Inliners International Forums General Discussion Engines 1955 GMC what engine?

My Brother just bought a 1955 GMC 1/2 ton panel truck or it may be called a delivery.
It has the original GMC in it and I want to get him to be a INLINER.
What size would it be? The door tag says 125 HP.
What ever size it is, what parts are interchangeable between the 248-270-302?

The GMC is one of those engines that you don't have to look up serial numbers or cast numbers to ID them, they have the cubic inch stamped right on the distributor pad, 228,248, 256 etc.....

Many things will interchange between all these engines, but the most critical thing is the head swapping. Some of these engines have domed pistons and most every engine cubic inch group had a different combustion chamber size and shape to match its engine. So you just need to be careful if you start trying to do head swaps.

As Scott says the first 3 digits in the number stamped on the flat by the distributer tell the displacement on GMC inlines. If it has the original engine there are four possibilities for 1955. They are 236, 248,270,& 302. '55 was the last year for the 236 and I have never seen one. 302s were only used in big trucks. So you will find it is a 248 most likely but maybe a 270. There are only four GMC heads. One was used only on 302s it has huge ports and a D shaped chamber. One was used on the later 270 and maybe later 248. It has smaller ports and a similar chamber to the 302 head. Next is the 270 "H" head it has the same huge port size of the 302 but has a domed chamber. All other GMCs of this family used a small port head with the domed chamber. The 270 and 302 have a 4" stroke and a taller deck similar to the 292 idea.

While all of this is fun if the door tag is right it is a 270.
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Well , he read me the #'s from the dist pad. It is a 248.

Do we look for a 270-302 or work with the 248?

Work with what you've got, unless you are on an unlimited budget. You can find a 302 at a later date and upgrade.

We did a father / son project when my kid was a teen. I was a little apprensive about putting lots of power in the car so we settled on a 243 ci inline. Now that he's grown up and a (slightly) more matured (40-something) he's getting a 305 ci six that should knock over a second off his quarter mile times.

The 243 gets re-purposed into one of Dad's lame projects.

The only two things wrong with a 248 are #1: There are two bigger engines and #2 No one makes pistons for them. Actually California Bill liked the 248 crank for a street engine because of it's shorter stroke and smaller ports. If the 248 will go out to 3 15/16 like the 270 you'd have about 278 CI. If you are not going to race it it'll be fine. I ran a 248 for a while in my '53.

There is nothing wrong with a 248 if you want a dependable driver. I have been averaging 5000 miles a year in my '54 GMC Suburban.

Heres' some good info. on the GMC inlines.

http://www.oldgmctrucks.com/page0311.html

Patrick's has a great write up in their catalog on GMC pistons, heads, blocks.

Talk to Patrick's. I'm sure he can help you with parts.
And answers to your questions.

Tom,

Is the 248 running good? What are his plans for it - occasional driver or ?

It is just for a occasional driver.

He wants to improve the front suspension. We will do that by either installing a 2004 Crown vic front suspension or the Jag conversion.

It will get a auto with overdrive too. He seems to like the idea of keeping the old GMC 248.

If the 248 is running good and he is not after more go - do a pertronix conversion, late model alternator and call it good.

The alternators that came in the late model cavaliers etc are ~70 amps. and tiny.

+1 on the pertronix, and get a reasonably hot coil to go with and starting, idle and drivability all benefit compared to the stock setup.

The only problems I have ever had with stock ignition on a GMC came from a worn out distributer. A Pertronix kit won't fix that. I have never been left on the side of the road by a points set up but I have by Petronix and Mallory Unilite. It's not like that 248 is going to be turning 8 grand. At this time I am running a Unilite but there is a spare module in the glove box and a complete points distributer and coil under the seat. The benefit of electronics here is maybe a little easier starting and you don't have to maintain the points.

Is it still 6 volt positive ground? I'd change that.