OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Hello Everyone

Hi,

I joined today because I have an old Scott Bonnar model 45 that use to belong to my grandpa and I wan't to restore it.
Hopefully, this forum will provide some tips, info and help on my journey.








Hello to everyone!

Welcome to ODK Xhall,

We are very lucky to have a few SB Reel experts on the forum, and I'm Sure when they come to your post shortly, each will be only to happy in answering any questions you have, and help to make your restoration journey a smooth and enjoyable one.

Mal...

Hi xhall,

Welcome tom the ODK forum. I have looked at the images that you posted and noted that the cylinder still has a lot of life left.

My suggestion would be to have the cylinder ground as well replacing the bottom blade and ground as well.

If setup correctly (the cylinder and bottom blade) you will have a fantastic looking lawn.

Hi xhall, and a warm welcome to the forum. It's always nice to have another Scott Bonnar Model 45 owner on board, as we all like to learn new things and tinker here.
What a wonderful feeling you will have when your SB 45 is restored to its former glory....Your Grandad would be proud!
Your machine is the later twin-rail Series 2 design and is period correct in original condition, except for one minor difference....the knob on the cutter clutch engage handle should be black rather than red. This can be easily corrected.
We can't tell you the exact date when your machine was manufactured, but we can get close, if you could post the whole string of numbers stamped into the metal air-cowling of the Briggs engine...See the pic here for the location:

Looking forward to helping you with your resto,

Once again, to OutdoorKing.

Hi Deejay,

Unfortunately, the knob on the clutch release is the least of my problems . Well...at least I have hit the ground running. Attached is an image of the pull-start clutch cover and some other shots.













Unfortunately, I have been unable to get the motor to turn over (this is after dumping the old fuel and putting fresh fuel in the tank).
There doesn't seem to be any spark either (testing for spark with a brand new plug from a whipper snipper), but there is some compression. So, I have already started to disassemble and clean the motor and carburetor. In the process of checking/resetting the gap between the points and condenser. From the looks of it the captive cotter nut is missing and there has been damaged done to the key way on the output shaft, probably when the cotter nut got thrown from the clutch. The clutch half also sits very loose on the output shaft (also had to file the burrs off so the key would fit) and looks like the flywheel key needs to be replaced as it has been crushed on both sides and is no longer rectangular.

My main focus now is to restore the engine to working order, as my dad says it was working at some stage 3-4 years ago.
Do you think that this restoration might be too costly, given the condition of the motor, clutch, cutter cylinder and bottom blade?
Just checked for a new crankshaft #497232, $104US + delivery.

Hello,

So today I took off the cylinder head and the bore of the cylinder is perfect, no scratches/scoring. Can still see the honing marks.
Is it safe to re-use the head gasket? I am going to try cleaning and lapping the valves and valve ports, remove all carbon deposits using a soda blaster and sandblast and powder coat the block and head. Inside the tank looks good except for a couple of rust spots. Here are some more pics... If the engine comes good, I will definitely get the cylinder and bottom blade sharpened.

Answering the easy part first, that metal-covered-both-sides head gasket can normally be re-used safely, provided it doesn't show any signs of coming part and has never leaked, especially through the middle layer between the metal layers.

Another easy one: the diaphragm from the carburetor is long, long past its use-by date and needs to be dropped into the skip with a long pair of tongs, so you won't get nastiness on your fingers.

The crankshaft extension (called the Power Take-Off, or PTO) is fairly severely damaged at the keyway. To become a good crankshaft it would have to have the keyway milled out wider then you'd have to make a stepped key, to fit both it and the clutch that slides onto it. This is not worth doing except to a hobbyist-machinist with a milling machine and time on his hands. Generally, the most economical way to repair that mower is to get a second-hand engine of the same model, and just keep a few spare parts from the original engine.

The saddest part of the story is the clutch that mounts on the PTO. Over time the captive cotter has run loose, that condition was neglected, and now the crankshaft, engine-side clutch half, and key are all destroyed from the clutch rattling and shaking on the PTO for a period of years. By some miracle the mower's frame rails do not look cracked (they are usually cracked on SB45s with loose clutches, apparently due to the vibration). Most likely the thrust bearing in the center of the clutch will have been damaged: the key had probably slid along the keyway until it hit the thrust bearing.

