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

Hi all,

I have just completed my first rebuild of a Scott bonnar 45, I did a complete strip down and rebuilt the clutch etc. I fired it up for the first time today and noticed that it has a lot of vibrations especially at low idle, I've looked over everything and it looks pretty good, the only thing I can think of is that the clutch or engine shaft is bent. Has any one got any suggestions of what I should be looking for ?


Thanks

It's just a first guess Ozziesim, but SB45s with the original clutch that uses a captive cotter to retain the engine side of the clutch, are very prone to running out of true after some wear develops in the cotter and the fit deteriorates between the engine's PTO (output shaft) and the aluminium clutch. When there is slack between the shaft and bore, when the cotter is tightened up it forces the clutch out of true with the PTO, and that makes the clutch run out of balance. The vibration itself is the least of the problems - it causes the engine mounting deck to fracture at the engine mounting bolts, due to metal fatigue. Of course the out of true condition gets worse over time, due to fretting wear between the shaft and clutch.

Ok yes it is the older cotter pin type, so do you recommend I change the whole clutch or just replace the half on the engine side ?

First, check that the engine side clutch half is actually loose on the PTO shaft when the cotter is loosened, and the captive cotter pulls quite a distance into the clutch half when the cotter nut is firmed up. Please post pictures of the PTO, the bore of the clutch half, and the cotter when it is assembled and clamped up. At that point Deejay, our SB45 man (and very proud owner of one) will explain how to examine in detail for some of the internal symptoms inside the clutch. To answer your immediate question, only the engine side clutch-half, and perhaps the key and the thrust bearing, are likely to be damaged. The later style of clutch-half is unlikely to give trouble if correctly installed - the design weakness was overcome in the later design.

Ok I did put a new bearing in the clutch, I will pull it apart and take some pics and post soon thanks

Okay, pulled it apart, the clutch half sits firm on the shaft with the cotter loosened, no movement there at all, tightened it up and it sits pretty firm. I put it all back together and it runs a little smoother however there seems to be be still quite a bit of vibrations. I've taken some photos hope they help

The end of the key in the lower picture (which happens to be the engine side clutch half, the one that is prone to the problem) looks as if it might have moved axially toward the thrust bearing and become chewed, which we have seen on other SB45s that had the captive cotter problem. Also, the captive cotter might be pulled through slightly further than usual, though it doesn't look severe.

You haven't posted pictures of the thrust bearing that goes between the two clutch-halves, which seems prone to failing when the loose cotter syndrome appears.

Let's see what Deejay makes of it all - he's been through this with his own mower.

Thanks grumpy, I didn't photo the thrust bearing as it was brand new.

Just in case Deejay is out of touch for a few days Ozzie, I'll pass on my suggestion for something you might try. It goes beyond what has been discussed before, so please regard it as an experiment.

There have been quite a few cases where captive-cotter clutch-halves have come to grief, ending up with the clutch bore loose on the PTO shaft, and the cotter moving further and further into the boss as the bore expanded. When dismantled these clutch halves seem to show signs of the clutch-end of the PTO key being chewed up. As Julius Sumner-Miller used to say, "Why is it so?" My new theory on that subject is that the (IMO rather dumb) design of the captive cotter system did not really clamp the key into position. With normal engine vibration the key tended to slide slowly away from the tightest part of the bore, which was directly opposite the cotter. So, the key found its way to the end of the PTO shaft at the thrust bearing end of the bore. It then encountered the thrust bearing. The key got beaten and mangled, and eventually stretched and damaged the bore of the clutch. Even before the bore was stretched, the eccentric forces on the key may have caused quite a bit of vibration, and fatigued the engine mounting deck.

Now, if this radical and completely untested theory is correct, you might be able to alleviate the vibration in your mower just by driving the key back towards the engine end of the keyway. Furthermore if you tried this and told us the result, we might have some evidence as to whether the radical theory is actually correct, or alternatively, like most radical theories it is complete moose-droppings.

Hi guys,
This seems to be quite a common prob with the captive cotter clutch half, the new design has the grub screw which secures the key in its keyway, which alleviates the problem, and no damage to the thrust bearing

You might find Deejay's thread on repairing his clutch interesting, Ozziesim:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=39040#Post39040

Thanks Guys,
Yes I have watched deejays video. Grumpy I will give it a try with the key and see how I go and see if I can prove that theory correct. If that doesn't work I will replace the clutch half and hope that sorts the issue. I will post what I do either way in case someone else comes across the same problem.

Thanks Ozziesim, hopefully that will at least move us further toward understanding the problem with the captive cotters. Don't forget to take a close look at the thrust bearing in the center of the clutch, in case it is damaged.