The Stovebolt Site Forums The Shop Area Engine Shop 261 question

Hey all, I'm a newbie on this site, And picked up a 1942 1.5 dump. It is powered by the 261 six. I see on other sites that it is a great engine, but what make the 261 so special?, Is it a bored out 235? It runs great,and is staying in the truck. I ran the #'s and shows it's a 58-61.

261 was very similar to the 235 but increased bore and i think had some parts beefed up??? i'm new to this, so anyone feel free to correct me..

261 article on 1954advance-design.com

The main difference is bore.
235 3 9/16"
261 3 3/4"
Same stroke on both.

Hey alltruck, I have the same 261 as you, from a 58-60 Canadian Pontiac. I am very pleased with it, have been running it about a year now. Excellent torque, just a bit more power than the 235. I think it's rated at 150 hp.

Almost the same 261s. I think the Canadian Pontiac 261s had hydraulic lifters - the USA truck 261s had mechanical lifters.

Very cool and informative site you created there Tim. Was looking at your 261 #s and the head #s are the same as mine. Block casting is 3769925.

Adding to Tim's posted page: the head is unique although a 235 will bolt onto block, it is internally different. The later 261 has larger cam journals. The water pump is special. You can tell a 261 block from outside. The pan rail is larger/stronger. Inside the webe are thicker. Most laters have oil filter, full flow outlets or bosses,too. Rockerarms are different. Can't think of other changes right now. David

David,

The rocker arms and valve springs were beefier in earlier 261s, compared to same year 235s. In later 261 years (56-63), were the rocker-arm assemblies different from 235s?

The rockers will interchange with 235's. The only difference I can see is the location of the oil vent hole.
Chev moved the vent hole around on the 235's through the years also.
Although 235's changed cam journal size mid 54 the parts book does not show this for 261.

235 heads will bolt up to the 261 blocks,BUT,the the 261's have a few more steam ports. If you take a 235 head as a replacement on a 261,the extra steam holes NEED to be drilled to match the 261 block. easy way: order a 261 head gasket,(a MUST),lay it on your "new" head,then scribe and drill your steam holes. There is some discrepancy,on the "Big bolt" forums, about whether or not these holes need to be drilled at a precise angle,or not. One more note of interest for all of us that run solid lifters: we NEED to use zinc additive in our oil, since the new formulated oils eliminated most of that additive,thanks to the E.P.A. I'm NOT selling anything here,just following advice given by Patrick's antique cars and trucks. You may call them at:(520)836-1117, for more info. Just trying to keep our engines in good running order. THANKS!!