OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers SB45 engine upgrade

G'day all,

Long time member, first time poster.

We have/had a Diplomat 590. The old B&S was getting hard to start and down on its luck, so I re-powered it with a Subaru EX17 about 6 years ago.

This involved turning up a new PTO shaft, coupler and pulley (mainly because I broke the original pulley doing something dumb and couldn't find a replacement). This worked well for years until I had the blades sharpened and the "engineer" said the bedknife needed replacing. He replaced it, but it turns out he "made a new one" out of mild steel, which duly bent. Time passed, stuff happened and I was tossing up having a new bedknife made by a local guy.

Before I got around to that I inherited my grandfathers SB45.

A close family friend owned a mower repair business for 40 years and he did a lot of 45's. He did this one up for Grandad 20 years ago and it had probably only been used a dozen times since then.

I got it home and got it going. Bit of a clean and some new fuel and it started right up. I had planned to leave it as-is, but that old B&S needed some love. The muffler was rusted out, the carb needed more work and to be frank I had a practically new EX17 sitting on another mower that is smooth, incredibly quiet and idles like a clock.

So I ordered a new 3/4" clutch half and swapped it in. The EX17 is quite a bit heavier than the old B&S. I couldn't find specs on the B&S, so for future reference I weighed it at 10.95KG wet. The EX17 is 15KG.

As our friend did what he did, and it was for my grandfather the motor may not be the original. It is stamped 80102 0233-0L 7407180. So it's only a couple of months younger than me.

Had the blade and bedknife sharpened up and it cuts a treat. The only thing left to do is fit a throttle return spring. The Diplomat had the solid steel cable, so it didn't need one when I fitted it on that.

It's a shame. In addition to the 3/4" engine mod, I also recently replaced all the bearings on the Diplomat, rebuilt the drive clutch and it was running like a dream. Unfortunately another family member picked up a stick while mowing and that bent the bottom blade again, at which point I gave up. I'll just put it on Gumtree sans engine and see if I get any bites.

As for the 45, while I had it in bits I took a good look at the rails and they are in great condition. I have no intention of restoring it, blinging it up or otherwise doing any more than keeping it well maintained and cutting. I will say reel maintenance and setting is an order of magnitude easier on the 45, so even if the Diplomat didn't need work I'd probably still get rid of it.

A few years on. A year or so ago Mr 10 year old put it away for winter, leaving it with a full tank of fuel and the fuel tap on. By Summer the tank was empty and the carb was full of gum. Several go-arounds led to :
- A new carb
- Home made gaskets (quality gasket paper)
- A new clutch half (the old one had no set screws left and was flogged out)
- A new clutch thrust bearing (the old one was grabbing even after a flush and re-grease)

So we're back on the grass. My idea of "raked" and his were different, so after what looks like a couple of stones and at least one nail the blades "need some work" and one of the cylinder bearings is screaming like a banshee. Those bearings were replaced last sharpen. This time I'll replace them with units from my local bearing house rather than whatever the local grinder uses.

The Diplomat is still sitting out the back with a full set of brand new bearings, but I've offered it to a guy for parts. Not that many of them around with functioning drive gears these days and it's just taking up space.