The Stovebolt Site Forums The Shop Area Engine Shop Engine Compression (and ID) Specifications for 261 engines

The topic of 235 vs 261 compression ratios was recently raised in another post.

Here is a compilation of 1954-1962+ 261 and 235 engine compression ratios, and ID information for 261 engines (USA Chevrolet truck 261).

The 1955-2nd series information is incomplete/confusing, because the 235 and 261 engines changed at some time in 1955. The significant change was the location and style of water pump. It is not clear if the head changed when the block changed (there was no need to change the head, but, we can only speculate, unless someone has GM documentation).

Chevrolet Truck Engine Information
please note: the casting codes listed below came from various web sources: they often conflict, and no source indicates that their casting codes come from GM documents.

If you use this information in another post/publication, it would be polite to indicate where you got it (as I indicate at the bottom of this post).

1954/1955st compression: 7.17:1 (7.5:1 in 235)
Head 3703414, 3702436, 3703414
Block 3635499 ???, 3703570, 3836850
1954 Engine Serial Number:
F54Q, T54Q (5000 series)
F54N, T54N (6000 series)
1955st Engine Serial Number:
F55Q, F54Q55 (5000 series)
F55N, F54N55 (6000 series)

1955nd compression: 7.17:1 (7.5:1 in 235)
Head ?? (earlier head ??)
Block ?? (later, 1956-style block ??)
Engine Serial Number:
1955-2nd series: F255N. T255N (6000 series)

1956 compression: 7.8:1 (8.00:1 for 235)
Head 3703570, 3836850
Block 3733950, 3836340, 3837012 (, 3738813 in 1956)
Engine Serial Number:
1956: F56TA, F56TD (P.T.) (6000 series)

1957 compression: 7.8:1 (8.00:1 for 235)
Head 3703570, 3836850
Block 3733950, 3836340, 3837012
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1957-t-ser.pdf

NOTES:
- the 1954-1957 261 might have had a by-pass oil filter system.
- the 1958 and later 261 supposedly came with a factory installed (standard equipment) full-flow oil filtration system (the first GM/Chevrolet/GMC truck full-flow oil-filter system).
- apparently, factory replacement 261 blocks (cast after 1958) came set-up for no oil filter (or, for a by-pass filter), but a plug on the lower driver's side of the block could be "punched-in" for the full-flow filter system.
- a 261 set-up for the full-flow filter had to be run with the filter installed (or with a hose/line between the two filter-line/ports on the block.

1958 compression: 8.00:1 (8.25:1 for 235)
Head 3836340, 3739365 (Jim Carter), 3759365, 3769717
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717
Engine Serial Number: http://chevy.oldcarmanualproject.com/chevyresto/58t074.htm

1959 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925
Engine Serial Number: http://chevy.oldcarmanualproject.com/chevyresto/59tsn03.htm

1960 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1960-t-ser.pdf

1961 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1961-t-ser.pdf

1962 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1962-t-ser-ID/

1963 and later (replacement blocks) compression: probably same as 1962: 8.00:1)
Head probably same as 1962
Block probably same as 1962
Engine Serial Number: any code (or even no code) could/might be used/found here (usually/probably the code from the engine being replaced)

please note: the casting codes listed above came from various web sources: they often conflict, and no source indicates that their casting codes come from GM documents.

If you use this information in another post/publication, it would be polite to indicate where you got it.
Much of this information is posted on a page on my website:
Casting codes/etc: http://www.1954advance-design.com/Stovebolt-engine/casting-numbers.html
Serial numbers/etc: http://www.1954advance-design.com/chev-specs/

I obtained most of the above information from GM documents (Shop Manuals & Specifications Manuals), and from:
- Block codes and series/sequence codes from Beck's Machine & Tool on Inliners International (and from Jim Carter) - some of the information from each source is probably incorrect
- Keith Hardy (The Old Car Manual Project - OLD ONLINE CHEVY MANUALS) kindly hosts GM engine serial number prefixes and suffixes and some GM Specifications Manuals
- Truck sequence/series codes/pre-fixes are nicely presented by John Milliman on The Stovebolt Page