coverThe Forgotten 261 Six Cylinder carter logo

Between 1954-62, Chevrolet produced their famous full oil pressure 235 cubic inch six cylinder in trucks and it soon proved to be one of the greats among engines. However, at the same time a lesser known "big brother" to this base engine was being used. This was the quality built 261 cubic inch six cylinder! This 261 was available only in 2 ton (6000 or C60 series) trucks and school buses. During it's early years (1954-57) it was an extra cost option above the standard 235 six cylinder. 

In 1958-62 (the 261 now had a full flow remote oil filter) it became standard in the 2 ton chassis up to 19,000 pounds gross weight. Above that Chevrolet substituted a V-8.

This larger six was not offered in US cars, however there was an exception in Canadian built full size Pontiacs. Their base engine, also produced only in Canada, was the 261 not the V-8 as in the US. This provided basic power, great dependability, and better gas mileage.

Basically, this larger engine was a 235 with the same crankshaft but GM engineers made various modifications to give it extra strength and horsepower. It's standard bore diameter increased from 3-9/16 inches to 3-3/4 inches. The connecting rods were heavier and attached to increased diameter piston wrist pins.

Its higher lift cam shaft, for better breathing, was shared only with the early 235 six cylinder Corvette. A modified larger Rochester carburetor was also a 261 only feature. Unfortunately most of these larger sixes have long since had their original Rochesters replaced with 235's and therefore do not perform to their full potential.

In pure big truck form the 261 has a larger thermostat housing holding a double acting thermostat. This is designed to circulate water through the block and head before the thermostat opens to allow hot water into the radiator. Thus, no internal steam hot spots during warm ups, especially in winter. This is particularly important with very cold coolant. Vital engine spots can become very hot before the total coolant becomes hot enough to open a normal thermostat on the front of the block.

 

6 Cylinder Engines
Jobmaster
Thriftmaster
Displacement
261 Cu. In.
235.5 Cu. In.
Bore
3 3/4 "
3 9/16"
Stroke
3 15/16"
3 15/16"
Firing Order
1-5-3-6-2-4
1-5-3-6-2-4
Compression Ratio
7.8 to 1
8 to 1
Horsepower
33.7 (AMA) 148 (Rated)
30.4 (AMA) 140 (Rated)
No. of Main Bearings
4
4
Wrist Pin Diameter
.927 inches
.875 inches
Rod Shaft Thickness
Front to Back
.595 inches
.595 inches
Rod Shaft Thickness
Side to Side
.975 inches
.760 inches
Crankshaft Journal
Diameter
2.435
2.435
Engine color in trucks
Green - some later yellow
Gray

The block and head surface have three pair of matching small "steam holes" [water/coolant passages] that allow any steam hot pockets to vent away from the open water cooled areas between the cylinders that are not solid metal. Of course, this means the 261 must have its own specialized head gasket.

"Steam Holes" [TCL: Ugh - call them water/coolant passages]

After four years into production, the major quality feature was added to the 261 engine. For the first time a Chevrolet [General Motors] inline six cylinder came standard with a full flow oil filter system. This improvement, used only with the later [1958-1962 USA truck] 261, forced oil through a remote filter cartridge before it reached the engine. It was not like the optional by-pass oil filter system as found on 216 and 235 Chevrolet sixes. This extra helped insure longer life to this larger six cylinder that was often subjected to heavy commercial use.

A full flow oil system has been a characteristic of almost all automotive engines for over 40 years but it was just beginning in the mid 1950's. With the 261, the disposable filter is remote and not built in as with later engines. It still resulted in a major design improvement.

As with the 235 light truck engine, the 261 came standard with solid valve lifters and an aluminum camshaft timing gear. The passenger car's 235 was equipped with hydraulic valve lifters and a fiber timing gear for quieter operation.

During the 1955-62 Canadian Pontiac application, the lifters were the hydraulic type, the cam gear was fiber not aluminum, and it did not have the full flow oil filter. These Canadian made 261's did not add the full flow filter in 1958 as in the U.S.

