Mods - can we make this a Sticky?
I will resize the large images shortly
This post attempts to draw together a staggering amount of information that is freely available on forums such as this about how to build a sturdy SAAB transmission. None of this information is mine but comes from a large group of contributors on the various forums. Particular contribution came from Dave Barrow (including most of the photos) and Emmett Jenner as well as a legion of regular contributors including birdiemango, old_saab_man, Ericsson, nutcase and too many others to mention...you know who you are!
The main issues with the gearbox are its tendency to flex at high torque and internal gearbox temperatures and lubrication failure.
Part 1. The Case
Chillcast
When it comes to talk of gearboxes the talk inevitably comes around to the elusive Chillcast case (or “kokill” if you are scouring the Swedish Ebay).
Chillcasting is a technique used in the casting process to control the rate of cooling to introduce a stronger grain formation in the alloy. For that reason they are deemed to be a stiffer box.
These 4 speed boxes were fitted in early 99’s from 1975 to around 1979. Being 4 speed they are perfectly appropriate for Track use although it is possible to fit a 5 speed pinion shaft and primary housing to convert to a 5-
speed Chillcast gearbox. Depending on the model year of the parts used this may or may not need slight modifications to the box to allow it all to fit.
There are three 4-speed Chillcast boxes – the most common being 870423 and 871239, the first number is the strongest of the two and should be used for high HP and torque. But, there are many sub-models and variations and not every Chillcast box is the same. There is also a Chillcast pinion housing and a Chillcast primary housing. It is rare to find all parts together on one gearbox.
870423 and 871239. Photo: D Barrow
What you see in both of the images is the three ridges that run down the primary casing. Each ridge is roughly 3cm wide with a 1cm gap between. On any other gearbox the ridges are about 1cm wide with a 3cm gap. This is the first thing to look for although it is no help if the gearbox is still mounted in the car!
The stronger of the two boxes shows three diagonal ridges on the corner of the oil pan. This is what you look for showing you it is the stronger variant.
If the box is in the car you will find the part number embossed on bottom.
Chillcast 870423 number stamp Photo: D Barrow
The stronger chillcasts seem to be found in ’77-’79. The date stamp is often found in the side. This one is from 1976:
Chillcast date stamp Photo: D Barrow
The third Chillcast box is from the early 99’s around 1972, part number 870352, it looks identical to the later box – with the three diagonal ridges – but you cannot fit the Chillcast pinion housing (more on that later) to this box, because reverse gear on the pinion shaft will bottom out on the inside of the casing.
5 Speed Chillcast
Converting a 4speed case to 5 speed can be done by simply using the primary gear housing from a 5 speed box. Some modifications may be needed to the inside of the Chillcast casing to allow the internals to fit but this depends on which parts you are using.
Of the 5 speed primary gear casings anything after ’88 is good as from then onwards the input shaft was made larger and an additional bearing set was added to the upper primary gear.
Alternatives
A Chillcast gearbox is not the only option however. Over the years SAAB made several changes to both the box and the internals. More recent gearboxes have a higher magnesium content in them which makes them brittle and more susceptible to corrosion but they are significantly lighter than the chillcasts.
Of the non-chillcast boxes the later years are stiffer than the earlier models.
Modifications
Two options for modifying the case have been welding extra support or fabricated cradles.
Welding extra alu sections to the exterior of the case has potential to increase stiffness but welding cast aluminium is not so straightforward; cast alloy has lots of voids which does not make for a particularly good weld and heating aluminium above 450° can cause it to loose its temper (not ‘it gets angry!’ ) which can actually make it weaker. Not that it can’t be done. Below is a photo of Will Gollop’s gearbox case:
Welded Strengthening. Photo D. Barrow
A few people have constructed cradles which support the outside of the case – most of the flex is at the differential area. Steel diff plates (see below) are extended to ‘wrap’ around the sides of the gearbox to provide additional stiffness.
A more recent consideration is cryo-treating the case. Aluminium is extremely responsive to cryo-treatment and this could be an avenue worth pursuing.
