Flexible Hones Used in Preparation of 4 Cycle Kart Engines Ensure Proper Seal of Piston Rings

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The Brush Research Manufacturing Flex-Hone® Tool is an integral part of the proper preparation of a Briggs racing engine

With the newer Raptor III piston and ring sets used in the Briggs L head engines raced today, the proper finishing of the all aluminum cylinder or cast iron sleeve – depending on the version of engine – with a flexible honing tool is critical for creating the best ring seal possible. Without the proper finishing, a poor seal can lead to wasted HP and quite possibly a lost Kart race.

For this task, the Flex-Hone® Tool from Brush Research Manufacturing (Los Angeles, CA) is the recommended honing tool for final finishing of Briggs racing engine cylinders by Jimmy Glenn of Glenn’s Engines. Glenn is a championship engine builder, author of “Building a Race Ready 5hr Briggs,” and technical inspector at national karting events.

According to Glenn, the primary 4-cycle Kart racing engine used today is the Briggs and Stratton “L” or flat head 5hp horizontal shaft engine. While these engines have been replaced by the Briggs Intek OHV engine for commercial applications, the Motorsports division of Briggs continues to supply complete L head engines (Raptor series) designed for racing.

Ultimately the OHV engine will replace the L head. Briggs Motorsports Division has produced its first racing version of the Intek and labeled it the Animal. It is a cast iron sleeve engine but its ring set is a bit different from the older L head design.

On all Briggs L head engines raced today the newer Raptor III piston and ring set is being used for either cylinder type.

This piston was designed to eliminate a problem with the older style pistons having wrist pin boss failures at higher RPM levels. Today it’s not uncommon to see an L head turned in the mid 6000 range and the older piston (designed for commercial 3600 RPM operation) was failing. These failures are all but eliminated by the new piston.

Along with the new piston came an entirely new ring set for the Raptor III piston. This ring set was much smaller, softer and had very low wall tension when compared to the older style piston/rings. With the softer rings, cylinder finish has changed from years past.

The ring set on the OHV animal is a bit different. It’s a much harder material than the Flat head rings set so a more coarse grit flexible hone should be used for final finish.

Proper cylinder preparation begins with a straight cylinder perpendicular to the crankshaft. Align boring the cylinder to 90 degrees referenced off the crankshaft is essential to the best possible performance. Once this operation is complete, rigid honing should be used to bring the cylinder to its proper size and clearance.

Final finish for either cylinder type is done with the Flex-Hones of the proper material. 320 grit Aluminum Oxide on aluminum bores and 600 Silicone Carbide for the cast iron sleeves are recommended based on experience with the resulting ring seal.

The resulting finish is referred to as a plateau finish where the cylinder hone removes the small peaks of metal left by the honing stones leaving a smooth, flat wall (plateau).

For the OHV Animal we suggest a final bore finish of 320 grit due to the rings set used today. Simply follow the procedure outlined for the aluminum bore flat head.

The Flex-Hone cylinder hone can also be used for carburetor bores, final fit of valve guides, for clearance and finish of connecting rods for the wrist pin and crankshaft areas, the sleeve bearing on the Flywheel side, and for intake and exhaust ports.

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Heather Jones
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