature of the float bowls when the engine is running.

Tim------


Even with the heat shield in place, I think that too much heat is transferred to the carburetor with the heat slot active. It may solve the one problem you mentioned, but it creates a whole bunch of other problems. GM abandoned the heat slot after 1969 for ALL applications and never went back to it.

[Timothy B.]

Re: heat slot revisited

I agree with you Joe, and there is a BIG difference in carburetor temperature with the slot blocked.