The Stovebolt Site Forums The Shop Area Engine Shop 302 coolant bypass tube

Hi Guys -

I am reworking a military 302 to go into my '54 GMC 2-1/2 ton dump flatbed. I actually bought two old 302s in various states of teardown and borrowed parts to make a whole engine. One of them had a coolant bypass tube going from the thermostat housing back to the rear portion of the head near the water temp sender, the other did not. My guess is that this is designed to prevent a big temperature gradient down the head and more evenly cool the engine to prevent warping and the like. The existing tube is crimped and will need to be replaced - I take it it's pretty important? (The tube is labeled 'AZ' in the exploded diagram that is first to appear in this site:

http://www.alfaheaven.com/MilitarySection/M135Mech.html)

I have a military 270 that has that tube. Not sure what it's for.

I'd suggest going to http://www.oldgmctrucks.com/ , sign up for the forums and post the question there.

Follow Cletis's suggestion and inquire at the GMC site. However, I have seen lots of civilian 302's and have never seen the tube and civilian engines probably worked harder than the military ones did.

Thanks, guys -

Here's the link from oldgmctrucks.com:
http://oldgmctrucks.infopop.cc/eve/forums/a/tpc/f/633004236/m/8311043482?r=2011065482#2011065482

This is going to be a working truck, hauling a lot of "stuff" for our horse farm, so I think I'll install it if it aids in more efficiently cooling the head.

Update: I decided to upgrade the carburetor on this engine and installed Fenton cast iron headers and a Clifford intake manifold with a Holley 390cfm 4 barrel. The manifold is heated with engine coolant, which I routed with hoses through and through the manifold on the path taken by the bypass tube described above. This is not as efficient at heating the carb as the stock paired manifolds or an electric heat plate, and the truck is a little cold-blooded, but once it warms up, it delivers pretty muscular torque. The rig is a '54 GMC 2-ton and will haul a load of gravel no sweat. Passes everything but a gas station . . .

It's been too long since I sold my M-135 in a fit of responsibility to remember accurately but I'm pretty much certain it had the tube still in place - so I can't tell you if it's important to keep it or not.
The 216 engines used in Canadian military trucks WW2 had a "steam relief tube" that ran from the rear of the head to the top tank of the radiator that was intended to ensure that steam pockets wouldn't develop at the rear of the head when operating on down slopes. Most of these steam relief tubes have been removed or blocked off post-war, either because they got in the mechanic's way or when the radiator was swapped for one that lacked the fitting for the tube to keep the truck running. Removal doesn't seem to have caused any problems that I've seen. (From my perspective today, that's a pain because they also often tossed the fitting that went into the head and also held the temerature sending bulb so it's now awkward to put the tube back in place for restoration.)
I suspect the tube on the 302 was intended to serve the same purpose as that on the 216.