Replacing Connecting Rods or Pistons

What is Engine Balancing? explains that certain engine parts are organized in matched sets, by weight, before they are installed in an engine being built at the Lycoming factory. Occasionally, it may be necessary to replace one of these parts during the operational life of the engine. When this happens, mechanics in the field should know that Lycoming parts which are matched by weight when an engine is built should only be replaced with parts which fall within design specifications for service (replacement) parts.

Starting with connecting rods, the system of marking specifies that each part will be marked with one of these letters: A, B, S, D or E. Connecting rods with the same identifying letter are installed when the engine is new; if only one or two rods are to be replaced in the field, they must be those marked with the letter S. The S indicates a “service part” which will always have a median weight. Use of a connecting rod stamped with an S will ensure that the weight match between rods is always within the tolerance allowed by the Lycoming specification for that particular part number.

If a piston is to be replaced, it is strongly recommended that the power plant mechanic refer to Lycoming Service Instruction No. 1526. This service instruction shows that certain piston part numbers are designed to fall within a specified standard weight range. Any genuine Lycoming piston with that part number will be a satisfactory replacement, although the weights of each piston should be compared before installation.

Other pistons are marked as A or B weights and are used in matched sets when an engine is built. When these pistons must be replaced as a partial set, “service part” pistons are provided by Lycoming. These will be marked with AS or BS and are restricted to a very narrow weight range that falls between the A and B weights. Use of these pistons as replacements will ensure that the entire set, both old and new, fall within overall weight limit tolerances set by Lycoming.

When pistons are installed in an engine, another consideration for standardization of weight is the piston pin plug. Lycoming Service Instruction No. 1267 describes three piston pin plugs currently in use. Although these three are interchangeable, they must be used in complete sets of the same part number because each part number has a different weight.

Connecting rods and pistons are one more example of the need for appropriate reference materials when working on Lycoming engines. Overhaul Manuals, Parts Catalogs or service instructions are the source for those details we cannot retain in memory.