OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Gears n Chains n bits n pieces

Hey all,

long time reader new member here. I have just scored myself a new (actually pretty old) SB45 20" mower. I have just pulled it to pieces and have discovered that the sprockets and chains are really worn and ideally need replacement. Q: Where can I find replacements at a reasonable price? Additionally I want to have a go at sharpening the cylinder and BBlade myself as I have access to a workshop with a decent lathe although only with standard machine tools. Has anyone tried this before and how did it go?

Any help will be much appreciated, and keep up the great work as this is a terrific forum.

Thanks,
Graham

Way back in the old days people used to resharpen mower reels using an ordinary toolroom lathe and a high speed steel tool, Graham, but that was backyard practice, not something a greenkeeper would do. At best there will be burrs to remove, and you would have to use a large nose radius tool, a fine feed, and extremely light cuts (or you will deform the blades where they attach to the frame of the reel, and along their length). An excellent job can be done with a toolroom lathe using a toolpost grinder, provided it has a large grinding wheel - say, at least 4" diameter. The limitation on using a more conventional small toolpost grinder is that the wheel will wear down during each cut, and the diameter of the reel will therefore vary slightly from one end to the other. This is not disastrous, because you can adjust the clearance of the two ends of either the reel or the bedknife (depending which model mower) independently.

I think the main reason to have the job done by a specialist, using a special purpose reel grinder, is that the chances are nothing will go wrong and you aren't risking using up a lot of the "meat" on your blades through having to repeat the operation a few times before you get it right.

You can sharpen the bedknife with a surface grinder, but to get it flat along its length it needs to be attached to its holder when you do it, and supported at the two ends, as it is in service. This means using very fine cuts, or it will deflect downward when grinding near the center. I am not clear on how this was done in the old days - it may have been usual to just replace the hardened steel bedknife, then lap it flat after attaching it to its holder. Remember, the quality of the final mowing job depends on a straight, parallel reel, a straight, flat bedknife, and a straight front edge of the bedknife that is precisely parallel to the axis of the reel. No burrs on the reel blades, of course.

If your mower has had its blade adjusted many times you are likely to find that the adjusting screws have their threads stretched out of pitch at the precise point where they are usually adjusted. This makes adjustment just about impossible. (If you have ever adjusted the tappets on a very old car that has suffered from ham-fisted mechanics over the years, you will be familiar with the problem.) I suggest you inspect the adjusting screws carefully, and replace them if necessary.

Thanks Grumpy will check those screws and see how I go. Any ideas on the chains and sprockets?

Graham, I suggest you browse this thread:
https://www.outdoorking-forum.com.au/forum/u...547&Words=sprocket&Search=true#Post22819

In it, our resident Scott Bonnar historian, Deejay, says all mower parts are still available for genuine SBs, though not necessarily for the Rover-built ones.

Thanks Grumpy,

I see you are the B&S guru. Are you able to identify my motor the numbers are 81232 0208 01 79011 703, SB Model no 45 12501.

Thanks mate.

I can help somewhat, Graham. First, look at the third block of numbers (the code), 79011703. These tell us that your engine was built on 17 January, 1979. (Ignore the final 03, that relates to which production line it was built on at the US Briggs factory.) Second, look at the first block (the model), 81232. These tell us that your engine is 8 cubic inches displacement (130 cc), is a second generation design (first generation was 0), has a horizontal crankshaft, has a Pulsa-Jet carburetor (the type with a fuel pump built-in), apparently has a flange mounting with ball main bearings (that is the 3 in the number, and seems quite strange for the application), and has a rewind starter. As I said, I don't really believe it has a flange mount for a gearbox around the output shaft, or ball main bearings, since it is fitted to an SB45 and they normally have neither - if you read that 3 correctly, someone may have fitted the wrong cooling air cowl to it along the way, or for some reason it happened to be fitted with an up-optioned engine, perhaps due to a shortage of the correct type. A picture of the crankcase cover where the output shaft comes out of it should show whether it has a mounting for an external gearbox. Having ball main bearings is something of an advantage provided you keep the oil clean, but it is not necessary in lawnmower service. Note that because your engine was built before 1983, it has breaker-points ignition: the Magnatron breakerless ignition came after yours was made, but can be retrofitted if it ever becomes necessary.


By using the model number (first block) together with the type number (second block) we can get the operator's manual and illustrated parts list from Briggs and Stratton:

Operators' Manual
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=lrwBF-wU7eL7hAxB91ObQ

Illustrated Parts List
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=Z6mmyAVJ1DajI

Wow, thanks Grumpy

Hi Grazzaa, and a warm welcome to the forum. It's great to have another Scott Bonnar owner on board.
Any probs you may have just post here and it would be nice to have a pic of your machine as well, if you can post one or two, as all members like to see them.
Once again,