Visually the 261 looks almost identical to the 235. It perfectly replaces the smaller engine and in stock condition increases horsepower from 140 to 148.

Those planning on a major rebuild or adding performance options to their Chevrolet inline six should seriously consider locating a 261. Often there is no extra cost in purchasing a re-buildable unit, and the results will be rewarding. If you plan on adding additional carburetion, a higher lift cam, or just want additional performance and more lower end strength in your daily driver, the 261 is for you!

Locating and Identifying a 261

Though last placed in larger Chevrolet trucks almost 40 years ago, this now scarce engine can still be located and often at a price no higher than for the smaller 235. Many still remain in the original Chevrolet trucks and are now setting in salvage yards or behind farm buildings. In Canada, the big Pontiac cars are sometimes in the back rows of more isolated older wrecking yards.

Don't overlook the wrecked and badly rusted Chevrolet cars of the 1940's and 1950's, particularly those showing signs of some past exterior customizing changes. The Chevrolet enthusiasts of that era knew about the 261 and its potential for added performance. Some of these will already have had extras added such as a higher lift cam shaft, extra carburetion, or dual exhausts.

When you have found what you suspect might be a 261, check a few specifies to verify you have the real thing and not the visual almost identical 235. Casting numbers, not stamped numbers, on the 261 head are very visible beside the rocker arm cover. A different set of numbers relate to the 261 block. These seven digits are located on the right side between the fuel pump and starter except for 1954 where it is located forward of the fuel pump. See chart below.

YEAR
ENG. SIZE
BLOCK NO.
HEAD NO.
54-55
261
3703414
3733950
3703570
3836850
55-57
261
3733340
3837012

3703570
3836850

58-62
261
3739365
3769717
3769925
3836850

Watch for the "Captain's Bars!"

The 261 has two pairs of parallel raised 3/4 inch long bars cast in the block. This is not seen on a 235 except 1954 [TCL: I think JC is incorrect: tI think the 1953 pasenger car Powerglide-equipped 235 seems to be the only 235 with the bars]. One pair is above the starter and the second pair is at the top middle of the left side of the block very close to the head. See photos below. The one exception is the early 261 produced in 1954 to mid 1955. It has only one "Captain Bar" above the starter but keeps the pair on the left side.


Most used 261 blocks are rebuildable, however often their cylinder heads will have a few very small cracks in the combustion chamber. This is typical due to occasional abuse of over heating in past years. If you choose not to add to your expense by having the cracks repaired, an alternative exists. The more common 235 head is the same except for the three pair of internal steam holes [TCL: Ugh, water/coolant passages is a better description]. These can be manually drilled to make the water flow just like in the 261! Sorry, but some 235 heads can be cracked even more than the 261 because they lack heat releasing steam holes.

"Warning" When Installing a 261!

The [USA Chevrolet truck, see below] stock remote filter system has two very visible 3/4" lines threaded into the block. One is from the pump to the filter and the other from the filter back to the block. Oil must leave and return to the engine by these lines (even if the filter is eliminated) or the engine will fail from lack of lubricant. Many 261 engines have been quickly seized after persons plugged the two oil line holes. Those people had many years experience on Chevrolet engines without the full flow oil system. Some thought it was an easy fix to just remove the 3/4" lines if one was leaking and cap the holes. This procedure was acceptable on the older 216 and 235 but never on the 1958-62 [USA Chevrolet truck] 261 truck engine. 

[Canadian 261 engines were available in Pontiac cars (1954-63??) and in 1954 and later Chevrolet (and maybe some GMC) trucks.  The Pontiac 261 had hydraulic lifters (and maybe a fiber camshaft gear).  The truck 261 engine, like the USA 261 engine, had mechanical (solid) lifters and a metal camshaft gear.  The 1958 and later Canadian truck 261 still used a by-pass oil filter (supposely), not the full-flow oil filter used in USA truck 261 engines.]

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 The notes/corrections in the above document in italics and withing brackets "[notes]" are by TCL, Tim Lederman.