OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Briggs and Stratton 3.5 or 5.0 is one better

Hi All,

I am new I here so please forgive me if this subject has been done to death.

I recently purchased a SB45 and from the research I have done I think it is about a 1973 build judging on the decals and the solid engine mount frame and also the Kirby engine.

It needs a new engine and all my research says the B+S 550 series 3.5hp is a direct bolt in replacement, I can buy one online for about $270 delivered however I can also get a 750 series 5.0hp for only $15 more.

My question is will the 750 series also be a direct bolt in replacement and if so will it be a better choice of engine?

Cheers

Personally I would be hesitant to put a 5hp engine on the scott bonnar 45 frame, the frames have a tendancy of cracking as it is without the added weight and torque of a more powerful engine.

I have and a fellow forum member run a honda G100 engine which is 2.5hp 92.0 on the early ones) and they rip through thick grass like a hot knife through butter, recently I have bought another scott bonnar 45, which I have sourced a chonda (replica honda) g100 engine for, time will tell how that one goes.

At the end of the day the choice is yours, just check the measurements in the diagrams that are available for the new engines and go from there.

Hello SYTerra and Joe C

I agree with Joe, the larger engine will not be necessary, assuming
your machine is a 14" or 17".

The frame issue will not be relevant here, as you have described the
earlier, solid deck - or single rail machine.

The biggest issue will be crankshaft diameter.
The PTO side could be either 5/8 or 3/4 inch.
My understanding is that the Kirby engines had the 5/8 inch,
but Moderator BB may need to confirm this.

So, bottom line.
- The engine should be an OHV design, preferably Briggs or Honda.
- If your clutch inner housing is in good condition, make sure
you measure its ID, to match the crankshaft diameter you purchase.
[Otherwise you will be up for a new clutch housing of the right diameter]
- reel mowers need less hp to work than rotary mowers. Larger
replacement engines are, generally, not necessary, and weight may
upset the balance and feel of the machine.

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

Hello SYTerra, Joe C and Mod CJ,

Firstly, have to support what Joe has suggested about not requiring a 5 HP unit on a SB45. It's just total overkill, bit like putting a 350 Chev engine into a Mini Cooper. (not that it would fit anyway)

Just to confirm with Jack that all the kirby units that were fitted to SB45's only had 5/8 PTO shafts. It wasn't until Briggs came on line across the whole range that the 3/4 shaft did appear from time to time. This was due to special pricing offers put to SB by B&S when having larger quantities of engines on hand that they offered at a special price to clear them out, thus SB had to come up with a clutch body that had the slightly larger hole. Apart from that there was no technical improvement with a 3/4 over a 5/8 unit, just necessity for fitment that's all.

Now the other issue that is very important is that most of the later engines will no fit directly onto a series 1 machine that utilises the solid deck as the handle bar was a straight up design that will clash with the engine opposed to the later Briggs fitted units that had the larger tank on the 3HP units and required the kinked back handle bar to be used on the 17 inch models. I think you can get away with it on the 20 inch series 1 model as the engine is mounted more inboard and thus it doesn't interfere with the handle bar.

Briggs do supply a special adapter plate for use with the series 1 machines but they are rather hard to come by and only the very knowledgeable mower shops know about these plates. I will photograph a pair that I have on hand at the moment so that you can all see what I mean. I'm just doing a job for a friend that requires his Kirby that's thrown it's conrod to be replaced and his son works for Briggs here in Australia, thus he sourced all the bits and pieces to do the job properly.

Cheers,
BB

whichever engine you get, your mower will now be heavier on the engine side and will cut deeper on that side, i got the 3.5hp on mine, and its almost a run to catch up with it at full speed, cant imagine how fast a 5hp would be

Drive speed is not set by horsepower it is set by engine speed, the beauty of a scott bonnar 45 is the clutch is meant to be slipped so you squeeze the drive lever to the desired land speed.

Twin rail or single, I have seen pictures and examples of machines with broken decks from both sides.

Don't be too sure about that ...



Not exactly the original driving position but gotta admit it's pretty cool

I don't know how you come to that assumption about the weight as I've just replaced an old HK25 Kirby with a brand new B&S 3.5 that was supplied to me to do the job and there's nothing in it. There's so much alloy on the new ones that weight hasn't increased and thus shouldn't affect it all.

Also I support what Joe Carrol said about the speed as that's all based around the hair trigger clutch arrangement as it's designed to slip to slow the land roll down and the more you squeeze it the quicker it goes, also the speed of the engine can be reduced thus not supplying the same amount of rotation to the land roll.

BTW it's great to see our cameraman JC back in action. LOL !

Cheers,
BB