I have a 1951 Chevrolet 3600 that at some point in the past had its engine replaced. Based on inspection and my research I am fairly certain I have a 261. I would like to verify that and also know the year of the motor, but am confused by some of the casting/serial numbers. Below are the numbers I have on the engine block and cylinder head: Engine Block Castings: 3733950 (behind fuel pump) CON4 G 29 0 (behind starter) Engine Serial Number: S0901 (rear of dipstick) Cylinder Head Castings: 3836850 CON12 G 21 0 The combination of casting numbers (3733950 & 383650) indicates a 261 from 1954 - 1957. The problem I have is that the block codes for both the engine block (G 29 0) and cylinder head (G 21 0) would both indicate a 1960 date from my understanding.... Also, I do not have "captains bars" cast into my block, but do have a cast circle directly above the block code. I have read a few places that the early 261's ('54 - '55) may not have the captains bars. These years would match up with the casting code date ranges, but not the block codes..... Any help/insight would be greatly appreciated!!
Might be a factory replacement engine? But posting a clear picture of the stamping on the flat machined pad at the base of the distributor, will be a big help in figuring it out. The block and head casting numbers show up in my old Chevy parts book as later 50s 261 parts
yeah, I could tell by your low post count That is a slightly weird looking stamp...normally there's an engine plant code (usually V, F, or T) followed by the month and day numbers, then the engine suffix. No suffix on this one. My slightly educated guess it that it's a replacement engine, possibly a short block and separate head, from 1960, to replace the engine in an older truck. Does it have the big single hole for the water pump, or two smaller holes?
I will have to take a look at the water pump, guess I am not sure.. Pulled the water pump, block has 2 smaller holes.
Interesting, it's not the same as the later 261 that has the single large hole, and fits the later trucks.
A 261 should have Captains bars cast in the block above the starter. S0901 looks more like a rebuilder number stamped in the serial number than an actual Chevy 261 serial number. This shows what most Chevy six cylinder engine serial numbers from 1941 to 57 could look like. https://chevy.oldcarmanualproject.com/models/engine.htm You can go year by year through the pfds for Chevys here and find the info to decode an engine serial number. https://www.gm.com/heritage/archive/vehicle-information-kits This is straight from the GM or Chevy horses mouth real deal info an not what someone thinks it is. https://www.gm.com/heritage/archive/vehicle-information-kits If you have seen me post info on Chevy six cylinder serial numbers this is where I got it most of the .time.
The number looks too much like a Chevy stamp, to be a rebuilders number. I also found a reference to the S prefix, it's Saginaw Service. Which to me, explains why there's no suffix, if it was just a fitted block, not a complete engine, and the date doesn't match when that casting was normally produced. Strange engine...as long as it works, I wouldn't worry too much about not being able to solve the origin mystery.
A bit of 261 Id info here https://oldchevytrucks.com/blog/2010/02/the-forgotten-261-6-cylinder-engine/
Does it have an oil filter attached? Some, not all ,261s had full flow filters and those can be id'ed by having oil holes that accept a 1/2"FPT fitting in the main oil gallery in two locations on the left side midway or slight more to the rear. If it has those it is a 261. Not having them isn't a positive means of excluding it as a 261, but having them is a sure thing as the 235 never has them. I think all trucks for 58up had that feature.
The truck has a byp*** filter on it but it’s not hooked up. Fittings are 1/4”, front one is plugged, back one has what I ***ume is the oil pressure gauge hooked to it.
Are you near the Canadian border? The Canadian Pontiacs used 261's and they have minor differences to the US versions.
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