I have a 1941 GMC Truck which I do not have a proper engine in. I know it is suppose to have a straight 6 cylinder engine, but I do not know what size it needs. The model number is AC504 and serial number is 4006 on the truck. If anyone could help me it would be appreciated. Thank You.

Jim, While waiting for a response you might try this: Click on Links on the sidebar on the left of page; Click Automotive at the top; scroll down to Rob English Old GMC Trucks Page. Good Luck and Welcome.

Hi Joe . . .

It's going to take some research to figure out what kind of engine was originally in your truck. The model number is a good start.

From what I've read GMC got into light trucks starting in '36. These vehicles were powered by an Oldsmobile 213 flathead. In '37 they again used Olds, the 229. In '38 (for one year only) they used the Pontiac flathead (a 223 I think). And then went back to Olds on the pickup trucks.

I think that it was in '39 that the true GMC overhead valve design made the scene. But I think they continued to use Olds design flatheads into the early 40's on pickups. By '41 the GMC 270 was in the steet but I think it was only in BIG trucks.

My 1949 Chilton's Flat Rate manual offers only the following:
1939-41 Models AC100, AC200, AC250, AC300, AC350
An I6 with a 3 9/16" bore and 3 13/16" stroke (which I think is a 228)
1939-41 Models AC400 and AC450
An I6 with a 3 23/32" bore and 3 13/16" stroke (which is I think is a 236)
1939-42 Model AC850
An I6 with a 4 3/8" bore and 5" stroke (which sounds like a big Group II engine)

The only entry close to the model number you've given is for
1939-40 Models AC500, AC550
An I6 with a 3 5/8" bore and 4 1/2" stroke (the stroke is too long for 256 or 270 - dunno could be a misprint)

Happy hunting.

regards,
stock49

Thanks for your replies, the truck that I have is accually a Fire Truck on a GMC chasis if this makes a difference in the research process.

Are talking about the one on Ridge Road in North Royalton?

This is an original truck for the department that I am on. Some people several years ago took the original engine out of it and junked it because it had a cracked block. They then put a different engine in it that was not made for the truck. These people are no longer around and there is no one who knows what size engine is suppose to be in this truck. We want to restore this truck to be as close to the way it originally was.

Given the GVW rating of a fire truck it may well have came with what was called a Group II engine. Much heavier block then what was put in the pickups (and transplanted into hot rods Good luck with the restore. It should be great cruiser for parades!

You might want try one of the enthusiasts sites like: Ted\'s Engine House