This pertains to the GMC. (I believe any head and size) All of the ones I have used have have two 1/2" pipe tap holes back at #6 and one a little farther forward. For many years I put the my temp guage in the rear one and plugged the one farther forward. At El Mirage and more at Bonneville I always showed high cylinder head temp; more than I wanted. About 10 years ago I spotted a stock 302 in a truck and noticed the rear taped hole fed water to the thermostat housing and the one farther forward went to a temp sender. All of a sudden a light went on in my head and I started plumbing all my stock head engines like this. All my "hot" head problems went away.
I am now in the process of assembling a mild street engine for our 40 coupe. I'm going to use this really neat old home made 3-2 log style manifold I purchased in 1974. It was for racing and I'm going to street it with hot water using those same lines. I'm considering welding in a 1/2" tube right thru the plenum chamber to heat up the air. The other way would be to weld the tube/pipe to the bottom of the intake plenum.
The intake was from the 50's and used with Nicson headers of which I have an original pair and planning to use. I plan on using one of the 302 heads I've been hoarding all these years since I normally run the 270H heads when racing. 25 years ago I was able to buy a set of 4-1/8" McGurk pop-up coated pistons still wrapped in 1955 newspaper and it's finally time to put them to good use. Now I just need to figure out a cam that will do good on the street. I'm looking to some of the RV grinds of the 70's and 80's to do the trick.........Good luck on your projects.............JD
