OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar 45 twin rail clutch

Hi,
I’m new to the forums.

Just a quick background before I get to my question.

I’ve inherited a Scott Bonnar 45 and a Scott Bonnar Diplomat. Both weren’t running and need work. A week ago I knew nothing about these mowers but have been on a steep learning curve.
After reading the forums for a while it looks like the 45 is the best one to try and get into usable condition.

I thought I was up for a new engine as the old ones oil looked like a chocolate thickshake and it wouldn’t start. After repairing the pull starter clutch, de-carboning the head and valves, upgrading points to electronic ignition coil and rebuilding the carb and changing the oil twice I’m pleased and a bit surprised that the motor now starts easily and runs really well.

My current issue is with the clutch. After starting and running the engine the clutch was disengaged but but was randomly engaging and making a screeching noise before the cotter pin flew across the yard(luckily missing me).
The cotter pin had scored the output shaft and obviously wasn’t locking the onto the clutch half.
I have slid the clutch half down to a fresh piece of metal on the output shaft, tightened the cotter pin, adjusted the clutch and done a test mow.
The mower works which is great but the clutch doesn’t look right as there is a bit of a wobble to it.

I’m wondering if I have fitted the clutch incorrectly or if it’s worn beyond repair?
I’ve attached a photo of the clutch half.

Thanks.
Brent.

Hi PTT,


It's had it's day and shouldn't be used any further. Clutch bodies need to be around 3 mm away from the engine block and not positioned any further away. Any clutch that has an inherited wobble is considered cactus and will only inflict stress fractures into the chassis. This is the biggest enemy to the model 45.

Cheers,
BB.

Thanks for the reply BB,

I have an old 3hp Briggs and Stratton motor I can use as a replacement. I guess the motor and clutch both need to be replaced. Is it normal to just replace the output shaft half of the clutch or do both halves normally wear out? The cork clutch pad seems ok still.

Thanks,
Brent.

It's just the engine side that has the issues, but the original ones are currently not available and you can only purchase a turned up unit from a couple of outlets, namely Bonmods which is George here in Adelaide and Trident's in WA.

I've documented on this site about what's gone wrong with the original type that's brought into this country via GA Spares and at this stage there seems to be no solution to that issue as it again involves our wonderful trading partners in China..

Cheers,
BB.

Don't discount the Diplomat.
They are a great mower, nice and simple just let down by a dodgy drive gear! But they're available for about $50 now. The drive belt system is great as is the rear mounted catcher. So much better than a 45 on a tight lawn or a lawn with obstacles

Thanks guys,

I’ll order a new clutch and maybe start looking at the diplomat while I wait for the 45’s clutch.

I can see what started out as a bit of curiosity to see how those old mowers worked before selling them slowing turning into a restoration hobby.
It should be cheaper than restoring a car from the 70’s and takes up less room in the garage too.

100% they've replaced my vintage motorcycle restorations for a spell as they are simple and cheap but just as rewarding.

Welcome to the forum PTT.

You're in good hands with Bonnar_Bloke giving another of his "no BS" replies. I've learnt 99% of what I know from him and I don't believe there's a thing he doesn't know.

Add a few pics when you've fixed your new obsession!

Cheers,
TC.

Just a follow up post to say thanks for the advice and let you know how it went.

I was able to get the mower running properly and usable. An engine swap and clutch half did the job.
The Scott Bonnar 45 actually cuts quite well.

I plan on using it as is for a year, maybe two then fitting a more modern Briggs and Stratton engine and possibly rebuilding it at that time.

Thanks,
Brent.

Hi PTT,

If I may ask, from whom did you source the clutch half from ?

Also it seems you have no stress fractures showing in the rear rail as yet ? if not I would change to the later more current Briggs engine asap as the currently fitted engine won't be doing the rails any favours and may end up costing you more money and grief than a new engine would.

BTW. Not sure how many of these new Briggs are left but I'm well informed that they are very short to nil supply left here in Australia due to COVID and the Briggs ownership / Position issue is also now a major issue.

Cheers,
BB.

Awww shucks, you're making me blush TC

Hi BB,

I got the clutch through SB fabrications. It was almost too good to use on the mower.

I wanted to use a new Briggs 3.5hp motor but wasn’t able to find any stock so this old 3hp replacement engine will have to do for the time being.

I can see what I think may be a small hairline crack starting to develop on the rear rail. I might put a modern Briggs engine on it when they are back in stock. If I can’t get one with a 5/8 output shaft I’ll most likely get a chonda and upgrade the clutch again to 3/4 inch.

Hi PTT,

Yes pretty much how I read it.

Don't expect any new ones any time soon, if anything I'd keep an eye out for a lightly used one as that rear rail is only going to get worse from that old unbalanced Briggs and Rattler. Honestly I've given up using them for the obvious reasons.

Sadly all the twin railers are at that point in their lives where unbalanced thumping force is critically bad for them.

Cheers,
BB.

They definitely are a tough but rough old motor. I bench tested this Briggs before fitting it and it was like a jumping bean. In comparison I’ve got an old Honda gx160 that is nice and smooth when I bench tested it. Just a shame it has a splined output shaft otherwise it would have been perfect for this mower.

I’ll keep my eyes open for replacement engines.