261 Truck Engine

[Grant Bowker]

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.