Briggs and Stratton 5S Engine
Article and Pictures by Mark Trotta
With company reports showing over 1.5 million sold, the 5S model may well be Briggs & Stratton's most produced vintage small engine. The single cylinder, 4-cycle motor featured a cast-iron block with aluminum oil sump and cylinder head. Although designed to run on gasoline, they could be converted to run on kerosene.
The name 5S (five-S) is derived from the engine's displacement of five cubic-inches, and the 'S' is for suction-type carburetor. All 5S models have a 2" cylinder bore and 1.5" piston stroke.
The factory output rating is 0.83 to 1.00 horsepower (depending on engine speed). Most examples have a 1/2" output shaft, but they were also made with a 5/8" shaft.
The basic 5S set point is 2800 RPM, rated one horsepower, but will also run well at 3200.
Briggs 5S vs 6S
The 5S motor looks similar a Briggs 6S (6 cubic-inch), but the 5s has a five-fin block with a 1.5" stroke. The 6S has a seven-fin block and a 2" stroke. Both have a 2" bore.
The motor in the picture below has a tensioner pulley mounted to the accessory post. Although most 5S Briggs were rope-start, some may have had lever starters mounted to the post. Sometimes a belt guard was mounted to the post, or nothing at all.
The Vacu-jet carburetor was fitted to several small Briggs models, including the 5s and 6s. There is a slide choke, and it relies on vacuum to pull gas into the engine. A single tube draws fuel up from the gas tank below it.
The flat, kidney-shaped gas tank was fitted to many Briggs small engines, and was seen from the 1940's through the 1980's.
The air cleaner is an oil-bath style, and is secured by a wing nut on a rod which is threaded on top. The bottom of the canister gets filled with oil.
Champion J8 Substitute
Briggs & Stratton recommends the Champion J8 spark plug for many of their old engines, which are now obsolete and expensive when found. The modern replacement is Champion J8C, which looks slightly different, but works the same and takes the same 13/16" wrench size. It is sold as stock # 841.
Briggs 5S Production
The Briggs 5S was produced from 1949 to 1957. Both Sears Roebuck and Montgomery Wards sold Briggs engines during this time, and were likely re-tagged with their own numbers.
The motor in the pictures is an unrestored early example. The low six-digit serial number, according to factory archives, was produced in January of 1950.
Briggs 5S Repair
The good news for small engine restorers is most anything can still be gotten for these engines. The 5S uses the same basic block as the WMB series, so most of the parts are interchangeable. All 2" bore Briggs use the same rings. This include Models W, 5S, 6S, I, and maybe others.
Briggs 5S Afterlife
Like many small gas engines, 5S motors were originally fitted to reel lawn mowers, garden tillers, orchard sprayers, and other outdoor gas equipment. Because of their small size and light weight, many were fitted to bicycles and minibikes back in the day.