OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Reel-Type Mowers Diplomat 590 Repair

Having started on an Alroh 17BDB a few weeks ago for my father in law I now reckon it is too much machine for him (at nearly 80)so I started looking for an alternative. A Diplomat 590 for $50 seemed a good starting point (knowing that it was without drive and the original catcher).
The '76 model Briggs 2.5hp is a little smokey on start up but only took a few pulls to actually get going.
As an added bonus the seller chucked in a dead Masport Lawn Marshal with an '04 Briggs Classic 35 for nothing.
First upgrade to the Diplomat has now been done - it now has the Magnetron ignition from the Masport installed.

Hi Diablo,

The Scott Bonnar Model 590 was first introduced in 1976 and was manufactured in the Thebarton factory in South Australia. Both the Model 430 and 590 were a radical design departure from the cylinder mowers that were popular at the time...eg. the Model 45.
In some quarters, they are considered an abject failure, and most spare parts are now like hens teeth, some almost impossible to come by.

If you could post the model, type and code numbers stamped into the metal air cowling of the Briggs engine, we can give you an approximate age of your machine.
The numbers are in the location usually near the intake manifold...please post the whole string of numbers.


There is a good thread here on the Model 590 drive problems that may prove of interest to you....Click HERE

If you are chasing a metal crown drive wheel for this machine, I will send you a PM with the details, just look for a flashing envelope in the toolbar above next to 'My Stuff'.
Hoping this is of help,

Thanks for the info Deejay. I had already read the post you recommended and it was the detail in there that made me decide I could fix the drive problem. The engine code details are: 80102 0233-01 76040901.

The engine was made on 9 April 1976, and the mower was probably made say 6 months later, though the exact lag would depend on shipping schedules from the US, and the amount of engine stock held by SB at the time.

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6jmvGVJ1DajI.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/lrvvA-wU7eL7hAxB91ObQ.pdf

That engine has the Vacu-Jet carburetor, a simplified version compared with the Pulsa-Jet, which had a fuel pump built-in. Your engine has a shallow fuel tank, and the (pump-less) Vacu-Jet is perfectly adequate for that situation.

You will be aware that your engine, being a first design generation type (second digit of the model number is zero) has a metal tab which flips across and shorts the top of the spark plug to ground, rather than a conventional kill switch. It also has a pull-out choke of a type which I have always found rather awkward. Generally though, it should be both durable and very reliable.

Hi Diablo,
This machine could be of historical interest to the Model 590 archive.
Your Briggs engine was manufactured on the 9th April 1976. This is interesting because the exact launch date of the Scott Bonnar Model 590 is known. We know the following is fact:
" On Wednesday 7th July, 1976 at the Scott Bonnar factory at Thebarton, the Premier of South Australia, Mr Don Dunstan, launched the revolutionary Scott Bonnar Model 590 Rear Catcher Reel Mower. This was the first mower to win an Industrial Design Award from the Industrial Design Council of Australia."

Could your machine be the one he pushed through the ribbon on that very day? I guess we will never know!
All very interesting.

Please tell me he not only pushed the mower through the ribbon but he actually cut it with the mower
Here is a pic of the ID plate.

Hi Diablo,

I am led to believe that Don did actually cut the ribbon with the machine....and also that a photo of the occasion was taken...I am doing some research to try and find a copy.

Unfortunately, we will never know exactly which machine is was....but it could have been yours. the timeline fits.

Made a good start on stripping the machine down to fix the drive ring gear tonight. It went smoothly except for removing the engine. I cant get the engine pto shaft to separate from mower drive shaft. I removed the grub screw, drowned the parts in RP7 and still no joy. I used 2 blocks of wood, 1 against the engine, the other against the driveshaft and tapped a ball joint separator between two but it still didn't drive them apart. In the end I left them stuck together and moved onto the ring gear. Here is the end result of my efforts:

Well, the ring gear was trashed so you haven't lost anything by having it break during removal. I'm told they usually do that, so you shouldn't feel badly. Now for a metal one. In my view the aluminium one is better than the stamped steel one, but it is no longer available.

Hi Diablo and grumpy,
I have never seen an aluminium ring gear for a 590 or Diplomat 430. Turf Machinery Services that was fabricating the stamped steel version has ceased trading....so here's hoping that they are still available.

We've had a couple reported on Outdoorking Deejay, this was the most recent:
https://www.outdoorking-forum.com.au/forum/u...ords=aluminium&Search=true#Post47397

Just from what I've seen in threads, the stamped steel gears only appeared after the aluminium ones became unavailable. The aluminium ones had cut, involute-form teeth, not the simple triangular cogs/pegs the steel gears use. They should have been a great deal gentler on the mating pinion.

Hi grumpy, sorry mate, I should have made myself clear...I have heard of the alloy gear, but I have never laid eyes on one...I don't know how they were fabricated nor the tooth form or who manufactured them. As Keith has gone out of business (TMS) we could be in trouble here.
We have no parts for the 430/590 models in the online Store.

Our earliest thread on that gear isn't completely clear but it seems to suggest that Scott Bonnar may have replaced the nylon gear with the aluminium one during production as a running change in design:
https://www.outdoorking-forum.com.au/forum/u...amp;Words=gear&Search=true#Post16128

Bruce may remember whether this was so.

So it sounds like I need to be on the lookout for a later model diplomat that is good for parts. It would be great to confirm if the change happened and when so I can have an educated guess when looking at the mower (based on engine production date and maybe Scott Bonnar serial number).

