OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar 20" - to re-engine or not

You can find some slightly more detailed specifications for the 550 here:
http://www.briggsandstratton.com/us/en/engines/other-engines/550series_other

It has a cast iron cylinder sleeve, and has a reported gross power output of 3.56 hp. On paper it is a decent engine, but you have to know its history in the field to form an opinion on its reliability and durability. You also have to see it, hear it, feel it and smell it before you know whether you would like to use it regularly. Its capacity, layout and concept are quite similar to the Honda GX120.

The side valve and OHV Briggs engines, and the Honda ones, are physically interchangeable (same mounting, same output shaft position) but all of them are produced with various options of output shaft diameter and length, so you have to look at this detail when selecting one.

Dropped the blade and base plate off today, looking forward to getting them back. Have decided to replace the engine as it would be just as easy spending $270 getting the old one working and you get (hopefully) a more reliable engine.

So have removed the old one but as always if it is left untouched for 30 years things get tight so a new inner clutch housing as I was suffering a recalcitrant cotter pin

Hi Jim,

I'm assuming you've decided on the ebay Briggs engine? I'll be keen to hear your opinion of it once you're back up and running. I've got a few SB45's myself and I might try one also.

Cheers, Michael

That cotter pin is captive: it cannot be removed, it can only be loosened then the clutch is slid off the crankshaft extension. (It is often necessary to soak the crankshaft extension in Penetrene for a few hours before trying to move the clutch off it.) The replacement clutch-half will not have a captive cotter, it will have two setscrews at 90 degrees to each other, which is much better. Also, when you buy the new one you will have a choice of 5/8" or 3/4" shaft diameter. Just be careful to buy an engine and a clutch-half that match each other.

The Briggs 550, being OHV, will use considerably less fuel than the side valve engine it replaces. It should also run rather more smoothly, since it will not miss shots all the time, and idle roughly.

I'll be getting it through Five Dock Mowers as they are getting the blade sharpened as new inner clutch. Best to support those that are helping me.

Trip to Five Dock and great service and head home with new clutch housing and new motor, blade and base plate sharpened and sprayed. (And a few $ lighter)
Base plate & Blade

Clutch on waiting for engine

Cylinder installed

Adjusted and sharp - piece of plain paper, works with newspaper just as well


Engine in - fitted perfectly


Will fire it up on the weekend and is it is a bit late now

Great work Jim

Looking forward to reading your thoughts on the engine and how the machine performs now.

Jim,

That was quick you only picked the parts up this afternoon. Just one thing when you set the blade up just make sure that it spins freely and not tight.

Also when you put the blade in did you find it easy to setup for cutting?

Hi Jim, I presume you used the video that Joe Carroll and I did to so the reel adjustment...to cut paper like that usually means there is contact between the cylinder reel and the bedknife (bottom blade). If so, here is something to consider....

You will find that there are 2 schools of thought here....(1) that you need slight contact and can cut paper and (2) (which is my method) that you have NO contact but minimal clearance. here is a brief extract from a professional turf care magazine:

Myth: You must have contact between cylinder and bedknife.

Fact: �You do not need contact between the reel and bedknife to cut turf grass. Contact between the reel and bedknife will generate heat which will have many effects on not only the turf grass, but also on the traction unit. The heat generated between the reel and bedknife will tinge the leaf blade. Also that heat can cause the bedknife to expand which will tighten the cutting unit up even more. The tighter the reel - bedknife contact, the more strain this puts on the traction unit which can cause hydraulic hose failure and premature failure of the hydraulic system if fitted�.

Contact between cylinder and bedknife causes drag requiring greater effort to turn the reel and in turn this greater effort has an adverse effect on drive-lines and engines. This drag also leads to rapid wear and loss of sharp edges, as well as a poor quality of cut. With contact you have the undesirable �scissor� action instead of the desired �scything� action. It is the scissor action which damages the turf plant and can prolong the healing process.
You do not need to cut paper...the cylinder reel is designed to cut grass.

There is a lot of joy in working on correctly engineered and designed machines. I hate the crap that is produced these days that you can't fix. Hence the SB is a great machine and after 40 years stil comes apart and goes back beautifully

It only took 3 beers to reassemble. However I got the blade back in and adjusted but then the chains wouldn't fit, however if you look at the orginal photo's they didn't go over the tension roller which I later found out was adjustable. So wound up the roller then put the chains on then readjusted it, then reset the chain tension

To fit the engine I had to unbolt the lower handle bar assembly but it fitted perfectly and all bolted down until I saw the bearing shaft key lying on the ground. Off it all came again and rebolted everything again

Will fire it up this afternoon

Fired up second pull - first one I had cut off switch on, gears worked and even the blade spun the right way
And Bruce I put the oil in so no coming back for a warranty job !
But bucketing down with rain so I guess first cut will be tommorrow

It works (after one hiccup when a chain came apart), but is a bit fast

Notice slash cut at the bottom right from the normal mower

Jim, when you said "a bit fast", did you mean it moves over the ground uncomfortably fast? Remember, it is designed to have you slip its drive clutch all the time: full clutch engagement is "attack speed", only used in emergencies (maybe when you are late for afternoon tea?)

Hi Jim, the lawn looks great mate.... at getting your machine up and running....just set the throttle to achieve a reasonable engine speed, it doesn't have to run flat-out; engage the cutter-clutch and gently squeeze the drive clutch handle until the mower moves across the ground at a comfortable pace that suits you.

Well pulled the old girl out for its first outing and no joy getting it started. Any ideas would be appreciated. Does make a funny knocking sound when trying to start. Just a major pain to get it to a shop. Easier to unbolt it