I was given an SB45 flat bed 17" mower with a Kirby engine years ago. It has been derelict but under cover for some 10 years. Lots of surface rust but everything seems to twist and turn as it should.
I plan to scrap my 35 year old and very tired Morrison 600 and move its 4 year old B&S engine to the SB. The B&S is a 3/4" shaft and the SB is 5/8" shaft with a cast iron engine clutch.
I have searched for a cast iron 3/4" clutch outer without success so am left with two options. 1 - fit a new aluminium cast clutch engine side matched to the cast iron clutch body, or 2 - a new 3/4" clutch assembly.
I am asking for advice on the mixing of the two metals. I would prefer to retain a full cast iron clutch for the rotational mass but I suspect my preference will not count.
If any members have knowledge or experience of this change I will appreciate an answer.
Yes, to summarize: you need a 3/4" bore clutch body (the fixed outer housing with cork lining) to mount on your Briggs engine crankshaft, and an alloy version is the practical choice since cast iron is scarce. Your existing iron inner cone on the( PTO shaft ,12 in image) pairs perfectly with it.
A 3/4" alloy clutch body (lots of online stores sell them) fits directly on the crank with matching keyway and grub screws, engaging reliably with the iron cone via cork friction—no metal contact issues arise.
The alloy body is lighter than cast iron, but the difference is minor for SB45 mowing: a lot of the inertia comes from the iron cone, reel, chains, and gearbox. Users report no noticeable effect on cut quality, drive smoothness, or engagement once adjusted—many Briggs conversions run this setup for years.
I think I saw a a 3/4" bore clutch body for $110 new.
