OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers What Method is best for badly Cracked Rails on 45

Hi All,

I have another 45 on the go that I picked up on Saturday and it has both rails severely cracked. I will post up photos tomorrow so you can all see to what extent, but this is what happens when the clutch becomes flogged out and the balance of the clutch injects massive vibration. How some people can use a machine without noticing that something is wrong is beyond me, but all I can do is deal with the situation and repair it to better than original for strength. I have some ideas in mind but as this is the first chassis that I have obtained with this level of failure I thought it best to ask the folk who've done this repair many times before as its quite common.

Cheers to you all,

BB

BB, to set the scene for this, the pictures you intend to post of your mower and the cracked rails will be very important. We have had plenty of reports of SB45s cracking the front edge of the deck at the inboard engine mounting point, but I don't recall any previous report of a twin rail machine cracking both rails. It would improve our archives to see it in detail. Presumably if someone kept operating the machine after the front rail had more or less broken in half, the rear rail would then fail as well, but it would be useful to see the detail and try to work out what had happened. A key point: did this machine have the captive-cotter style of engine-side clutch half? So far the cracked rail reports I have seen may have been confined to captive-cotter machines, as have the reports of loose engine-side clutch halves causing clutch malfunction and machine vibration.

With regard to repair technique, you might take a look at this thread:
https://www.outdoorking-forum.com.au/forum/u...mp;Words=crack&Search=true#Post33516

Hi BB, you're quite correct in saying it is a common problem....it is a design flaw in the later Scotties with the adoption of the twin rail engine deck....previously all machines had a solid deck and no probs.
Just to clarify, not all rail fatigue fractures and cracks are caused by clutch out of balance problems, I have seen small cracks radiating from the engine attachment holes because of the Briggs inherent engine vibration.

All Scotties, to my knowledge, had the captive cotter style of the inner clutch half up until the production the Rover 45, and subsequently all after market replacement clutch halves have the opposing grub screw arrangement.

Hi Grumpy and Deejay,

This machine was most definitely a captive cotter clutch issued machine. When I picked it up on Saturday the previous owner was still using it with the fully flogged out clutch and the cotter pin pulled out, thus no clamping force on the output shaft causing the excessive wear on the clutch bore and finally out of balance vibration which is causing fatigue failure of the under strength poorly designed rails. I have seen a repair done where a 5mm plate the size of the engine base is welded between the two rails, this way it can't be seen but adds additional strength and stability.

Anyway those are my thoughts I'll let the photos do the talking on how bad the failure of this machine is.

Also while I'm at it, would I be correct in saying that about 6 out of 10 machines that are being flogged off via Gumtree and eBay are suffering from this symptom as they seem to be the machines that never have any maintenance done on them and just been dumped in a shed and forgotten about, much of this was cause by the drought and many people not watering and looking after lawns and older people getting their lawns professionally mowed ?

Cheers to all you guys at Outdoor King,

BB.

Hi BB, yes mate, those rails have really suffered from fatigue...but your repair idea should do the trick without cosmetically altering the outwards appearance of your machine. Over engineering in this case would be a good thing. A new inner clutch with the opposing grub screws, a new thrust bearing....plus maybe a new cork clutch lining (if necessary) will see you up and running again sweetly.

I'll leave the discussion of the machine to Deejay, who is our reel-mower guru, and just comment on the welding.

As you will find if you read the thread I nominated in my previous post, there are two main issues in the welding project. The first is, ending up with the mower frame correctly aligned rather than corkscrew-shaped. This is by far the more difficult part. The second issue is making the machine strong enough to prevent another failure.

The alignment issue was handled in some detail in the other thread. Essentially, you need to clamp the frame into correct alignment, tack it, unclamp it, realign it by twisting, re-clamp it, weld it a bit more using short runs which compensate for each other in the distortion they cause, unclamp and align it again, and so forth. The core technology here is to carefully put each small additional weld in a position where the shrinkage distortion will twist it in the opposite direction from the previous small weld. Of course it becomes more and more difficult to correct the twist by physical force as you put more welds into place, and you have to do it by putting the welds closer together on one side than the other, so there is more total shrinkage force in the required direction. While this kind of thing is fun, it is very frustrating to have to keep grinding off welds after you make mistakes that cause distortion. A genuine tradesman welder (I do not mean an experienced welder, I mean someone who completed the welding apprenticeship then practised the art for a decade or so) can do this kind of thing routinely without breaking into curses and starting again repeatedly.

The other issue is to end up with the machine strong enough to not break again. Presumably you will be using a grub-screw type engine-side clutch-half next time around, so the strength of the original SB design might actually be sufficient, but if you just weld a reinforcement underneath the original rail you probably won't even achieve that. The reason the original rails broke is that the vertical flanges on the rails were neither deep enough nor thick enough to adequately control the vibration-induced deflections involved. It is difficult to change this situation without making new rails, of deeper section and thicker steel. If you did this by laying the new rails inside the old ones it would not only look pretty awful, it would only be possible to weld the new rails to the side plates on one side. A series of spaced stitch-welds on both sides of the rails would give you a stronger result together with a better opportunity to control the shrinkage distortion.

