The Stovebolt Site Forums The Shop Area General Truck Talk Maybe a mystery

I haven't been on for a while, since my truck has been away getting body work and painting done. It's finished now and is beautiful, I'll post pictures later. What has me wondering is, a buddy stopped by yesterday to check out my truck and he drove up with a 55 2nd series GMC. The truck had recently been rescued from a family farm yard and he's been getting it running. Now for the mystery. It appears to be completely original, with an automatic transmission and a 270 engine. In the work I've done on my truck, I thought the 270 didn't come out until 56. It also has a 6 volt positive ground electrical system, which I think was from 55 2nd year trucks. The questions are, was 270 available in the 55 2nd series? How rare is the auto trans with the 270? I've seen auto trans with the Pontiac 8s,but not 6s.Thanks.

The 270 was used in larger trucks since WW2, but 55.2 was the first year it was used in smaller trucks.[ <-- This is a mistake on my part, corrected a few posts down. ] 1952 was the first year for the Hydra-Matic. 6 cylinder 55.2 GMCs stayed with the earlier 6 volt positive ground electrics while V8s went to 12 volt negative ground. All were 12 volt in 56.

I have a 1955.2 Series GMC Shop Manual (X-5503) dated October 1955 that lists the driveline options for all models 100-500. According to the listings, Models 100 thru 350 list the 248's as what looks to be the standard engine with a 288 V-8 option. Various model Hydra-Matic transmissions (models H-M177C, H-M210U and H-M220Y) where offered in this model range.

The 270 6 cylinder and 288 V-8 are listed for the Model range 370 thru 410, all with either Hydra-Matic Model H-M220Y or standard 4 speeds for transmissions, with an optional 5 speed manual (Model 205V).

Just a guess on my part, but I think the 270 in his truck is likely a transplant from a bigger truck, but still maybe a 1955 original model year.

Here's some added info from another 1955 GMC Sales resource pamphlet that lists the engines and models info (scroll down to page 15 of the pamphlet for engine info and page 17 for electrical info):

GMC Trucks New Models and Features for 1955

My mistake about the 270 engine in 55.2 small trucks. It was a '56 thing.

Attached is the '55.2 sales brochure for 1/2 ton.

RBS56: I assume your friend thinks this is a 270 because the engine serial number stamped on the machined flat spot just to the rear of where the distributor mounts starts with "270". If it does, check the other side of the engine block for a 7 digit casting number and a 3 or 4 character casting date. The date will be in the form O M D Y O or O M DD Y O where M is month (A = January, B = February, etc), D or DD is day of the month and Y is the last digit of the year. The O actually looks like the head of a slotted screw.

Some GMC heads had similar casting numbers and dates on the flat top surface between the carb and the valve cover. All this info is useful in determining the birthdate of the engine.