OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Thrust Pad Replacing it

Hi Guys

I would like to replace the thrust pad (black plastic component) situated in the clutch fork assembly on the engine drive shaft. It looks to be the wearing part when the the clutch is engaged. It has "melted" and is out of shape.

I seek guidance on how to best to make the replacement. I read an earlier post to rossf about disconnecting at the engine drive shaft end and wonder if that is the path I should be taking.
The bearing and its housing at the chain end look to be difficult items to remove.

There looks to be two setscrews at that end to deal with and some avdels to release the clucth fork assembly. Youir advice appreciated


I promise some photo's as soon as the new cylinder has been back lapped and the front roller replaced.

Rob Hocking

Rob, would this be a Scott Bonnar cylinder mower, with a manually-operated clutch in the drive shaft from the engine to the chaincase on the left side of the mower?

If so, you have to remove the engine to remove the right half of the clutch, then to thread something over the drive shaft on the left side of the clutch, you'll have to remove the left half of the clutch from the drive shaft as well. I haven't heard of anyone doing this except as part of an overhaul, which normally includes removing the drive shaft from the chain case. However I don't see why you couldn't remove the left half of the clutch once you've taken off the right half, leaving the drive shaft sitting in its bearing on the chaincase end.

Loosen the captive cotter pin in the right half of the clutch, or undo the grub screw if it has that instead of the captive cotter pin. Remove the four bolts that hold the engine down to the mounting plate(s). Disconnect the speed control cable. Slide the engine to the right as far as possible (until it hits the right side plate). If this separates the clutch completely without removing the right side of the clutch from the crankshaft extension, all the better - you won't have to remove the right side of the clutch from the crankshaft extension. If necessary though, slide the clutch-half to the left, off the crankshaft. Take the entire engine away and stash it temporarily. Next you have to slide the left half of the clutch off the drive shaft. It will be retained by something, probably a grub screw, and this will have to be loosened first.

Please take pictures as you go for our archive (I don't believe we've specifically covered the thrust bearing before), and if you get stuck or I'm wrong somewhere, show us a picture of the problem. A picture of the mangled thrust bearing would also be good.

Hi Rob, its been quite a while since I have had that part of my machine apart, but I think I remember the procedure...Here goes:

(1) Remove the 3 nuts and screws from the cutter clutch.This will split the clutch in two.

(2) Remove the 4 bolts that secure the engine to the frame and slide it off the machine to the right, with the right hand segment of the cutter clutch in situ.

(3) Slide the clutch outer complete with the clutch cone off the engine shaft.

(4) Slide off the cactus thrust pad and discard.

(5)slide on the new thrust pad.

(6) Check the cork clutch lining in the clutch outer and if badly worn, replace.

Reassembly is the reverse procedure.

(7) re-adjust the cutter clutch by use of the locking set screw at the bottom of the clutch fork, so that the clutch lever dis-engages the clutch correctly and the cylinder reel does not move.

Please let us know how you get on, Rob.

Well done Deejay, I seem to have left out Step 1. Hmm.

Also, doesn't he have to free the left half of the clutch from the drive shaft before sliding it off? I don't recall this operation being handled here before, but I had the impression it was locked onto the shaft.

No grumpy, not from memory, it slides off exposing a woodruff key, on the engine shaft.

Also do not undo the captive cotter pin or grub screw under any circumstance or the whole clutch set-up will need re-adjustment...not an easy task for the new-comer to these machines.
In this case grumpy, it is good to get advice from someone who owns one of these machines as there are some hidden pitfalls for the unwary.

Hi Grumpy

Sorry, my post wasnt specific enough and I may have misled you. Its a Rover 45 (Scott Bonner !) red with the flash black plastic cover over the clutch assembly and shaft. One of the later ones that were made I think.

The plastic thrust pad (is not a bearing) or a part of the clutch proper sits in about the centre of the shaft in teh clutch fork assembly and when the clutch lever is activated / deactivated "pushes or releases " the clutch body (chain side) to disengage / engage the clutch. I would say its the sacrificial part that wears over time. I think there is a oil lube point in the black shield that sits over it.

Talk of taking the clucth and unbolting the motor scares me a little to be honest..This follows roughly along the line of a previous thread except the motor wasnt being removed.

Thanks

Hi Rob, removing the engine is easy peasy, just slide it to the right and sit in on a box of approximately the same height as the frame rails...it has to move in order to allow the rest of the cutter clutch assy to be removed which then allows you to slide the new pad into position.
Just take it easy and if you get into trouble just post here and we will help.

Rob, I think the new thrust bearing (thrust pad if you prefer to call it that) is a full circle. That is the reason it has to slide on from the free end of the shaft. To get access to a free end of the shaft, the engine and clutch have to be removed first. Removing the engine is the easy part, just 4 bolts and the speed control cable. However you have to split the clutch or remove it from the engine's output shaft. Deejay is recommending splitting the clutch rather than sliding it off the engine - a number of people have reported difficulty removing the right side of the clutch from the engine shaft, due to rust on the shaft, so Deejay's way should be easier than mine would have been. Then you still have to remove the other half of the clutch from the drive shaft, to get a clear, free end of the shaft to slide the thrust pad over.

This is all easy stuff unless the left half of the clutch is rusted onto the drive shaft. If that happens, it gets a bit more difficult. Even so, it is an easy repair as these things go, not one that needs an expert.

I managed to follow Deejays instructions and as suggested it was pretty easy. I would make the following points. The 3 bolts (setscrews) holding the clutch together where not tight (but loose) and when I re-assembled I tightened them slightly. The bolts holding the engine onto the frame were not that easy to undo (or redo) as they are placed awkwardly particularly the rears as I hadn't removed the fuel tank. The 2 front bolts (closest to the cylinder) had a nut and loose bolt and the rear nuts were fixed. The clutch separated and went back together fairly easily.. I moved the motor just off the frame and it wasn't a problem.

Pictures show the pad before and after. Hope this helps anyone looking at changing that part or renewing cork in the clutch.

Thanks Guys


Per Deejay

Geez I wished I could spell check !!!!

I tend to type some of my longer posts in MS Word, to spell check them, then I paste them into here, However, i dont always end up doing it for all my posts, so sometimes I'm quite well written, othertimes I am not so great!

Don't worry Rob, I went in and fixed it for you.
Great to see it all worked out so well and now it's off for the back-lap. Please let us know how you get on.

Thanks for posting those pictures Rob, this thread will help others who dismantle SB clutches in the future.

Thanks grumpy for your help also, this is a good archive for the future, as it is all complete I will now close it.