Hello SYTerra, Joe C and Mod CJ,
Firstly, have to support what Joe has suggested about not requiring a 5 HP unit on a SB45. It's just total overkill, bit like putting a 350 Chev engine into a Mini Cooper. (not that it would fit anyway)
Just to confirm with Jack that all the kirby units that were fitted to SB45's only had 5/8 PTO shafts. It wasn't until Briggs came on line across the whole range that the 3/4 shaft did appear from time to time. This was due to special pricing offers put to SB by B&S when having larger quantities of engines on hand that they offered at a special price to clear them out, thus SB had to come up with a clutch body that had the slightly larger hole. Apart from that there was no technical improvement with a 3/4 over a 5/8 unit, just necessity for fitment that's all.
Now the other issue that is very important is that most of the later engines will no fit directly onto a series 1 machine that utilises the solid deck as the handle bar was a straight up design that will clash with the engine opposed to the later Briggs fitted units that had the larger tank on the 3HP units and required the kinked back handle bar to be used on the 17 inch models. I think you can get away with it on the 20 inch series 1 model as the engine is mounted more inboard and thus it doesn't interfere with the handle bar.
Briggs do supply a special adapter plate for use with the series 1 machines but they are rather hard to come by and only the very knowledgeable mower shops know about these plates. I will photograph a pair that I have on hand at the moment so that you can all see what I mean. I'm just doing a job for a friend that requires his Kirby that's thrown it's conrod to be replaced and his son works for Briggs here in Australia, thus he sourced all the bits and pieces to do the job properly.
Cheers,
BB
