OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Supercut 24' engine

Hey Guys,

Thanks for the welcome DJ, you guys have an excellent forum here.

Oops I did it again. Just accidently bought a 24' supercut on ebay for $100 (Thought it would go for more, thatl learn me). I haven't finished my 45 20' yet is still at the sharpeners. I am now in the predicament as to which one to keep/sell whatdya think?

I am considering finishing the 45 20' and selling and putting a brand spankin motor on the supercut. What motor dya reckon? there are 3.5hp B&S motors on ebay for $275 http://www.ebay.com.au/itm/Genuine-...rs&hash=item3f0df816b3#ht_1693wt_952 , would these be OK?

The supercut also has a busted roller. Are these available or should I get the welder out and patch it?

Looking forward to hearing your thoughts.

Thanks, Graham

Hi Graham, the Model 17 Supercut was the Rolls-Royce of cylinder mowers in their day and restored they make a brilliant mower and do an excellent job. They are very heavy and sturdy in construction and use gears in the side case instead of sprockets and chains of the Model 45. They were first introduced in the 1950's and some are in use today showing the extent of their reliability.

I have a 18" Model 17 Supercut electric here that would suit you for parts as most Model 17 chassis parts (except for handle and cylinder reel) are interchangeable with the 24"...It has both rear rollers intact and in fact the mower is in serviceable condition. Yours is a 3 segment rear roller, mine is 2 but my rollers are the same as your 2 outside ones.
We could do a deal if you want, just send me a PM.

I am sure that grumpy can advise you on the Briggs that would suit, if you decide to proceed. In my opinion, the Supercut would be the way to go!!

Graham, in the first picture the current engine looks like it might be a Honda (because it has a red cowl). It is important to work out whether the mower was made surprisingly recently and that engine is the original one, or the machine is from the 1950s and originally had some ancient British engine. This is important, because the mountings and crankshaft height are likely to be different. Was it made originally with a modern (Honda) engine, or has it been converted? Also I take it that the engine is defunct, or you do not want to rebuild it? Essentially, there are three ways you could go:

Option 1: restore the mower to its original specification, whatever that was. By doing that you make your machine into a potentially valuable collector's piece.

Option 2: repair the mower at minimum cost. That would involve identifying the current engine and then finding a second-hand four stroke engine that is physically interchangeable and has at least as much power.

Option 3: fit a new engine. You would still need to establish what type of engine fits the machine, then buy a reputable engine that is physically interchangeable. It would need to be a 4 stroke to suit the kind of service it will be used for. Between them, Briggs and Stratton and Honda produce the bulk of the world's engines of that general type, and both make excellent engines, subject to some quality control questions in the case of Chinese-made versions of both brands.

So, the first thing you need to do, is decide what you are trying to achieve: just mow the lawn, or acquire a classic mower and preserve it for posterity. The best classic mowers are restored to their original configurations, so you would need to find out its exact specification when new, then make a plan to return it to that condition. Before you go too far with plans, you need to verify the condition of the parts that would be difficult to restore if they are badly damaged. Examples are the drive cover, which presumably has to end up oil-tight, and the gear train inside it. If bad things have happened in there, the Scotty 45 will start to look like a really good option.

With regard to the roller, cast iron is usually regarded as quite weldable unless it is "white" cast iron (i.e. chilled during casting, to make it hard). However to end up with a good quality, robust, cylindrical outside surface is quite a lot of work.

Hi grumpy, as usual, you have given good options there mate...I have some comments that may throw some more light on the matter....

Back in the 50's when this mower was first introduced, the first petrol engined Model 17's had a Villiers Mark 25C,147cc 2 stroke fitted. It was offered in 14", 16" 18" and 20" cutting widths. Later a 24" version was produced with a larger Villiers fitted and was a 4 stroke side valve...(The mark number escapes me).
Most of the 17's in service today have had Briggs or Honda's retrofitted as has this one....You can tell by the petrol tank and cowl in the picture...also the reduction gearbox on the engine.

Most machines also have a full length 'counter' shaft with pulleys (usually double) that can slide and lock in any position on that shaft as it runs the entire width of the mower and at the right-hand end has a dog for engaging a crank handle to start the engine (original configuration prior to recoil starters).

Engine mounting on these models is pretty versatile as is the horsepower rating used...I have seen 3.5 to 5 hp engines used successfully utilised.

I'll come back to your specific question, Graham: whether the 3.5 hp Briggs 550 series engine would be a suitable direct replacement for the current engine. The answer is no, because the current engine has a reduction gear on its output shaft, to make the output shaft speed suitable for the mower. I'm guessing that the original engine was a thumping old English 4 stroke that did its best work at about 2,000 rpm. Modern equivalent engines, including the 550 Briggs, do their best work at 3,500 rpm, so you need an engine with a 2 to 1 stepdown gearbox built-in. Both Briggs and Honda make suitable engines, but you need to find one. The 3.5 hp output level is, in my opinion, barely sufficient for a 24" mower. You might find it a bit reluctant to climb significant slopes whilst mowing (that is a heavy mower).

