SaabCentral Forums banner

1 - 20 of 73 Posts

161 Posts
Discussion Starter #1 (Edited)
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 cam
e 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 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


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.


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.


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.


161 Posts
Discussion Starter #2
Part 2: Choosing Parts


With the gearbox case decided upon and any modifications you may chose to make, the choice of components can have a cumulative effect on the longevity of the gearbox.

A lot of parts nowadays are becoming NLA but there is still a supply of 99s and 900s being broken for spares and can be found in scrap yards.

Some of the parts are cheap while some, more esoteric, custom items are significantly more expensive.

What follows is not a rigid order of importance but a suggested route based on availability and increasing costs.


  • Over the years various changes were made to the gearsets and other internal parts. Making a wise choice as to the internals you choose adds to the cumulative strength of the gearbox.

    Post 88 gear sets are reputable as the synchos were slimmed down at this point to allow for slightly wider gears. At this point Saab also went back to self centring shift shafts. For a period during the early 80’s Saab
    moved the selector shaft centring mechanism out of the gearbox and onto the floor inside the car. Earlier and later boxes included a self-centring mechanism built into the gearbox.

2. Rear steel diff cover

  • Generally made from ¼” or 3/16“steel it provides a significant increase in stiffness to the rear of the gearbox. These should be fitted with the longest possible bolts. The original bolts will be too short and will have to be replaced. A proper gasket and liquid gasket is recommended during fitting as the case and thick cover might not meet flush when fitted and the thick steel cover will not bend to fit the shape of the gearbox.

    There are various suppliers of these in limited number – in the USA or JamSaab or Malbrad in the UK ( or )

    If you plan to install an oil cooler then the steel diff cover is an ideal place for the inlet and outlet for the oil cooling circuit. Either ask your supplier to fit an inlet and outlet or drill holes in the cover and fit your own. The
    supplier will charge extra for this if offered and may ask you to supply your own fittings.

3. Chillcast pinion housing - or custom steel housing

  • A big area of weakness is the pinion housing. . 5-speed gearboxes from around 1984 to 1987 sometimes came with Chillcast pinion housings (part number 870723) which are made using the Chillcast process instead of normal casting. There is no guarantee that every box from this era will have one so you will have to remove the differential to check. Broken gearboxes are sometimes a good source for parts even if one of the parts has failed inside the box.

Chillcast Pinion Housing on the left. Photo: D Barrow

  • To use the Chillcast pinion housing you will have to use gear sets from before 1991 as the later design is different.

Oil Cooler

  • The design of the gearbox case includes the engine oil sump. There is a lot of hot engine oil in the engine sump and this wraps around one side of the gearbox. At high temperatures the oils properties change which reduce their ability lubricate properly. As such an oil cooler is critical. The oil temperature should not really exceed 75- degrees centigrade if good shifting performance is to be maintained.

  • When thinking about designing an oil cooling system there are some things to consider.

  • First, a list of parts you might include in your design.

    1. The oil cooling radiator.
    2. The electric oil pump.
    3. Hose to link the components
    4. Oil filter
    5. Fittings to join the components and to connect to the gearbox
    6. An oil temperature gauge
    7. A fan for the oil cooler
    8. An oil temperature sensor
    9. A switch to activate the pump and fan

    You will need power for the electric oil pump (which can be thermostatically controlled if you wish) and a suitable location for the cooler – generally either behind one of the holes at the lower front valance of the engine bay or mounted in front of the radiator. Cars with automatic transmission already have a gearbox oil cooler (mounted on the opposite side of the engine bay from the engine oil cooler.) An automatic transmission oil cooler might be used to cool manual transmission oil. All the fittings and the cooler itself could be taken from
    an automatic car.

    The SAAB Works department experimented with various locations for the exit and entry ports for the pipe-work and an ‘oil out’ of the bottom of the diff plate and ‘oil in’ at the top proved to be the most efficient. Initially this seems slightly perverse in that it would appear to circulate only a small amount of oil around the differential – rather than through the entire gearbox – but it is the differential which generates most of the heat in the gearbox oil. Especially so with a Limited Slip Diff. You would also not want this area of the box to be starved of oil. This part of the box is always operating regardless of gear selection.

