There are multiple casting numbers on the 261 head. The one on the passenger side by the side cover usually has 848 with 850 below it. The one you actually want to read is on the driver's side between the head bolts.

However, even if it is an 848 head it is common to transplant them onto 261's because they offer slightly higher compression and everything else bolts up. If the engine ever needed a valve job or something the head could have gotten swapped.

You can look at the oil lines on a '58-'62 block to tell. The USA 261 of that time period has 1/2" NPT pipe threads and 3/8" or bigger ID for the oil lines and a big flow-through filter with a spring bypass valve in the body (extra "port" on the housing plugged by a big bolt). The filter takes the same element as the early V8 cannister filter- it's about 8 inches tall. The 235 has small lines. Earlier 261's and Canadian 261's of all years share the same style lines as the 235.

There are a few 235's with captian's bars, too. It's hard to tell.

Big truck 261's also usually have a big bypass thermostat housing, but not all of them do. I don't know if any 235's came with that option.

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