The OS 61SF is an older version of the OS 61FX. The last of the 61SFs' ended about 1997. The price for one then was about $135.00. They were not cheap, but they were better than anything currently on todays market in the same class. (in my opinion)

While you are breaking in your Tower 75, keep in mind that it is a ringed engine, not an ABC engine. Do not get it hot!! You heat up an ABC engine for break in, and just run lots of fuel in a ringed engine at a rich and cool setting to do the same. It will not be completely broken in until it has at least 3/4 of a gallon of fuel through it.

In any case, don't get me wrong. Both the Tower and the MDS are good engines fro their intended purpose. Sport flying.

Silversurfer