I have spoken to Bruce, he couldn't remember whether it was a factory replacement part or after-market.

However I just searched the SB 590 Parts List and found that this page has a pen amendment on it that says"Used to be nylon now alloy"

This sheet came from Rover...see the header and date.
So it must have been a factory produced part.

What a shame that they are no longer available.

Hi Deejay, thanks for the further info. By the way when did Scott Bonnar cease and Rover take over? Thus will give me a date reference to work back from. I had a look in the company history section and did not find anything about this change.

Don't worry about the question above. A google search found an ODK page with the info I was seeking.

Deejay, that is the reason I've been pretty confident the aluminium gears had a proper involute tooth-form. I can't imagine SB in those days, selling gears with triangular teeth.

Hi grumpy,
It looks like it was only available as a spare part, there was no product recall for retro-fitment, so perhaps it was available to mower repairers to fit a machine with a broken nylon gear. Anyway, it is consigned to history now as spares for the 430/590 are now almost extinct.

A bit more progress tonight. Started with separating the rollers and got most of it done. I still need to get the right roller off the shaft but the bearing will not be moved just yet. In the mean time I made a 'pusher', the opposite of a puller, to separate the engine from the driveshaft. I tightened the nuts on each bolt bit by bit and could not get over just how much they had to be tightened. They gradually crushed the plywood packers (to spread the load on the engine) and wasn't sure they were pushing the engine and driveshaft apart. I undid the 'pusher' after seeing fhe ply crushed so much and half expected no movement in the parts but it had moved a small bit. I reset it but this time I also used some steel packers between the bolt and the ply. I also gave the parts regular sprays of RP7 and it finally moved things apart a bit quicker until they separated completely.

Here are the before and after shots which show the home-made 'pusher'. I had terrible getting them to post in the reply above.

That was an ingenious solution, Diablo, well done.

If you have difficulty adding pictures, check the window beside the word "Markup". If it does not include "using UBB code", operate the dropdown menu beside it and tick either "using UBB code" or "using HTML and UBB code". The picture-adding software is a special feature Bruce had written by the UBB programmer for Outdoorking, and it relies on being able to use UBB code.

Thanks for the compliment grumpy. Always a great feeling to come up with a solution to a problem and have it work as expected. Although it did also require a few taps on the tool to break the grip enough for the bolts to do the rest of the work. And best of all it was $3 for the angle bracket and $6 for the bolts. Money well spent.

As for the photo upload problem, I think it was due fo a network problem with my phone company. Living in a semi-rural area means data speeds are poor.

Almost to the starting point of the rebuild phase.

I would have got a bit more done today but I have to prune a few branches and needed something to help with the job (thank you gumtree). I already have a Honda line trimmer but it has no pruner option.

I've finally returned to the Diplomat. I have considered quite a few options to fix the internal ring gear that provides the drive and narrowed it down to two. First is making a silicone mold of the part and then casting a replacement in polyurethane around the boss. This would give an almost exact replica of the original. The second is the one I decided on doing because I had the parts. I have cleaned up the ring gear and shaped a piece of 4mm plate steel to serve as the connector between the boss and ring gear. I will weld the boss the the centre of the connector plate and then place the ring gear around that. To stop back and forth movement of the ring gear I will drill a number of 4mm holes from the outside to the centre. Into these I will insert pins that align with and will be welded to the connector plate. I will post more pics as I progress but here are a couple of the parts as they are now.

Oh forgot to mention. By taking this option if I hit any problems then the rebuilt part will be better suited to use for making the silicone mold. Always good to have a back up plan. And I may just make the mold to test the first option anyway. I've done some epoxy resin casting for my windsurfing fins but not tried polyurethane so it would be a good test.

It will be interesting to see what happens. I am not confident that polyurethane would be strong enough to make a functional gear, but I haven't tried it. Meanwhile your project is very interesting: where did you get the steel gear you have used as a mounting for the ring gear made from the old nylon gear?

If you can find a practical solution to the gear replacement problem for the SB430 and 590, I think quite a few people will be keenly interested.

Hi grumpy,
My research indicates that you can get room temperature cure 2-part pourable polyurehtane with a shore d hardness just under nylon (which I am assuming is the material used for the original part. Even allowing for the slightly softer material I expect the part to last long enough to justify the time/effort/money in making it and once the mold is done I can turn out a spare or two. Being slightly softer it may not be as brittle as the original and I will only have to deal with wear not breakage.
As for the metal gear, that is just 4mm mild steel plate I had lying around that I have cut by hand.

Hand-filing about 60 teeth to a decent-looking profile must have been a hassle, Diablo, I don't think I'd have been prepared to do it, nor would I have ended up with all of the teeth seeming to fit pretty well. Congratulations.

I'll with-hold judgement on the viability of poured urethane gears - I don't think the Shore hardness gives a reliable indication of the stress and durability capabilities, though it is certainly a useful start. I hope you make one and report here on its performance, it would be a very interesting, and potentially very useful, project. I have never shared Deejay's dislike of the SB430 (I'm not so keen on the SB590, due to the lack of a second clutch) and it seems to me if the drum gear breakage problem, and the catcher availability problem, could be solved these would become quite interesting mowers.

Having to resort to hand filing is what took me so long to decide on this option and they are not quite done. Also a close up would show they are a bit rough. So much so I'm thinking of redoing it. I placed the ring gear on the plate and used a marker to outline the teeth then cut them out. I think I need to measure and mark each tooth and then drill each point so it is more uniform. There will still be sme hand filing but the end result will be a lot better.