The Stovebolt Site Forums The Shop Area Engine Shop Military 302 Crankshaft vs. Civillian 270 GMC Crankshaft

I am converting a military 302 to civilian use. For the time being I lost my build notes and I can't remember if it is required to change out the stock military crankshaft and replace it with a civilian 270 or 302 crankshaft.

If I recall, the only advantage to this switch is primarily a weight savings; the civilian 270 crank being lighter.

The 302 military crank and bearings I have are in great shape and wondering if I need to switch this out at all - to save some money.

I'm assuming that the military crankshaft is the same length as a civilian 270 crank? and there should not be a fit issue with the crank pulley/balancer and the radiator?


Can someone refresh my memory about these details?

thanks. Tom

If the 302 crank is good, use it. The 5 to 10 lbs. difference isn't a big deal. The biggest problem may be the longer length for extra pulleys that big trucks used, depending on what you are installing it in. My 302 crank is 2" longer than the 270.

Joe

Thanks, Joe. I agree if it works I will use it - the fit into a smaller engine bay may be the problem. Whether it goes in a 51 GMC or, a 41 GMC I believe the fit would be the same.

And it may be impossible to have the length of the crank (at the pulley end) cut, drilled and tapped.

I never heard of anyone doing that...



Tom

Just posting this note for someone converting same motor. I had a good talk with Patrick Dykes of Patrick's Antique Truck Parts etc. He said the crank length isn't an issue in an Advance Design GMC. It is 1 1\2 inches longer. He did say to switch out to a 3\8" narrow belt harmonic balancer from a 235 and replace the pulley on the GMC water pump with the same.

Reason being that some GMC harmonic balancers have a hard rubber sleeve that tends to wear out over time and could cause problems down the road. (Didn't know that).

Tom

A GMC 302 crank is a 302 crank regardless if it came out of a military or civilian. I have some of both. The military vibration damper used a 7/8 pulley which means you need a 7/8 water pump pulley and generator pulley and they are difficult to change because the clearance between the fan and the radiator. GMC used a 3/8 belt in their pickups from 54 on up and they fit on a military crank. The 1 1/2 inch difference is the difference between Chev and GMC. GMC 228, 236, 248, 270 and 302 are all the identical length and these engines will bolt into a GMC from 1939 through 1959 up through 450 series.

I have known Patrick for over 30 years and I still have a NOS 302 that I bought from him at a time when he needed money. He has always had the idea that GMC vibration dampers slip. After they get to be 60 to 70 years old, they probably do, but they can be rebuilt to last another 50 years.

Where the idea came from that a 270 crank weighs 5 to 10 lbs less than a 302 crank is beyond me. If you look in a GMC master parts book, a 2404929 crank fits both 270 and 302. The military block is different in that it doesn't have a hole for a fuel pump, but the lobe is on the crank. It is possible to cut a hole in the block for the fuel pump. Also there is a locator block on the military block that was unique to the transmission setup used in the military trucks. The block must be ground off to mount a civilian starter.

I think the drilling and tapping myth got started with the older 228, 236, 248 engines that had a press on vibration damper. They also have a 4 bolt flange for the flywheel. Later 248, 270 and 302 have a 6 bolt flywheel flange. It is a simple matter to drill and cut threads in the old cranks to accommodate a bolt for safety.