GM2 Hydraulic Motor Specs

I haven’t been doing a great deal to the site for quite a while now, but have been trying to pin down some methods of keeping older RT45’s alive. 

Despite the lack of new material we still seem to have a respectable number of visitors.

My machine is fitted with two old SAI MT500 drive motors. They weep a bit of oil and show some signs that seals are on the way out. Seals are the only parts for these motors which can be sourced new today. 

Failure of the drive motors seems to be a common enough problem and without replacement parts you can find yourself with a useless machine which has no immediate prospect of repair.

The latest SAI motor of the same family, the GM3A is considerably larger than the MT500 and looks as though it would require at least the relocation of the fuel tank, oil tank and the muffler of the diesel engine in order to fit.

I have had some correspondence with SAI and one of its Australian suppliers. From that it appears that the smaller and lighter GM2 500  motor would actually provide better performance than the original MT500s. Most of the engineering required to fit looks like changes to the hydraulic lines to suit the distributor on the new motor and the machining of the original or fabrication of a new, mount to the brake housing etc.

  • The solution looks quite doable but would be expensive as SAI retails the GM2 at over six thousand spondulas a motor.  Given that the existing motors in the older RT45s have seen service since being turned out in the seventies the cost of the replacement could well be justified in terms of projected service life.
  • On the other hand there are quite a few purveyors of GM2 copies based in China and Taiwan. I checked with one producer who gave me a price of $580 USD per motor plus freight.
  • The big issue is whether you can trust Chinese hydraulics?

Any thoughts on this would be appreciated.

Replacement of the newer motors in late model RT45s is complicated by the fact that the assemblies are a combination of a proprietary Austrak casing and third party internals which can be very hard to identify. Most of us will have seen the machine on Ebay which was in that situation and I know of another which could be in the same boat.

With the Wildcat I have often wondered how the parts situation is going for those neat Daiken-Sunstrand transmissions and how they could be replaced if parts are no longer available.

If you have any ideas about keeping our machines functioning, or have come up with mods that keep the old clunkers running, please post it here as we can all benefit from each others knowledge and experience.


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  1. Jeff Bussell says:

    Hi all i will have soon a drive motor/brake assy to suit newer austraks for sale as i cant find a new motor for other side so doing a conversion using rexroth angle drive motors replacing the missing motor and the this unit, it is a shaft drive vmhr494 powered shaft drive austrak, 1987, approx 4700hrs, motor was in a good running condition when taken out no leaks or metal in system, if one desires i can reconnect hose’s & a flow meter to inspect it running. from what i can gather/source the internals have southcott components/austrak casing. motor/brake assy,
    serial number: 129w-89-157, model number: 137-505-004
    Im welcome to offers/inquires please ring me on 0408293427
    cheers Jeff.

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