Proper Engine Break-In by S&S


When a discussion about engine life of the Harley engine occurs, the participants, directly or indirectly, are usually talking about how long the pistons survive. At S&S it is no different. Many of the performance kits we offer have engine life ratings based on how long we feel the pistons will last. It is for this reason that "proper engine break-in" is critical and be addressed, because without a good foundation the structure collapses.
The expression "proper engine break-in" is often misunderstood by many riders and enthusiasts. Misunderstanding what "proper engine break-in" means can lead to a variety of mechanical problems, the most common being 'scuffed" or" galled" pistons. While defective parts are sometimes the culprit here, the trouble usually is traced directly to improper break-in. To clarify "proper engine break-in" and minimize damaged pistons, we must first look at a few important related elements. These are: piston fit, engine assembly, ignition timing, carburetion, and engine maintenance.

Piston Fit

Essentially, piston fit is the measurement, or clearance, between the piston skirt and cylinder walls that enclose it. The object for long piston/engine life is to fit the pistons to as tight a running clearance as possible which allows the pistons to function without generating excessive heat.
Basic piston design elements dictate what the running clearance range of a piston will be. Looser than minimum piston fits mean the pistons are free to move around in the cylinder bores more than usual. Tighter fits eliminate these extra movements. Less movement means reduced wear on the skirts and better piston ring life since the rings will have to work less to contain the upper portion of the piston during movement. S&S supplies two fitting ranges for each piston we sell. This is done to accommodate the many riders and riding applications.
Close fit - Fitting pistons to the minimum side of the clearance range requires accurate assembly procedures - careful measuring of the pistons and precision boring and honing of the cylinders for proper fit. Close fit pistons also require a careful break-in. The minimum side of the clearance range is recommended for the patient street rider who puts a lot of miles on his machine and wants the most out of each engine overhaul. Generally speaking, cast piston types offer the rider better longevity although numerous reports of excellent service from customers using S&S forged pistons have been received.
Loose fit - S&S "loose fit" specifications give the rider more break-in leeway as they allow him to run the engine at higher rpms sooner without generating significant amounts of extra heat. Less heat means the pistons will be less likely to "seize" in the cylinder bores with the resultant galling. The sacrifices that he makes are: less total miles because the clearance/wear established over many miles of operation is taken away right from the start, poorer piston ring seal and less overall ring life, and more piston noise because the pistons are free to move around more in the cylinders. Looser piston fits are recommended for the rider who desires a minimum of break-in time such as in racing applications. If a lot of racing is intended, the rule of thumb is used forged pistons because they are stronger.
As a last word, try to fit the pistons more towards the tight side rather than the loose side.

Engine Assembly

While lubrication is important for every motorcycle engine, newly-built motors usually require an extra supply of oil to reduce the friction and heat that occurs during break-in. This is because the "new" surfaces are actually rough. Eventually, after break-in, these surfaces wear smooth, which in turn reduces the amount of friction that causes excessive heat buildup inside a new engine. To insure that the pistons get proper lubrication during the break-in process, the builder must put a crosshatch pattern of fin