I will resize the large images shortly
This post attempts to draw together a staggering amount of information that is freely available on forums such as this about how to build a sturdy SAAB transmission. None of this information is mine but comes from a large group of contributors on the various forums. Particular contribution came from Dave Barrow (including most of the photos) and Emmett Jenner as well as a legion of regular contributors including birdiemango, old_saab_man, Ericsson, nutcase and too many others to mention...you know who you are!
The main issues with the gearbox are its tendency to flex at high torque and internal gearbox temperatures and lubrication failure.
Part 1. The Case
Chillcast
When it comes to talk of gearboxes the talk inevitably comes around to the elusive Chillcast case (or “kokill” if you are scouring the Swedish Ebay).
Chillcasting is a technique used in the casting process to control the rate of cooling to introduce a stronger grain formation in the alloy. For that reason they are deemed to be a stiffer box.
These 4 speed boxes were fitted in early 99’s from 1975 to around 1979. Being 4 speed they are perfectly appropriate for Track use although it is possible to fit a 5 speed pinion shaft and primary housing to convert to a 5-
speed Chillcast gearbox. Depending on the model year of the parts used this may or may not need slight modifications to the box to allow it all to fit.
There are three 4-speed Chillcast boxes – the most common being 870423 and 871239, the first number is the strongest of the two and should be used for high HP and torque. But, there are many sub-models and variations and not every Chillcast box is the same. There is also a Chillcast pinion housing and a Chillcast primary housing. It is rare to find all parts together on one gearbox.
870423 and 871239. Photo: D Barrow
What you see in both of the images is the three ridges that run down the primary casing. Each ridge is roughly 3cm wide with a 1cm gap between. On any other gearbox the ridges are about 1cm wide with a 3cm gap. This is the first thing to look for although it is no help if the gearbox is still mounted in the car!
The stronger of the two boxes shows three diagonal ridges on the corner of the oil pan. This is what you look for showing you it is the stronger variant.
If the box is in the car you will find the part number embossed on bottom.
Chillcast 870423 number stamp Photo: D Barrow
The stronger chillcasts seem to be found in ’77-’79. The date stamp is often found in the side. This one is from 1976:
Chillcast date stamp Photo: D Barrow
The third Chillcast box is from the early 99’s around 1972, part number 870352, it looks identical to the later box – with the three diagonal ridges – but you cannot fit the Chillcast pinion housing (more on that later) to this box, because reverse gear on the pinion shaft will bottom out on the inside of the casing.
5 Speed Chillcast
Converting a 4speed case to 5 speed can be done by simply using the primary gear housing from a 5 speed box. Some modifications may be needed to the inside of the Chillcast casing to allow the internals to fit but this depends on which parts you are using.
Of the 5 speed primary gear casings anything after ’88 is good as from then onwards the input shaft was made larger and an additional bearing set was added to the upper primary gear.
Alternatives
A Chillcast gearbox is not the only option however. Over the years SAAB made several changes to both the box and the internals. More recent gearboxes have a higher magnesium content in them which makes them brittle and more susceptible to corrosion but they are significantly lighter than the chillcasts.
Of the non-chillcast boxes the later years are stiffer than the earlier models.
- The 89-'90 box would be the strongest of the more recent gearboxes as they have a larger rear pinion bearing and stronger shift forks than the later boxes - but the pinion bearing housing is not as strong.
- The 91-93 gear box also has a good reputation. They slimmed down the profile of the synchros and enlarged the gear faces on these boxes. The pinion bearing and layshaft bearings were also enlarged.
Modifications
Two options for modifying the case have been welding extra support or fabricated cradles.
Welding extra alu sections to the exterior of the case has potential to increase stiffness but welding cast aluminium is not so straightforward; cast alloy has lots of voids which does not make for a particularly good weld and heating aluminium above 450° can cause it to loose its temper (not ‘it gets angry!’ ) which can actually make it weaker. Not that it can’t be done. Below is a photo of Will Gollop’s gearbox case:
Welded Strengthening. Photo D. Barrow
A few people have constructed cradles which support the outside of the case – most of the flex is at the differential area. Steel diff plates (see below) are extended to ‘wrap’ around the sides of the gearbox to provide additional stiffness.
A more recent consideration is cryo-treating the case. Aluminium is extremely responsive to cryo-treatment and this could be an avenue worth pursuing.