Jeremy, I think you need Deejay's involvement to do the set-up and adjustment of your clutch. He posted a thread on it recently though, in the Scott Bonnar reel mowers forum, and that will set you on the right general track:
https://www.outdoorking-forum.com.au/forum/u...p;Words=clutch&Search=true#Post39243The SB45s are pretty old now, and we are seeing a rash of clutch halves that use the captive cotter design, coming very slightly loose then rattling on the crankshaft enough to wear out and become very loose. Fortunately the clutch halves now available do not use captive cotters - they use two grub screws at 90 degrees to each other, one of them directly above the key. That is a traditional British design that works very well. Can't say I have much respect for the captive cotter system. Hence if you replace the engine-side clutch half with a new one you should have a permanent solution.
Clutch halves are available with both 3/4" and 5/8" bores, to suit the two available crankshaft diameters. However as far as I am aware, just about all modern horizontal-shaft engines in the 2.5-4 hp range usually have 5/8" shafts, though other sizes may be available as factory options. I suggest you should look at various engines, choose one, check the standard shaft diameter it comes with, then order your clutch half. There are standard layouts for mounting bolts and output shaft positions for those engines and most common types will conform to them, so the engines will be a straight bolt-on fit to your SB45, but you need to check anyway. We had a recent thread where someone bought a nice but unfortunately non-standard small Japanese engine, and had to mount it on an adaptor plate. Nearly all the engines fitted to SB45s as replacements are small Briggs or Honda models, which fit straight on - but you have to watch the shaft length and diameter, because they have optional shafts and you might happen to choose an engine that has one.
The simple summary of new engine choices I would offer is that you can buy a classic side valve Briggs (probably of 8 cubic inches displacement), or a modern pushrod OHV Honda (probably a GX120), and either can work very well. The Honda is more elegant and pleasant but costs substantially more. I personally would avoid any Honda model that is GC rather than GX: these are OHC engines rather than pushrod types and are marketed as "residential" where the GX types are "commercial". In other words, there is a large difference in how many hours' trouble-free work they can be expected to do and how practical it would then be to overhaul them.
