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I have purchased a 1941 GMC CC250 1.5 ton flatbed. How can I determine what engine is in it? I believe it is the original engine. Gray I6 with GMC script on the valve cover.

Is there a list of casting numbers available?

Welcome to the 'bolt, Martin.
This may help, once you find the casting number. I "accidentally" found mine while cleaning the throttle linkage with degreaser; it's on the driver's side just above the oil pan.

You might need a good wire brush and some brake cleaner (cold engine) to reveal the casting #'s.
This is an incomplete list, but maybe your number is in here?

2135412 GMC 46-54 200, 270

2193980 GMC 52-54 302

2324002 GMC 55-62 248, 302

2324003 GMC 55-63 270

2324004 GMC 55-62 302

2404929 GMC 55-63 270

6107326 GMC 39-54 228

6107328 GMC 39-55 248

I thought all GMC 's had the engine size as a prefix on the engine numbers stamped right next to the distributer hold down on the small machined pad?

Tony, there's no such number on my '54 248; that may be true for other GMC blocks.

My 39 GMC 228 has the engine size as the prefix to the engine numbers stamped next to the distributor hold down as Tony mentioned. In addition there is a plate riveted to the block on the drivers side just above the oil pan and to the rear of the block casting numbers with the same 228 prefix and the engine numbers. Thanks for the GMC casting numbers Hedgehog.

Well, Martin did ask for casting #'s.......
The displacement is stamped on the machined boss to the left of the distributor and will start like "A248******" In this case the displacement is 248.
Also, on the door post should be the model # and serial #. Something like "354-24.....". The "24" would be to designate the truck came (in this example) with a "248". Martin, yours is probably a 228, but casting numbers will tell all.
This is how my '51 was, I'm not sure about earlier models. Check OldCarKook\'s site for more info.
Martin, Welcome to the 'Bolt!

Welcome to stovebolt land Martin.

Martin: Check this link on ID Plate locations on my website and then report back on the engine serial number.
GMC engine numbers will often, but not always, begin with a letter which denotes the bore and crank journals other than "standard" and then three numbers for the engine size, which in your case would normally be a GMC 228.

Sooo....

If no code letter then the engine bore and crank is standard. IF it has a letter, the following apply:
LETTER..........Cylinder Bore Crankshaft Journals
...............................Main.......Rod
A .............+0.005".....-0.010"....-0.010"
B .............+0.005".....Standard...Standard
C .............+0.005".....-0.010".....Standard
D..............+0.005"......Standard...-0.010"
E............Standard......-0.010".....Standard
F............Standard......-0.010".....-0.010"
G............Standard......Standard...-0.010"

Hedge: I have never seen a GMC motor that didn't have a number on it when it left the factory. Every GMC engine made carried a serial number and the 248s were stamped between the flywheel timing opening and the distributor mount on a flat spot facing up on the block. Its possible that your's was ground off and I am curious to know if your flat spot shows any sign of being ground down. Is this spot level and flat or is it by chance pitched away from the motor on a slight angle?

I have to beleive that your number was ground off by the chop shop that stripped stolen truck and then sold you that motor cheap!

Well, here's what I know for certain:

The casting number on the driver's side of the block tells me it's a '54 248, just like the long block I found in the back of the panel oozing oil out of a crack in the block. On closer inspection, the flat spot is canted about 20 degrees off horizontal, so it could easily be a chop shop special. I suspect the boat anchor in the back was the original engine, but the one in it runs strong so I'm not going to be too picky about it's origin.

There is an exception to this ID scheme. Service replacement engines did not carry serial numbers I have seen 3 NOS 302 short blocks, none had the serial numbers. 2 of the 3 had "302" punched on the distributor pad - the third had nothing at all.

Thanks for all the help. I found the pad next to the distributor and found B228 stamped there. Then below and somewhat to the rear is stamped 270.

I interpret the B228 as being a 228 cubic inch displacement and .005 off standard.

I am assuming this is a low oil pressure engine, but have not confirmed that yet.

Thnks,

Martin

I don't think any 228 or higher GMC engine would be a low pressure design. Chevy kept using those antiquated Babbit bearings forever, but GMCs were real trucks!

GMC sixes were all high pressure oilers, starting in '39 all the way to '62. This goes for the 228-302 series. I don't know about the larger sixes in the big trucks.

If your 228 is only building 30 psi of oil pressure, then either its a Chevy engine (go ahead Chevy guys...make my day) or it's not working properly.

GMC engines were 60# of oil pressure as stated above.

Yes, a "B 228" would have the cylinder bore punched out 0.005" over but the rod and crank journals would be standard as indicated in the shop or maintenance manual.

I agree with OCK, I always had 60 psi+ when cold and no less than 35-40 psi hot and idling at 450 rpm. This was with a '51 248.
Jimmy motors are tough, believe me.

Hi guys, I also have a 41 GMC one ton,havent looked at casting numbers yet but on flat area behind the dist. I have these numbers 579494colo what could that mean ? Also the valve cover doesnt have the GMC on top. Thanks Gary

BRAND NEW TO THIS SITE. JUST FOUND A 1956 GMC GVW=21,000#. I BELIEVE ITS I6 GAS ENGINE IS THE 302 C.I. OF WHICH SEVERAL SPOKE. ALSO HAS 5-SPEED (5=DIRECT). CANNOT FIND TIMING MARKS, FIRING ORDER. PLEASE HELP! NOT RUNNING, BUT TRIES TO FIRE. 12-VOLT. FLATBED. APPEARS TO BE EX-MILITARY. HAS GRILLE OF EXPANDED METAL FRAMED BY FLATBAR. WOULD LOVE ALL ADVICE! THANK YOU.

Maybe the Pooh Bahs should set up a new forum for engine IDs only !

GMC L6s are pretty easy to ID (At least the 228 through 302). All bets are off on the big 308 - 503 low RPM monsters.

1) Count the freeze plugs on the driver side of the block. GMC's have 5; Chevrolet will have 2 or 3.

2) Record the engine ID from the aforementioned block surface just behind the distributor. First 3 digits are the displacement.

3) Print the GMC numbers list from the Inliners site. Get under the truck, start scraping off the crud and somewhere you will find one of the numbers on the list. Keep looking, it's there somewhere. Guaranteed.

Is everyone ready for more confusion? OK, I have 2 trucks, a 1949 GMC 3/4 Ton and a 1960 Chev 1/2 ton long box Custom (currently going for parts). Both are Canadian built. I am trying to confirm the sizes of both engines, but am having difficulty. The 1949 appears to be a 216 (full height side cover). Casting #3836327, dist. stamp C359288. The 1960 is either a 230 or 250 as the casting # is 3850817, but the stamp is 12562453 and I can't find what that means. Any ideas?