Briggs 5-HP Engine Build
Article by Mark Trotta
Produced for over 40 years, 5-horsepower Briggs & Stratton flatheads are so popular that there are racing classes based on this engine. Although 25+ years have passed since production ended, these little motors are still in demand.
All year 5hp Briggs are similar and many parts interchange. With the exception of engine tin (gas tank, air filter, blower housing) there is little difference between a 1960's model and a 1990's model.
5HP Briggs Specs
The bore size, or the inside diameter of the cylinder, is 2-9/16 inches. The stroke, or the up-and-down distance the piston travels, is 2-7/16 inches. This makes a total engine displacement of 12.48 cubic inches, or 206cc.
5HP Briggs FunPower
The five-horse FunPower motor was offered for many years and found on dozens of recreational vehicles. They are a common sight on old go-karts. This particular motor had been sitting for quite a few years, and needed some work to get back to running condition.
Before I checked for spark or compression, I removed the head to get a general condition of the engine. The piston and bore looked good, as did the cylinder walls. There were no scratches, gauges or wear ridge, but there was a small amount of carbon build up.
I carefully cleaned the top of the piston while it was on the compression stroke (furthest up). Then I rotated the crank and cleaned each valve when in the closed position.
After seeing that the motor was in good shape internally, I went ahead and ordered an engine gasket set. Before re-installing the head, I cleaned the head bolts and replaced the head gasket.
Check For Spark
There's several ways to test for spark on a small engine. I find this Briggs & Stratton spark tester easy to use. Clip one end of the spark tester to the spark plug wire and the other clips onto a ground (usually cylinder head).
With points ignition, it takes very little RPM (about 350) to produce spark. This is easily done by pulling the rope start. As the flywheel turns, watch for spark in the clear circle on the tester tool.
If you see a spark, your ignition is working properly.
Before removal, I took several pictures of the carb linkage, because it's pretty hard to remember where it all goes back. The carburetor was disassembled, soaked in parts cleaner, then cleaned and air-dried. For reassembly, a new carb rebuild kit was purchased, which basically consists of a diaphragm and a several gaskets.
Read: Briggs Tank-Mounted Carb Cleaning And Repair
Before the carb was reinstalled, the inside of the gas tank was flushed with kerosene several times. The tank was then repainted black.
These rubber grommet(s) are a common source of vacuum leaks. Replace them if they're old.
Shop: Briggs Breather Tube Grommet Kit
Briggs Engine Identification
On the blower housing you should find a tag or decal. Later models have numbers stamped into them. This will give you the model, type, and code numbers.
Blower Housing Interchange
Since blower housing covers are easily swapped from engine to engine, you don't really know what you have until you take the cover off and look. You also don't know if the engine has had any modifications, so opening it up helps solve any mysteries.
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.
For most applications, all that's needed to mount an old Briggs flathead is four 5/16" bolts with washers and lock nuts.
Caution: Most 5hp Briggs flatheads have an aluminum base. If you over-tighten the mounting nuts and bolts you can crack the ears off.
As a precaution, I flushed the engine with thin motor oil, then added fresh 10/30 oil and a new spark plug. After 10 to 15 pulls, it started up.
Getting More Power Out Of A Briggs 5HP Motor
Although newer OHV engines outperform the Briggs flathead, there's still plenty of performance parts available for these classic small engines!
Read: Small Engine Performance