OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Model 45 - Clutch Adjustment

Hello,new to this site. You guys are a wealth of knowledge.I have a B&S 3.5 on a Scott Bonnar 17inch. I recently pulled it apart, replaced a few low $ value parts, cleaned it up & put it back together. Runs well except for the fact the clutch slips to the engaged position at times without touching the clutch engage lever. Also when the clutch is not engaged the shaft still spins rotating the reel.Believe the cork is still ok (have around 3-4 mm left). At present the clutch appears to be secured hard up against the engine side. Is this correct & should there be a few mm gap possible? Is there any other adjustment necessary? Any help is appreciated. Cheers- Ross.

Hi rossf, and a big warm welcome to the forum. It's nice to welcome another Scott Bonnar owner. We are a friendly bunch and we all love to learn new things and tinker here.

Now to your problem. You have correctly identified your main cause. The usual adjustment is by the screw and lock-nut underneath the cutter engage handle, but if the whole clutch assembly is hard up against the engine, you will run out of adjustment.
To understand how it all works, you really should download a illustrated parts manual from the Parts and Manuals area on the main forum. Instructions for accessing the manuals area is contained in the very first post in any topic on the main forum page.It will also come in handy if you need to buy spare parts.

To fix it, undo the nut on the top of the captive cotter-pin, (the nut and bolt you can see on the clutch housing, closest to the engine) until the nut is just flush with the top of the pin itself, tap the nut and pin until it releases (ie. it moves down and in doing so releases the clutch assy. from the engine drive shaft.

You will need to move the clutch assy to the right (looking from the front of the machine) it will be hard to do, as you will be pushing against the clutch spring and move it to approx 1 cm or 3/8" from the engine. Re-tighten the captive cotter pin nut which will re-secure the clutch housing to the shaft. Now engage the clutch lever to see that it is pushing the clutch cone in to enable the outer housing to move freely and no movement of the cutter shaft.

Any fine adjustment can now be achieved by the use of the screw and lock-nut, located underneath the clutch handle. you will find that the clutch cone itself, does not have to move very far to dis-engage the cutter clutch.

Rossf, once again and I hope this has been of help.

I'll explain the same issue in different words - Deejay, please put me right if necessary. The half of the clutch that is attached to the crankshaft is only retained by friction, using a captive cotter pin. Sometimes this slips a bit, resulting in the clutch-half moving up against the engine. The clutch then lacks clamping force even if adjusted with the adjusting nut. To fix this, undo the adjusting nut as far as possible. This releases the clutch spring. Then loosen the captive cotter pin, and lever the crankshaft-attached half of the clutch along the crankshaft until it is 1 cm from the side of the engine. Tighten the captive cotter pin, then use the clutch adjusting nut to properly adjust the clutch so it works correctly.

Yes,Grumpy, you've got the gist of it; the cutter clutch lever works the actuating arm that bears on the thrust pad that then moves the cone inward, disengaging the clutch...the adjusting screw comes into contact with that actuating arm. In other words, once the "clamping force" is lost, the actuating arm has no purchase, and the clutch cannot be disengaged....

thanks gents, will give it a go & let yuo know how i get on.

Cheers- Ross

No probs, Ross, best of luck with it and please post a pic of your machine, as all members love to see 'em.

a couple of pics of my machine. Paid only $100.00 for it from a fellow who brought a Rover 45. I replaced a few parts- reel bearings, diaphragm, throttle cable, a couple of gaskets, replaced spark plug, changed engine oil & had the reel set by local mower shop. Probably spent around $120.00 on it. So for around $220.00, money well spent I believe. What do you think? Except for a couple of scratches the catcher is near new!. Had to weld the frame where the engine sits as it was cracked. Assume this is/ was a common problem with these mowers? I may just have to replace the left hand side of the clutch assembly as the lock nuts onto the engine shaft are starting to spin. Also need to fiddle with the governor spring set up as I cant seem to get it right- cheers Ross.

thats not a bad deal considering its a later model engine.
personally i have never heard of them cracking because they are allot sturdier than anything else out there. did it crack on the rail or on a weld? considering they were made during the time or arc welders its possible it had a bit of slag or wasn't properly penetrated.
regards jay

Hi to rossf and mowernut, This problem of the rails cracking on the later model Scotties is quite common. Apparently it is caused by engine vibration and usually is found radiating out from one or more of the engine mounting holes....Rover 45's have the same prob. The early model SB 45's had a solid deck and the reason for the change to 2 rails was probably to save money...My 45 however, does not have the prob as the Honda is a smoother running engine and less vibration.

rossf, as to the clutch prob, I am having trouble understanding what you mean, as on the left hand side (looking from the front) has only the nut that tightens the captive cotter-pin that locks the clutch on to the engine shaft? Can you please describe the prob in detail, so as we may be able to help you

