Let’s change the gearbox, from the original FM57 to an FM55.

I had various reasons in mind for this change, the key one is I wanted shorter gears, the old FM57 gearbox is standard for the LPT and diesel models, and thus sports very long gears that are not best suited for a 310Hp car, the other reason was suffering from a noisy gearbox with some play in the differential bushing, that equates to some inexact gearing..

I got a gearbox from a 2003 93 Aero, reference nbr. FM55507 (the last one made for these models, with increased resistance to torque)

Here below, the gears ratios on the two different boxes, FM55 and FM57:


New box below....




To begin, time to take the driveshafts out.



Next take down the sub-chassis



And here below, the gearbox is ready to be taken out, we can notice the differential bushing is not round but oval, the hum and increased vibrations we felt are partly du to this.
Beware when taking the box down, make sure to disconnect the negative earth cable, the reverse indicator plug, and the gear linkage.




Down with it,



The clutch is in perfect shape, so after a good an thorough clean up...



And furthermore, a good clean-up and inspection of the new FM55 gearbox, we check that everything is correct, and push the box in, position the dowels, and tighten the bolts, put back the starter and the supports...



Tighten all bolts, put back the driveshafts, remeber to not connect any cabling nor the linkage at this stage, wait until the sub-chassis is in position.



Once the sub-chassis is mounted, connect the reverse indicator plug, the earth cable, and the linkage (don’t tighten the linkage, as we will have to adjusted when the car is almost done.)

The box takes 1,9 Liters of SAAB or GM gearbox synth oil, fill it up, check level, and tighten the plugs.



Main differences?

  • No unwanted noise now and less vibrations.
  • Gear ratios are shorter.
  • Better gear change feeling.
  • Sportier and exhilarating to drive.