Hi Grumpy,

Thank you very much for going in to detail about this. I have spoken to a friend of mine who welds garages together for a living and he has very much concurred with your statements and gone as far as saying basically spotting it with a MIG Welder along each crack but spotting it at alternate ends of each crack without heating the metal up to any real extent thus avoiding distortion and keeping it to an absolute minimum. Clamping the sides (with the sole plate and front roller installed) to a solid metal bench and then clamping a solid metal bar to the underside of the rail to hold it perfectly straight is all part of the exercise he claims.

I totally agree with you about the comments you made on the thread you pointed out about the structural integrity was completely compromised on that other earlier SB / Rover faker machine with the totally failed rail. That one would have been "shakin' all over" like a 1950's rock n roll outfit.

BTW I have already purchased a twin set screw clutch body to replace the defective old captive cotter unit.

Once again thank you very much Grumpy for your detailed input as its always great to get fresh ideas from a different source and angel, that's for sure.

Cheers,
BB. (Marty)

That should've read "angle" and not angel. Sorry about that.

Cheers,
BB

Hi BB, As this is a common problem with the late Model 45 Scotties and this repair schedule will make for a great archive on this topic, could you please take plenty of pics as you go along and post here.....If necessary, we could start a new thread and transfer some of the content of this thread to it.
Thanks mate,

Hi Deejay,

Sounds like a plan to me. I'm happy to provide a running pictorial dialogue on the progress. I won't be doing anything on it for a little while as I'm currently doing a full engine rebuild in a VS Statesman that is going to Perth for my son in law and then moving house but once I'm past that I'll get back to the Scotty and then the "Blue Meanie" Lawn King version that I have which has a minor case of the same syndrome around one hole.

For now cheers to all,

Marty (Bonnar_Bloke)

Thanks BB, and best of luck with the move and the engine rebuild.Will look forward to your return to the forum.

Hi BB, whilst trawling around the SB threads, I found this pic of the construction of the underside of an early (solid engine deck) Model 45.
Photo courtesy of one of our forum members Deviosi
As you can see the deck is bolted to the side frames, rather than welded and has reinforcing for the engine attachment bolts.

Deejay, it looks to me as if that deck has considerably deeper flanges than the twin rail, and it is also double-thickness on the horizontal deck part. You can see the arc welds at the side: two L-shaped reinforcements are welded underneath the flat top deck, and the heavy engine mount reinforcements appear to be spot welded to the L-shaped pieces. Comparing that with the twin-rail deck is like comparing a piece of heavy steel plate with a piece of steel sheet. The deep flanges (both around the perimeter and across the middle in two places) give the deck bending strength, and the double thickness deck plus engine mount reinforcements give cracks no opportunity to start at the engine mounts. The engineer who designed the twin rail deck obviously thought the engineer who designed the 45 was a dill.

It also seems to me that the best thing to do with a cracked twin rail SB45, is to fabricate a single rail deck to the original design, and bolt it to the side plates rather than weld it.

Yes grumpy, all is revealed when you see the underside isn't it...I agree wholeheartedly....that is better than welding the twin rail design.....the original engineer (Syd Bowditch) was definitely no dill!

Now we just need someone to carry out the pilot project by making and fitting a single rail deck to the SB design, then posting a thread spelling it out in detail, with dimensions and an explanation of how the bolt holes were located and drilled in the side plates.

Post-Edit: I suggest that someone should remove a side plate from a one-piece-deck SB45, lay it on a sheet of drafting mylar, and spray paint the mylar around the plate and over the various holes, to produce an accurate, dimensionally stable, template for the side plates on the mylar. This could be used to prepare an accurate drawing.

After that there will be a business opportunity for someone to start making and selling one piece decks and conversion drawings. The key requirement is doing it exactly to the Bowditch design, so the retrofitted mowers will be sort-of-authentic.

That would be a good plan grumpy, what we need is someone that is restoring a solid deck Scotty is to remove it and take accurate measurements and dimensions using both the deck and side plates as templates.
Don't forget that the clutch fork assembly needs to be accurately fitted to the deck as well, as that was originally spot welded in position.

It looks as if the engine mount reinforcing plates are spot welded too, Deejay. That means a spot welder is needed to do an authentic job.

By making the whole one piece deck then bolting it on, it is possible to straighten the deck precisely after completing it, before attaching it - to straighten the deck you don't need any equipment except a flat plate and some feeler gauges. With the side plate bolt-holes accurately located, the basic frame will then be square and precise - no welding distortion hassles. I think that will have been what the original design counted on.

Just a thought, there would have to be the slots cut and the captive nuts fitted for the grass deflector attachment, that would have to be accurately measured as well.