So, the simplest way to address your situation is to procure a second-hand engine, a new key, a new engine side clutch half, and probably a new thrust bearing but this awaits confirmation when you dismantle the clutch. You will also need to check the clutch generally, and the self-aligning ballrace at the sprocket end of the clutch shaft.

Hello Grumpy,

So, it looks like I have multiple problems with this mower then. I checked the frame rails and indeed there is a slight bend/sag in the rear rail. The front one near the cutter is ok though. I may already have a spare engine that I can use for parts but I don't know if its compatible. Its a newer 3hp Briggs and is being used on a edger, it has the newer electronic ignition, a magnetron and it is also very hard to start. Do you think I can swap out the crankshaft between the two engines, or should I just restore this one and transfer it to the Scott Bonnar?

The crankshafts are likely to be interchangeable, and so are the complete engines, which have build dates only 2 years apart, but the green one is a later model as well as having Magnetron electronic ignition. Since without a crankshaft the green engine would be scrap anyway, it seems to come down to a case of whether you are trying to restore your mower (best done by keeping the original base engine) or get it looking decent and running well (best done by using the green engine). Either way, you'd mix and match parts as necessary so long as they were not model-specific.

Can anyone tell me if the crankshafts are indeed identical and can be swapped.I would like to swap them over and should I also swap the connecting rods?

Update: I found out why I wasn't getting any spark. The spike on the clip (metal part that holds the spark plug) that digs into the lead and is crimped on, was bent and not contacting the wire core. So I ended up stripping the insulation back to expose the wires and soldered the wires to the clip. It should work now, coil is showing 2.85kohms on my multimeter, it was 0 prior to soldering. The condenser also holds a charge. Checked the old spark plug that was in the lawnmower on the edger and it works, lots of spark!
Some Pics of the rails, sole plate and catcher:

You have asked whether the crankshafts are identical. Briggs lists dozens of crankshafts in their parts list, sometimes dozens for just one engine size. The differences are some minor detail, and the changes were often made for some reason that is not apparent to people assembling or using the engines. So, I'm not sure anyone can answer your question - the best prospect I can think of is that AVB will know. So far as swapping them is concerned, in cases where the engines are of the same bore and stroke (as they are in this case) and were made close together in time (as, again, they were in this case) you nearly always can swap them, but there can be no guarantee without trying it, or finding someone who has already done it.

Now that you have spark, the next question is whether you have compression and whether it runs. If it has spark and compression but won't run, the first test to make is to put a small spoonful of petrol into the cylinder through the spark plug hole, and see if it starts then. It may only run a couple of seconds, but if so, you know your only problem is in the fuel area, usually the carburetor. We can discuss that more specifically if necessary.

Hello xhall,

Well it seems that you certainly have your hands full there. Most times these old Scotty's are pushed aside for a reason and I'm sure that yours was because it would've vibrated like crazy when it was last running and your Grandad had had enough of it.

A flogged out clutch, no proper contact between the motor and clutch, a bent rear rail and a bottom blade that looks like it's been used for cutting wire and not grass is just to name a few issues that you've encountered. I must admit I would be very surprised if there were absolutely no stress fractures in both rails underneath the green paint. With a bent rear rail like you have there the out of balance motor would've induced some fractures as they always do. I have about 3 machines that are fractured to buggery and are "straight as" in the rail department and believe me I've seen many in my travels assessing other people's machines.

Therefore could you please be so kind and take some good close up shots of around the motor anchoring holes, as this will give me a better idea of how you should approach the chassis part of the project. I won't interfere with the engine side of things as Grumpy is the expert in that department and will be able to best advise you there.

I look forward to the rail shots.

Cheers,
BB

Hello bonnar_bloke,

Here are the pictures of the rails you were asking for... Until I take off the rollers and remove the old rust and paint, I won't be able to assess the condition of the rails completely. There does appear to be some hairline cracks in some places.

At the moment I am focusing on the engine and waiting on some parts to arrive from Amazon. These are new gaskets, keys for the flywheel and PTO shaft and new pump diaphragm.

