There's a tech article somewhere . . .
Edit: There are several ways to identify 261's, but the most obvious difference is a set of parallel bars cast into the block near the starter, and a similar set of bars on the passenger's side near the oil pan rail about halfway back on the driver's side. They're often called "captain's bars" because they resemble the military rank markings. 261's were used in heavy trucks, school buses, and some "Pontiac" passenger cars in Canada in the late 50's and early 60's. Their major claim to fame is a 3 3/4" bore size, with the same stroke as a 235 engine. "There's no substitute for cubic inches"!
Jerry
As Pre '68 Dave has recently posted, the Captain bars are also found on some 1953 235 blocks
Claims to fame regarding the 261 depend on your perspective: they used the same modified lift cam as was used in 1954 and later Corvette 235 engines. And, from 1958-1963 the USA Chevrolet truck 261 had a full-flow oil filtration system.
All truck 261s had mechanical lifters and the Canadian Pontiac 261 had hydraulic lifters.