Deejay, spot on with the cracks radiating from the engine mounting holes. As we suspect, likely due to the engine vibration / bouncing around etc (they were there before I bought it) - I need to tune the ingine properly I guess. So I decided to weld the cracks and weld a plate to the underside for added strength. Although I am no welder I think it will hold- fingers crossed.
Mowernut if you have a look at either of the pics in my last post you can actually notice the blackening of the paint under the front of the engine. This is where I welded from underneath after removing the engine. The 4 x holes are where I originally riveted the strengthening plate underneath.
Deejay to my clutch issue, I dont have a captive cotter pin, I have 2 x "square headed" nuts that screw through the L/H side of the clutch housing and one tightens against the key on the engine shaft, and the other, directly opposite, tightens against the engine shaft itself.I think I have tightened both too much & both I feel are beginning to (slightly) spin. Maybe this type of set-up is incorrect here?? (may need to buy a newie) I can take a pic & post it when I get home from work tonight. I do have an old B&S/ 14 inch SB (not working) with the captive cotter pin but it suits a totally different size (smaller) engine shaft- therefore I cant use it.

Deejay, you can actually see the square headed nuts in the pics if you zoom in. Also there is a flat area on the left side of the clutch housing where a captive cotter pin would go & I thought about drilling a hole to suit the pin. Do you suggest I give it a go?? Maybe my only option if the housing is past its use-by anyway?
cheers Ross.

Those square-headed screws are hardened set-screws, and are capable of doing a better job than the captive cotter except in one respect: the cotter does not mark the shaft much, so you can adjust the clutch's position on the shaft over and over again. With the set screws, the one opposite the key, that bites into the crankshaft itself, means you have to file the burrs off the crankshaft each time you remove it, and you should try hard to put it back in exactly the same place each time or your crankshaft will gradually get eaten away, and then the coupling's position can drift around.
Drilling a hole to suit a captive cotter is not easy: it has to have exactly the right offset from the center of the shaft and coupling. I suggest you stick to the way S&B made it, with the set screws. At least you can be sure it won't creep along the shaft: once that set screw bites into the crankshaft, the coupling isn't going anywhere.

Hi rossf, I haven't seen a set up like that before on any 45's that I have come across.... I would be tempted to purchase a new left hand clutch housing with a new captive cotter-pin assy to suit. It may save you probs with it in future.

UPDATE: I now have the 2 set screws on my LHS clutch housing (as I had to buy a new half) and it is a much better fixing method than the "captive cotter" used previously.

Grumpy/ Darryl, what about drilling an appropriate size hole partly into the crankshaft to accommodate the set screw which screws to the shaft opposite the key ? There would be no chance of movement then, correct?

Thanks Ross

Ross, there is very little chance of movement anyway, when you use a hardened set-screw because the crankshaft is soft steel, and the set screw bites into it. Because you have a key as well, rotational movement of the shaft is impossible; the set screw is only there to prevent axial movement. The axial slippage your mower suffered at some time in the past must have happened when both set-screws were loose, which is a matter of operator error, not a design fault. It is fairly common practice with setscrew-retained shafts to dimple the shaft to give a specific axial location you can return to each time. If you decide to do this, keep the dimple shallow: depth to be no more than half the diameter of the set screw, and diameter to be the same as the set screw. Remember, though, if you do that you cannot use axial adjustment of the coupling position on the shaft: you will only be able to use the screw adjustment on the clutch. Personally I would not dimple the shaft; I would make a shallow circumferential scratch around the shaft on the engine side of the coupling, to show the correct location.

Grumpy,thanks. I will take your advice & leave it as. I think the reason the screws have begun to slightly slacken off is that I have simply overtightened them. Being hardened steel screws against the softer alloy housing, the screws will win every time. See how I go & if I need a LHS clutch housing, I will be much more careful!

cheers Ross

If the screws strip the thread in the clutch, you have three ways you can go: buy another clutch-half (if available), tap out the stripped thread and fit a larger set screw, or fit helicoils to the stripped threads. I like option 3 best, because the helicoiled threads are very unlikely ever to strip again. As I think you now know, it is not necessary to tighten set screws all that much; they bite into the soft crankshaft, and the soft key, fairly easily.

thanks grumpy. Grabbed a couple of larger screws & suitable tap. Will give it a go this week & let you know the result.
Thanks again & I appreciate the advice.
cheers!- Ross

Hi rossf, for your info, the clutch housing is available for Scotty 45, shaft diameters of 3/4" and 5/8". The item includes Set Screw, Engine Key and three clutch Body fastener screws and nuts.
Further advice is to double check engine shaft size before ordering. Also inspect thrust bearing and shaft end for wear as these must be in good condition for the clutch to operate correctly. The price is $65.00

Thanks Darryl, removed the clutch housing & presto, there was already a circumferential scratch around the shaft to show the location of the set screw as you mentioned earlier. I simply cut a larger hole in the LHS housing replaced the set screw (which now rests in the scratch), made some adjustments via the adjusting screw under the shaft & all seems ok now. Will monitor it after I next cut the lawn & if I have any issues will just go and buy a newie.
Thanks again for your help & advice here.
Cheers!! Ross