Thanks Chris. That Briggs 60102 was made on 5 June 1975. I think it has a fairly crudely made loose collar on the output shaft, slid up to touch against the step-up in crankshaft diameter that occurs just outside the crankcase oil seal. It should slide off easily when you remove the key. The engine seems to be the same one that was fitted to SB45 mowers, so you have a genuine choice. You might enjoy setting the mower up with each engine in turn, getting them both in tune, and making a comparison. It's nice to have a choice in these things.
