Briggs 3.5 HP Flathead Engine
Article by Mark Trotta
Produced from the 1960s to the 1990s, the Briggs & Stratton 3.5 HP flathead engine has powered lawn mowers, garden tillers, pressure washers, and many other types of power equipment. They were, and still are, durable little motors. Many of them are still around today.
Briggs 3.5 Specs
Displacement comes from a 2.56" bore and 1.75" stroke, making 148cc, or 9.0 cubic inches. The horizontal crankshaft rides on plain main bearings. Output shaft diameter is 5/8", and the shaft end is usually tapped for an 1/4"-28 bolt.
Early 3.5 Briggs motors were equipped with points and condenser, which are easy to replace and provide good service. Somewhere in the 1980s, all Briggs & Stratton motors were converted to electronic ignition. Early engines can be retro-fitted if desired.
There are three sets of identification numbers grouped together. Early models have them stamped into the blower housing. Later models have them stamped into the heat-shield.
Briggs categorizes 3.5 hp flathead motors as 90000 series. The first set of numbers will be either 91000, 92000, 93000, 94000, or 95000.
All 90000 Briggs models have rewind start and feature "Easy-Spin" compression release for easier starting.
The second group of numbers is the "type" number. This tells you the paint code, decals, and governor speed.
Depending upon the engine's intended use, the governor speed setting would be different on a lawn mower than a tiller, or industrial pump, etc.
The last group of numbers indicate production date and place. First two digits are year of manufacture, followed by the month and day. The last one or two digits will specify the manufacturing plant.
Mini Bike/Bicycle Motor
Most people who install an engine on a bicycle use a two-stroke because of it's lighter weight. An alternate would be a small, light four-stroke motor. The major advantage of the four-stroke is they are much quieter than two-stroke engines.
The Briggs 3.5 is a thin motor, just 9" from side to side (about 12" including the output shaft). That makes it a good choice for a motorized bicycle or small minibike.
When I bought this vintage minibike several years ago, it had a Briggs 3.5 on it. At first, I thought it was just a lesser-powered Briggs 5hp motor, but soon found out that there are more differences than similarities between the two.
3.5HP vs 5HP Briggs Flatheads
Aside from the lower displacement and output, the 3.5 is smaller and lighter than a 5HP Briggs. This is significant when looking for a motor to power a minibike or motorized bicycle.
A Briggs 3.5-hp motor weighs approximately 25 pounds.
A Briggs 5-hp motor weighs approximately 35 pounds.
Briggs Parts Swapping
Comparing the two engines side-by-side, there are not a lot of parts shared between the two. The carb and gas tank unit seem to be the only pieces that interchange.
Output Shaft - 5/8" diameter on 3.5-hp vs 3/4" diameter on 5-hp.
Exhaust Port - 1.5" ID on 3.5-hp vs 1.75" ID on 5-hp.
(You could interchange mufflers and pipes but you would need an adapter)
There are three different blower housing covers for the 90000 series Briggs. They can be swapped out for each other, but do not interchange with 5 hp motors.
From the factory, all 3.5 motors were fitted with a tank-mounted diaphragm carb. These carbs can be tricky to get working right. I have found that it's worth the effort to remove the carb/tank assembly from the motor when attempting to repair or rebuild.
Read: Briggs Tank-Mounted Carb Repair
These rubber grommet(s) are a common source of vacuum leaks. Replace them if they're old.
Shop: Briggs & Stratton Breather Tube Grommet Kit
For most applications, all that's needed are four 5/16" bolts with washers and lock nuts.
Caution: Most Briggs flatheads have an aluminum base, and can easily crack if over-tightened.
Spark Plug And Oil
Oil capacity for the little Briggs is 20 fluid ounces, or 1.25 pints. Recommended viscosity (depending on season) is either SAE30, 5W-30, or 10W-30. The best thing you can do to get maximum life from your engine is to change the oil at least every year.
Use Proper Oil
Small engines require oil specifically designed for air-cooled equipment. Most automotive oils manufactured after 2009 do not meet these specifications. This results in higher operating temperatures and reduced engine life.
Briggs recommends a Champion J19LM spark plug, but any name-brand equivalent will work. Spark plug gap is .030".
Old Briggs flathead motors are tried and true, simple and cheap. And because so many of these engines were produced, parts support won't dry up anytime soon.