I finally got around to cleaning the engine today, had been raining all week. I used a soda blaster to clean all the crud off the cylinder head, inside and out. Took about an hour and it was easy to remove the carbon deposits , but very difficult to remove the paint between the cooling fins.

I think soda blasting is gentle on the machined surfaces. But I may have to use a sand blaster to get all the dirt out between the fins. Tomorrow I am going to do the top of the piston and take the valves out and do them.

Does anyone know if you need to degrease the aluminum before priming and is lacquer thinners ok for cleaning? Also, should I apply a primer or not, as I am positive a heat proof paint is available that doesn't require primer, is that correct?

Pics of the prepared cylinder head, almost ready for paint. Might go with red or orange.

Hi xhall,

Firstly thank you for the extra photos as they are a great help. I am though a tad perplexed what the photo of the two holes is as that is not original if it is on the left hand side of the rear rail ? Please elaborate if you can or could you show me a wider shot so I can see where those holes are in relation to the whole machine

Well as I suspected those rails are fractured all over the place and will have to be rewelded using a TIG welder (to reduce heat) by a competent tradesman so that the chassis does not distort. You will have to find someone locally that is certain that he can do the job and not just have a crack and end up with a corkscrewed chassis, thus rendering the chassis as a throw away.

Under no circumstances should this chassis be used as it is. It must be repaired so that the engine and clutch are fully aligned again otherwise the whole exercise has been a waste of time and money.

If you require any further clarification don't hesitate to ask any questions.

Cheers,
BB

On the engine question, can you measure the PTO shaft diameter of the green engine? With that info, we'll be able to tell if a suitable clutch half is available.

All else being equal, I'd fit the green engine if possible; it's an Industrial/Commercial model, with a cast iron cylinder liner, and a dual-element air cleaner. These are much more durable than the 'kool-bore' all-alloy blocks.

Hard to start - the first items to check on a Magnetron Briggs, after fitting a new spark plug, are the valve clearances, and the choke operation.

Hi BB,

Yea, I am not exactly sure why those 2 holes are there either. I do agree that the cracks and damage was caused when the rear rail bent, eventually displacing the alignment between the PTO and clutch. Here is a pic:






Gadge,

The green engine has the same size crankshaft as the red one 5/8" or ~15.8-16mm. On another note, I lapped the cylinder head today using 400 grit wet 'n' dry stuck to a granite slab. It appears that this also has a crack (or a defect in the casting), near one of the bolt holes. Now I am not sure if I am risking damage to the engine if I try and reuse this part. I might end up using the green engine after all and just buy a new edger. What do you think? Pics:

That looks like a minor casting flaw to me, rather than a crack. Not of concern, if it doesn't leak gas to the outside.

You can probably pick up a 'fixer-upper' edger for not much coin, via FleaBay or Dumbtree.
The Victa Tilt-a-cut 2-stroke models are common, well built, and very repairable, if you don't mind the noise! They also do an excellent job, when fitted with the circular blade that the later models used.

Decided to continue with cleaning the top of the block, piston and valves. Came up really good with the soda blaster. Also removed the valves, there was 0.15mm clearance on the intake valve between the end of the stem and push rod. And 0.25mm clearance on the exhaust side.

Looks a bit sludgy in the valve chest there. Probably due to infrequent oil changes, which isn't at all unusual in secondhand mowers.

Correct tappet clearances are intake 0.20-0.28mm; exhaust 0.36-0.44mm. Edit: Incorrect; see next post.

So they'll both need a little off the ends of the stems. If you're going to lap the valve faces, do that job first.
Many blokes do this by setting up a guide on a bench grinder, to hold the valve stem at right angles to the wheel face. This method must be used with care, and a vernier caliper to keep tabs on the valve's overall length. It's very easy to take off too much metal!

There is also an 'oilstone method', which doesn't have that risk. But I won't describe it here, as it doesn't work well with ordinary Silicon Carbide [Carborundum] oilstones. Requires an Aluminium Oxide stone.

Points gap is 0.5mm. Data source: B&S 'Service and Repair Instructions'.