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alan chadwick 116/07/2020 18:44:30
2 forum posts
3 photos

Hi, not a model engineer at all, but seeking advice which I suspect some members of this forum might have.

Basically, I'm trying to repair a hydrostatic gearbox on a ride-on mower, the said gearbox was supposedly designed not to be repairable - basically just fit a new one because the labour involved was too high to make it economic. Problem is you can't buy them these days!

So my ride on gave up the ghost the other day and I stripped the unrepairable gearbox down and found the fault was that one of the pistons powering the hydrostatic gearbox had failed - see photo.. Basically there are 5 of these pistons, the piston is loosely attached to the plate by what can be best described as a rivet. In my case the rivet had broken. In the photo, the top half of the rivet is in a recessed part of the curved plate and the bottom half is half way inside the piston.

My plan is to fabricate a rivet with a conventional rivet head at the top (matching the profile of the existing one as near as |I can, BUT for the part inside the piston I was thinking of making up a small collar with ID slightly smaller than the rivet shaft. The I would heat the collar to red heat, freeze the rivet and push the rivet through the piston into the collar.

Just wondered if anyone thinks this would work?

Many thanks

Alan Cimg_5885.jpg

john halfpenny16/07/2020 19:00:05
57 forum posts
10 photos

Well, it's got two chances. Not very helpful I know, but an unknown.

alan-lloyd16/07/2020 19:39:14
168 forum posts

what make of mower is it, please?

Oily Rag16/07/2020 20:26:17
161 forum posts
77 photos

I assume the curved plate is the 'slipper' plate for the swash plate?

Could not the head part of the rivet, the part in the piston, be punched over if you had a new unbroken rivet?

It would also help to know the rivet material and the dimensions of the head and shaft, a picture of the broken parts might help as would an end view into the piston.

alan chadwick 117/07/2020 12:01:55
2 forum posts
3 photos


The mower is aCountax, although the hydrostatic axle in question was used in a number of other models.

Another couple of photos.

On the left is the top and bottom view of intact piston assemblies, on the right are the 3 components, piston, curver rectangular plate and bent and very broken "rivet" - the head is probably 5 or so mm and the shaft 2 to 3 mm.


This photo, on the left shows the top of the piston, the rectangular plate sits in this cup and and the "rivet" passes through the plate and the piston. The right hand photo shows the view looking up into the piston, the rivet is obviously in the centre and appears to be surrounded by a brass looking collet (??) which presumably grips the bottom of the rivet.

Any advice/comments welcome


Howard Lewis17/07/2020 12:40:22
3627 forum posts
2 photos

Given the sizes of the parts, "heat and freeze" techniques, to produce an interference fit, are going to require some extremely tight tolerances.

As an alternative, could not the replacement rivet be made with a small clearance (say 0.025 / 0.050 mm) and after everything has been thoroughly degreased, be coated with a high strength anaerobic and assembled?


Paul Kemp17/07/2020 22:05:05
562 forum posts
18 photos

I used to work on Countax mowers many moons ago, probably about the same vintage of your mower!! I seem to remember the transmissions were Eaton units and probably still available? Just did a quick search that showed up several current models that look vaguely familiar to me. That probably won't help much as I don't think they ever sold parts for them but.... If you can identify the marque you may find a used one that you could either use or rob for bits if it doesn't work.

Doesnt answer your question I know but it may be another route worth looking at.


Paul Kemp17/07/2020 22:05:47
562 forum posts
18 photos

Ha, deleted, just managed my first double post!


Edited By Paul Kemp on 17/07/2020 22:07:16

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