The obvious alternative to an engine with a built-in reduction gearbox is to use a pulley on the mower drive shaft (not the engine output shaft) of double the current diameter, and achieve the required step-down entirely in the belt drive, but it is evident that there is not room for the larger pulley, so the reduction gearbox looks like the best option. Note that because your mower uses a belt drive between the engine output and the mower drive, the height of the output shaft from the engine is not an important issue (within reason): all that happens is that the length of the belts may change. However I do not see a mechanism for tensioning those belts, which may be a problem.

There are certain obvious attractions in reconditioning the old Honda engine, provided you do not need to buy many new parts (Honda spares are reputed to be expensive).

Hi Graham, I have made some inquiries for you and sourced a second had left hand rear roller segment for your Supercut.
I will send you a PM with the details.

Now that I have become sucked in to this project, I thought I'd post what I would do. Such choices are always personal, and perhaps idiosyncratic, but I'll put this possible solution on record.

Step 1: Make a chalk mark on the existing engine's output shaft, and rotate the engine until the output shaft has made one full turn. Note how far you turned the crankshaft to do this. That will tell us the output gearbox ratio. It may or may not be 2:1 - we need to know for sure, to perform Step 3 below.

Step 2: Unbolt the current engine, move it backward 6", clamp it in place, and see how the mower feels with that slight rearward shift in center of gravity. Remember, it has to be reasonably stable, with weight on its front roller, when tilted backward 15 degrees, simulating climbing a maximum slope.

Step 3: If Step 1 indicates you can move the engine backward, that is the solution so you won't need an engine with a reduction gearbox. Source a double V belt pulley of the diameter of the current mower shaft one, one multiplied by the gearbox ratio determined in Step 1.

Step 4: Buy a new or second-hand engine (I'd choose second hand, of course) of about 4 or 4.5 hp, with an output shaft long enough to suit the V belt drive. Cut a piece of 1/4" steel plate that will cover the existing 4 engine mounting bolt holes in the mower frame, and provide room for 4 new engine mounting holes sufficiently rearward to allow the engine's output shaft V pulley to clear the large new V pulley on the mower drive shaft.

Step 5: Weld the plate to the two mower frame lateral rails. Bear in mind that one day, the welds will need to be cut to restore the machine to original condition. Drill the four mounting holes for the replacement engine procured in Step 4. Put it all together and go looking for a 15 degree up-slope so you can see if you got it right.

Thanks for you thoughts guys,

Grumpy I was wondering why you suggest moving the motor backwards. Does it change the centre of gravity, or produce more useable horses?

Thanks mate.

The only reason to move the engine backward is to leave room for a much larger pulley on the mower drive shaft, without it hitting the small pulley on the engine crankshaft. By the time you put the small pulley directly on the crankshaft instead of it being moved 3 or 4" backward by the distance between the gears in the reduction unit, you've effectively moved the engine pulley forward 3 or 4". It looks to me as if it would hit the bigger front (mower drive shaft) pulley unless you moved the engine backward that 3 or 4" plus another couple of inches because of the larger front pulley.

Yes it does change the center of gravity, but that is undesirable. That is why you have to carry out Step 1, to ensure the mower won't want to tip backwards when you drive it up steep hills.

It makes no difference to power, so long as you let the engine rev up to 3,000 or 3,500 rpm without the mower travelling so fast it leaves you behind and goes away on its own. It might never find its way home.

ahh I'm with you now, in lieu of a motor with a 2:1 reduction alter the pulleys to create the reduction. (I'm not to bright but I cant lift heavy things).

Thanks Grumpy all food for thought.

Hey Guys,

Grumpy, do you have any ideas on whether my existing gearbox will bolt up to a later model engine?

Thanks mate.

If it is like a Briggs and Stratton reduction gearbox, the gearbox attaches to the crankcase side-cover, so you would have to remove the side cover (including one of the crankshaft main bearings) and fit the whole unit to the other engine. As far as I can tell that is a side valve engine, which as far as Honda is concerned, pretty much went out of fashion a long time ago. In other words I don't know the answer, but given Japanese manufacturers' tendency to keep redesigning their products, plus the fact that you would probably be buying an OHV or OHC engine that has a rather different crankcase from yours, I think it is very unlikely it would fit onto a modern engine. If you can find the engine's model number (probably on the red cowling) you could ask a dealer, or the part number may be shown in one of the parts lists available on line.

Thanks Grumpy