Lauson Model Identification
Lauson engines were built in various models through the production period. The model
designations Lauson chose to identify their engines consists of a “major” model designation,
followed by a “sub-model” designation. These model designations were stamped or printed on the engine tag.
The format of the model identification is a major model designation, comprised of one, two , or three letters; followed by a series of numbers (usually three) to provide the sub-model designation. Optional characters can follow the sub-model designation to further identify the engine type.
Where there is a rule, there appears to always be an exception. Lauson produced a series of engines with major model designations of 55A, 55S, and 55AB. These are the only major model designations, of which I am aware, that do not follow the above description.
For example: for an engine with a tag identification such as “RSC591”, the “RSC” provides the major model designation, and the “591” provides the sub-model designation. In another example: “SLV23-W-T” the SLV provides the major model designation, and the “23” provides the sub-model designation. The “W” and “T” provide further information; in this case the “W” indicates a “Wico” magneto and the “T” indicates a Tillotson carburetor. Please note that the sub-model designations, as well as any additional characters are not standardized, and the numbers and following letters will indicate different things for each different major model designation. (in other words, for a RSC300 and a RSH300) the 300 may have different meanings.
Engines sharing the same major model designation will all belong to the same family of engines and will have the same bore, stroke, and physical characteristics. The sub-model will indicate “features” unique to that particular engine; such as rope, petal, crank, lever, or recoil start; base type; suction/float carburetor; gas tank location; etc.
It is common to refer to engines belonging to a major model group with their major model designation only. Therefore on this site, and in the nomenclature, you may hear the entire major model group of RSC type engines referred to as the “RSC” engines, the major model group of RSH type engines referred to as the “RSH” engines, etc.
There appears to be no rhyme or reason on how Lauson chose the characters to represent their major model designations and sub-model designations, though some patterns can be recognized. For example: (The RSH replaced the RSC, the TLH replaced the TLC, the PAH replaced the PAC, and the LMH replaced the LMC. Also, with the vertical shaft Lauson engines: the TLV is the vertical shaft version of the TLH, the RSV is the vertical shaft version of the RSH, the LMV is the vertical shaft version of the LMH. We can also see a pattern in the air cooled marine engines where the RLM is the marine verison of the RLC, the RSM is the marine version of the RSC, the TLM is the marine version of the TLC, and the PAM is the marine version of the PAC. I’ve also seen references to the letters of particular major model designations being an abbreviation for something, such as “SLV” means “Super Light Vertical”. All in all though, I’ve never come across an explanation regarding a general scheme for a Lauson naming convention.
If we dig a little deeper into Lauson engine model designations we’ll notice that Lauson seems to have identified a “base” model for each “major” model designation (such as the RSC591 being the base model for the RSC). Numerous Lauson specification sheets indicate that specific engine models were built as variations to that base model type, as configuration variations requested by specific engine customers.
We can assume that as customers approached Lauson with a specific set of requirements, Lauson and that customer agreed upon a specific engine configuration, signed a contract, and at that point Lauson assigned an engine model designation for that specific engine configuration (such as the RSH899: specifically built for the Roto Hoe corporation for use on their garden tiller). The configuration change from the base model requested by a customer might have been as simple as requesting a different throttle linkage, or it might have involved more complex engineering changes, such as the Jari engines with their different engine base and carburetor mount.
We can assume that once the customer and Lauson signed the contract, Lauson would continue building that engine for the customer to the specification on the contract (that specific engine model) until the customer no longer ordered the engine, the contract expired, or Lauson and the customer altered the contract by mutual consent (in which case a different engine model would be established).
A common pitfall we may become trapped in, is dating a specific engine by the “features” found on that engine. For example: If a RSC has a Zenith type carburetor we may be tempted to date it as an “early” RSC (because RSC engines with Zenith carburetors of the type found on RSC engines were first produced earlier than RSC engines with ML type Tillotson carburetors). It’s easy (and tempting) to confuse design dates with build dates. A Lauson model (such as the RSC541) with a zenith carburetor (Lauson part number 22267) was produced as long as the customer placed orders for it and their contract with Lauson was valid. Other RSC type engines (such as the RSC676, with a Tillotson ML1B carburetor) might have been built before, during or after the production runs of the RSC541. It all depends upon when Lauson customers placed (probably re-occurring) orders for particular engine models.