This is a guide showing how to replace the primary drive on a Saab 900 gearbox.
Why would you want to replace the primary drive?
The primary drive transfers the engines power from the flywheel through to the gearbox and the size of the gears used in the primary drive controls the engine speed v.s. wheel speed ratio. Saab labelled the gearboxes so you can see what the primary drive ratio is on the gearbox. They used numbers from 4 through to 8. With a type-4 primary drive the engine has to turn fast to make the wheels go slow. With a type-8 primary drive the engine turns slow and the wheels turn fast.
Saab made quite a few different primary ratios over the years. The primary drive ratio needs to match the engine performance. If you have a powerful T16 engine you need a type-7 or type-8 primary drive as the engine is powerful enough to move you easily with that ratio. If you have a base spec 8v carburettor model you need something like a type-6 primary drive or else the engine will struggle to move the car. The basic design of the box carried over from the Saab 99 and the same principles apply to the boxes used in those cars.
In the real world the times that primary drive is an issue is cruising on the motorway at 70 or 80 mph and when you’re accelerating through 1st and 2nd gear as you pull-away. The type-8 primary drive gives you a nice low RPM for motorway cruising, keeps the wheel-spin down a little and makes the gears last a slight bit longer during 1st and 2nd. When the wrong gearbox is mated to the wrong engine you get an odd miss-match where the wheels spin faster or slower than they should relative to the engine speed.
So that’s the conjecture out of the way and now I’m going to talk you through the method I used for changing from type-6 to type-8 primaries. Usual disclaimer of follow this guide at your own risk and don’t come running to me after you’ve dropped a gearbox on your feet. I can’t be responsible for any injuries you might cause yourself. I suggest you read the guide the whole way through before you start so you know the process. Hopefully everything in the guide is accurate but please to correct if you spot a mistake.
You can replace the primary drive with the engine in the car but you would need to remove the front bumper, headlights and radiator to get to clearance to operate at the front of the gearbox. In my scenario I’m also replacing the gearbox so I’ve got the engine out in any case. The transmission is resting on an old chair.
1. Loosen the 12mm bolts securing the cover at the front of the gearbox. I tend to find a good sharp ‘crack’ with a breaker bar is my preferred method for loosening old fasteners. Slow and imprecise spanner work can round-off the head on the bolt and then you’re a bit stuck so try to avoid that.
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2. Use a screwdriver or chisel to prise the cover off the front of the gearbox. You shouldn’t need much force and you could yank it off by hand but then the oil would spill everywhere and it’s better if that doesn’t happen.
![Image]()
3. Use an old container to catch the oil as it drips out the bottom of the primary drive case. The gearbox contains about 3 litres in total but only ½ a litre of that will leak out at this point. Be careful though as used gearbox oil has a nasty smell and you don’t want it spilling everywhere.
![Image]()
4. Take the cover off the primary drive case and put it down in such a way that it doesn’t get dirty. i.e. not on the concrete floor. You need to keep the gearbox internals completely grit-free.
![Image]()
5. Here’s how it looks under the cover. The sprocket at the top accepts the power from the engine and transmits it through to the lower sprocket which feeds through to the gearset. There is a chain tensioner in the middle pushing the chains outward to keep them taut.
![Image]()
Why would you want to replace the primary drive?
The primary drive transfers the engines power from the flywheel through to the gearbox and the size of the gears used in the primary drive controls the engine speed v.s. wheel speed ratio. Saab labelled the gearboxes so you can see what the primary drive ratio is on the gearbox. They used numbers from 4 through to 8. With a type-4 primary drive the engine has to turn fast to make the wheels go slow. With a type-8 primary drive the engine turns slow and the wheels turn fast.
Saab made quite a few different primary ratios over the years. The primary drive ratio needs to match the engine performance. If you have a powerful T16 engine you need a type-7 or type-8 primary drive as the engine is powerful enough to move you easily with that ratio. If you have a base spec 8v carburettor model you need something like a type-6 primary drive or else the engine will struggle to move the car. The basic design of the box carried over from the Saab 99 and the same principles apply to the boxes used in those cars.
In the real world the times that primary drive is an issue is cruising on the motorway at 70 or 80 mph and when you’re accelerating through 1st and 2nd gear as you pull-away. The type-8 primary drive gives you a nice low RPM for motorway cruising, keeps the wheel-spin down a little and makes the gears last a slight bit longer during 1st and 2nd. When the wrong gearbox is mated to the wrong engine you get an odd miss-match where the wheels spin faster or slower than they should relative to the engine speed.
So that’s the conjecture out of the way and now I’m going to talk you through the method I used for changing from type-6 to type-8 primaries. Usual disclaimer of follow this guide at your own risk and don’t come running to me after you’ve dropped a gearbox on your feet. I can’t be responsible for any injuries you might cause yourself. I suggest you read the guide the whole way through before you start so you know the process. Hopefully everything in the guide is accurate but please to correct if you spot a mistake.
You can replace the primary drive with the engine in the car but you would need to remove the front bumper, headlights and radiator to get to clearance to operate at the front of the gearbox. In my scenario I’m also replacing the gearbox so I’ve got the engine out in any case. The transmission is resting on an old chair.
1. Loosen the 12mm bolts securing the cover at the front of the gearbox. I tend to find a good sharp ‘crack’ with a breaker bar is my preferred method for loosening old fasteners. Slow and imprecise spanner work can round-off the head on the bolt and then you’re a bit stuck so try to avoid that.
2. Use a screwdriver or chisel to prise the cover off the front of the gearbox. You shouldn’t need much force and you could yank it off by hand but then the oil would spill everywhere and it’s better if that doesn’t happen.
3. Use an old container to catch the oil as it drips out the bottom of the primary drive case. The gearbox contains about 3 litres in total but only ½ a litre of that will leak out at this point. Be careful though as used gearbox oil has a nasty smell and you don’t want it spilling everywhere.
4. Take the cover off the primary drive case and put it down in such a way that it doesn’t get dirty. i.e. not on the concrete floor. You need to keep the gearbox internals completely grit-free.
5. Here’s how it looks under the cover. The sprocket at the top accepts the power from the engine and transmits it through to the lower sprocket which feeds through to the gearset. There is a chain tensioner in the middle pushing the chains outward to keep them taut.
