261 Truck Engine
 

Looking to get a bit more bang out of your CMP?
261 truck engine spotted on Kijiji. The casting numbers verify that this is a truck block and not the more common Pontiac car engine.
These are getting very rare to find and are physically identical to the old 216.
Kijiji # 1331564370

Chris,
I'm curious which numbers are the confirmation that this is a truck engine. I checked today on a Pontiac 261 I bought a couple of years ago and the cylinder block, cylinder head and intake manifold numbers in the ad all match my engine. Why do I believe my engine is a Pontiac engine? The seller said so, he had removed it from a car that he still had to put in a V-8 and he had before and after photos of the car.
I don't see what I think of as one of the prime identifying points of the truck engine on the advertised engine - the large diameter oil filter lines. The automatic choke also seems more car than truck in that era.
I agree that the 261 is an easy (and noticeably more powerful) swap in for a 216 but "physically identical" is a bit of a stretch. The water pump is in the block instead of in front of the block and head so an adapter plate is needed to relocate the 261 fan to be centered on the radiator similar to the 216. The carburetor is different and the tinwork for the pushrods and valve cover requires some camouflage. The 848 head is also differently shaped so the CMP oil filter mount doesn't quite fit, even with the special CMP head bolts.

Grant. From the research I have looked at on US websites, the GM casting number 3769717 indicates a 1958-62 261cid truck engine.
The heads on the Kijiji one are 3836848 are 235cid heads, a common swap in the day, especially for hotrod guys looking to increase compression ratio.
While not a total swap, yes some creativity would be required; short water pump and adapter to relocate it. A few other bits.
As far as the choke, not unusual for the original Rochester B to be swapped out over the years. Matter of fact, many 261s actually have 235 carbs and most guys don't even know the difference or notice it at all.
For the difference in HP unless you are a total purist, a 261 is an ideal swap into a CMP or any other underpowered GM of the era.
The 261 was also well known in the hot rodding circles of the era.
Grant, the Canadian Pontiac 261 came stock with '858 heads. I can see how it may be deceiving. Your block is a '9717 casting?
This surely needs some more investigation as your points do bring up a couple points

I suspect that the same block might have been used for both car and truck versions depending on where holes were drilled to permit or block access to oil galleries etc.

In the US, people tend to think of the 261 as a truck engine while the same trucks in Canada often got the 235. The 261 in the Pontiac seems to have been used rather than a V8 in the low end of the spectrum in models that might have gotten the V8 in the US.

So Grant, externally what is the best way to decipher the mystery?
To surmise, the Chevrolet truck 261 used a 9717 block with '50 heads although the 858 head was a common swap.
The Canadian Pontiac also used the truck 9717 block with 858 as standard equipment.
Is there a definitive Pontiac identifier on a true Pontiac engine?
I understand the internal differences, eg solid vs hydraulic lifters and aluminum timing gear vs fiber.
I talked with the seller and he has no idea of the history on this engine.

I suggest that the most obvious external difference is the size of the oil lines to the filter. I'm not 100% sure the Pontiac had a filter as standard equipment but any filter supplied for it used small diameter lines (similar in size to those used on the 216) and was of bypass type. Truck engines seem to have had full-flow filters using much larger lines (about 1/2" ID). See the attached image taken from https://www.stovebolt.com/techtips/e...al-excerpt.pdf
Engines intended to have bypass filters as either standard or optional (often dealer installed) equipment can be safely run with the filter system removed and the block ports plugged. DO NOT TRY TO DO THIS ON A FULL-FLOW SYSTEM (the large line truck system). On full flow systems, if you want to run without a filter, a line is required from outlet to inlet on the block to prevent oil starvation. On the other hand, bypass systems require a small, restrictive orifice to prevent excess flow through the filter.
The Pontiac engine I've been describing had rubber cushioned mounts on each side of the block, about half way along the block (not at front corners) and I believe the truck engines used a front mount plate very similar (maybe identical) to CMPs although the plate has been found on car 235/261 engines either with or without the hold-down bolt holes pierced.

My C15A has a 261 (I believe from a Pontiac) that has been disguised as a 216. Not original but nice to drive. No oil filter system installed (but the block is set up for a bypass system using 1/8" pipe thread fittings) and it hasn't seized so far.