OutdoorKing Repair Forum Forums Collectors Corner Cylinder Scott Bonnar 45 motor replacement query

Hi I am a newby,
I recently purchased a 1966 17" SB to which I have and the cutter bar replaced, reel ground and other minor bits and pieces.
The 2.7hp Kirby motor is original and "dusted" due to the previous owner not replacing the intake foam filter.
The motor now has no compression and cannot cut.
I am therefore at two minds, repair or replace.
1. I would love to keep the old and do it up (but am time poor & need to mow ASAP)
2. Finding a small hp motor replacement is hard.
Question: Can I now place a 5.5hp Honda (that I have reconditioned) onto the frame or will it damage the mower etc?
Question: if yes to above how do I prevent the exhaust from blowing directly at the operator?
The mowing surface is a grass tennis court.
Any advice is appreciated.

Hi BASimmo
I am only a newie at this site and there are much wiser heads than mine that I am sure will come in with more detailed advice.

I put a 5HP Honda GX 140 on my 1968 SB45 in 1980 - and No - it didn't damage the mower and it has been in constant (domestic) use since 1980 with the 5HP Honda.

Yes - the exhaust is a pest - so I welded a short copper tube elbow on to the muffler to direct the exhaust out to the left.

The installation is a big / fiddly job but if you are carefull with the mark out detail - it works out OK. I have attached a series of "happy snaps" of my SB 45 "beast" with its 5HP honda onboard. These will show you how / where we mounted the Honda on to the solid engine deck and relocation of the clutch assembly etc.

For some further comment / explainations as to what was involved - have a look at my Thread "How Much Meat Do I Need on SB 45 Reel" in this forum and discussions / response to "Grumpy" 23 November 2012.

Best of luck and hope it works out OK

Hi BA, I don't know of a 2.7 hp Kirby Lauson, in horizontal crankshaft versions they had the HK25 (2.5 hp) and the HK30) 3 hp) - I don't know of any others.

Parts are available from the US, as Tecumseh spares, for your Kirby Lauson engine. There has been a thread here not long ago detailing an overhaul of one of those engines, and giving sources and prices for each replacement part. However, you can easily fit a more modern engine as a replacement, either permanently or while you overhaul the Kirby Lauson. The only point that needs to be considered, with regard to popular 4 stroke engines from either Briggs and Stratton or Honda, is that the Kirby Lauson had a 3/4" output shaft while the smaller engines from other manufacturers tend to be 5/8". Scott Bonnar made engine-side clutch halves for both Kirby Lauson and Briggs and Stratton engines, but replacing your 3/4" one with a 5/8" one would cost you around $80, while choosing a replacement engine with a 3/4" shaft may not cost you anything.

So far as your plan to fit a Honda GX160 goes, that is of course a nice engine, as all of the pushrod Hondas are. I do not know its output shaft size: if it is neither 3/4" nor 5/8", Scott Bonnar did not produce a clutch-half that will fit it, and you would have to have one made or altered to suit.

Unless your mowing conditions are quite unusual, the most suitable Honda engine would be the GX120, with an advertised 4 hp. A larger engine such as the GX160 (advertised as 5.5 hp) would only impose higher loads on your mower if you actually used its greater power. Something you should perhaps consider is whether you have a single rail engine mounting platform (found on very early SB45s) or twin rails. The twin rail mowers are somewhat prone to fracturing the front rail at the inboard engine mounting bolt. While this is not especially difficult to repair, and even to reinforce, it might be something that is more likely to happen with a more powerful engine.

Changing the design of a Honda muffler would be something of a hassle. Briggs used to have an interchangeable muffler with an external deflector to direct the exhaust sideways instead of rearward, but I do not think Honda does anything similar.

I think all of the small Hondas (GX120, GX140, GX160) have the same layout and the muffler outlet points in the same direction on all of them. I also think the engine external dimensions are much the same. The starter on the right side of the engine will of course project well beyond the right side of the mower with any of these engines - probably including your Kirby Lauson.

BladeButcher, the complex conversion you did to fit that Honda GX140 was, as far as I know, entirely due to your wish to not have the engine project past the right side of the mower. Small Honda engines, Briggs engines, and Kirby Lauson engines all have the same bolt positions and the same output shaft position: except for output shaft diameter, interchanging them is just a bolt-on procedure.

Yes Grumpy you are correct - all that fiddling around with the Honda engine location / relocate clutch was due to the need to have the engine entirely inboard of the chasis. If an overhang wasn't going to be an issue - then changing to the GX140 would have been a straight bolt on job.

My muffler conversion was just a silver solder braise exercise with some 1" thin wall copper tube - its been there since 1980/1981? as I had the same problem with the exhaust blowing up into my face, which is the pits.- The elbow fixed it - not pretty I know - but it works and has stayed together for 30+ years

You make a good point regarding the exhaust elbow Blade Butcher. If I did it I'd have handled it slightly differently (a curved elbow fitting, with lower back pressure, mounted on a flange screwed to the muffler body) but if you asked a dozen Outdoorking members, you'd probably get nearly that many solutions. If it has worked for 30 years, it would be difficult to classify that as anything but a success.

Hi BASimmo, and a warm welcome to the forum. It's great to have another Scott Bonnar owner on board.

Just for your info, Scott Bonnar first introduced the Model 45 in 1968 and as grumpy has explained, the Kirby would be the HK25, 2.5 HP engine.

If this is the case, your machine will have the solid engine deck (as opposed to the twin rail of the later model).
As has been explained in the earlier posts, if overhang is not an issue, the Honda should bolt on easily....the inner clutch half may or may not have to be replaced....that will depend on the Honda output shaft diameter.

What we need now is to post some pics to this thread, using our inline uploader (Instructions HERE ) of your machine from different angles and some of the chain-case after removing the chain-case cover.
This will allow us to have a look at the chains and sprockets and give you an assessment.

Once again BAsimmo, to OutdoorKing.

I note your comments on the Honda GX series engines. I have a Scott Bonner 17” chassis and am exploring options for a new engine. Briggs and Stratton engines are in short supply in the West so I was looking at a Honda option. I note that in addition to the GX engine series Honda also produce a GP series. Any views re fitting a GP 160 onto a Scott Bonnar chassis.