OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar 45 - Fracturted Rails & Bedknive

Hi all , have acquired my fathers scott bonnar 45 twin rail hk30 powered 17 " mower.

Have dismantled most of the machine as had no spark and very little compression!
Also thought to get the reel sharpened and to replace bearings as required.

As have read a number of discussions odk regarding fractures in the rails -am I correct in saying they are primarily caused by the captive cotter style of clutch housing? And the fix is to replace with grub screw type? Or can you discard the pin and drill and tap your own threads to retro fit to a grub screw set up or is the whole housing out of balance thus requiring replacement?

Also do I leave the bed knife attached to the sole plate for sharpening or remove it?

Thanks in advance for any replies.
Grame

Hello Grame
A warm welcome to these forums from me.

I don't know if the original cone clutch design is the primary cause.
I do feel it is a combination of factors that have come into play:-

- size of the machine [larger the width; more likely failure]
- the specification of the steel in the rails
- the hours of use
- the rpm range the machine was run at
- the mowing conditions
- maintenance of loose bolts and nuts

It really is a contested issue.
The cotter style is fine if the cotter is kept tight.
Any bicycle kid from yesteryear would know that cotter pins do loosen
over time. Scott Bonnar never used nyloc nuts on the cotter. Perhaps that
would have made a difference...

Rover changed to square head screw in the 1980s, when production moved
to Brisbane. The thing is, they retained the original die casting,
but without machining to take a cotter pin. In other words, some have argued,
the clutch housing was not balanced. I don't know.

It's best you leave the bedknife attached.
The main thing is that the new blade is faced after fitment.
Too many repairers do not do this!

Hope this helps.
--------------------
Jack

Hey thanks for my warm welcome Jack, a cooler one would nice though,

Sorry for slow reply!

The rails aren't cracked as yet, cotter pin was tight as!

Would you recommend plating up the under side of rails now or wait till they crack? Keeping the cotter pin tight!

As I said this was my fathers machine bought in the early seventies, used for probably 14 years before moving to a new house with unsuitable lawn. All original and almost complete, missing the plate on the handlebar
The engine is a Kirby HK30 h4-5a, does anyone know the difference between this model and the h4-4a as I haven't seen too much of other 5a's

Cheers Grame

Hello Grame
That's good news.
The best approach is to do no plating (that is a repair approach.)
It appears that this machine is fine and free of rail cracks.

I can't exactly answer that - but I know Kirby made changes to
the carby at some point.

-----------------------
Jack

Hi Grame,

G'Day also from myself !!

I fully support what Jack has suggested regarding the rails, the old saying goes like this, "if it aint broke don't fix it".

If you have a good well running clutch body that's running totally concentric then your rails won't be subjected to fatigue fractures. Using the "Gusset Repair Method" is only to be used to salvage a machine that's been exposed to abuse, resulting in eventual failure.

Cheers,
BB.

Goodo thanks for my replies cj & bb.

I'll have to get the old girl up and running again, waiting on parts and some more time, so I'll get there eventually.
Thanks again for your time and advice.

I'll be in touch no doubt!
Happy Australia Day!
Cheers.
Grame