5. New bearings

  • Bearings wear out. Replace them. New bearings will run smoother and generate less heat inside the gearbox. Timken taper bearings are the bearings Saab originally used for the pinion bearing on the pinion shaft and for mounting the diff. SKF or AFG and other makes of bearings were used in other parts of the gearbox. Gearboxes vary in design but the boxes typically contain around 10 to 15 bearings. To properly rebuild the gearbox you should replace all the bearings but if you wish to economise then perhaps just some of the more major bearings such as those on the diff and the pinion bearings. Be suspicious of cheap gearbox rebuilds. The builder may simply be charging you labour for disassembling, checking and reassembling and the ‘rebuild’ might not include replacement of worn bearings.

    Rebuilding a gearbox is not to be undertaken lightly given the opportunities for error. Precision measurements of positioning and torque settings are critical as is a large dust-free environment to work in. Otherwise get a professional rebuild from an experienced Saab 900 gearbox builder.

6. Steel pinion spacer

  • The pinion crush sleeve is exactly that and can be damaged causing loosening of the pinion bearings. A solid sleeve can be made to the following spec:

    id. 33.77mm
    od 43.00mm
    length 19.20mm.

    This length should set the pre-load on the bearings perfectly. But don’t take my word for it. Check.

161 Posts
Discussion Starter #3
Part 3: Upgrade Ladder

The First Stage of Gearbox Strengthen

At this point cost comes into play. So far – for a few hundred quid you could build a durable gearbox based on:

  • 870423 or 871239 Chillcast Gearbox casing (with ’88-’93 primary case if converting to 5 speed.)

  • ’89-’90 Gearbox casing
  • ‘88 ’91 Gearset
  • Chillcast pinion bearing housing
  • Steel diff plate
  • Oil cooler system

After this the cost goes up as you will need custom work and custom parts to further strengthen the gearbox.

The Second Stage of Gearbox Strengthening

Gear treatments

  • The gears themselves can treated to increase their strength and wear resistance.

    Cryo-treating is a full penetration hardening process which will increase their strength – unlike Tuffriding or Nitriding which are only surface hardening treatments.

    Shot-peening is a stress-relief process which remove minute surface imperfections which can lead to failure. Shot peening should always be the final treatment in any application although rumour has it that Saab gears from the 99/900 series transmissions are already treated.

Steel inner drivers.

  • The inner drivers can be re-manufactured by various tuning companies and machine shops, capable of holding more power transferred to the driveshafts. GKN Motorsport for example.

    A cheaper fix would be to have a steel sleeve pressure fitted to the outside of the drivers.

    Similarly machining steel housings for the inner driver bearings would increase stiffness to the sides of the gearbox.

9. LSD.

  • There are three types of limited slip differential to choose from. You can either use a very cheap ‘diff locker’ like a Phantom Grip which just attaches to the original diff and limits it action. You can use a ‘clutch’ type LSD which Eriksson sells. Or you can use a Quaife ATB diff. The ATB diff contains a unique Quaife design from gears. The advantage of the Quaife ATB diff is that it does not use a clutch so the wear-rate is similar to the original diff unlike a clutch-type diff. The disadvantage is that the Quaife diff will only work if both wheels are actually in contact with the tarmac. This is fine for street use or circuit racing but not good for off-road use when some time is spent with a wheel off the ground or on surfaces so loose that the diff is unable to bias the torque. The Quaife design is OEM equipment for some of the latest cars such as the Ford Focus RS.

    To fit a Quaife differential you will need to extend the ‘plungers’ that sit on the inside ends of the drivers that plug into each side of the gearbox.

  • The simple way is to use four plungers in the box instead of two plungers as originally provided with the box. Fit the additional plungers to each end so each spring has a plunger in either end. Compress the assembly in a vice to make sure the spring is able to fully compress like it should. The tails of the plungers could connect with each other and in this case the length of the tails should be reduced to make sure the plungers will still work.

    With the Quaife diff fitted the plungers will push against each other. With the original diff the plungers push against the pin that runs through the middle of the diff. The plungers are required to keep the inner drivers in
    position as they are not latterly fixed and are susceptible to lateral movement if the plungers are not fitted. In other words, the drivers will fall into the gearbox and possibly lead to catastrophic failure with bit of twisted metal all over the place.

The Third Stage of Gearbox Strengthening

  • Jorgen Eriksson makes several high performance parts for SAAB gearboxes, among them:

    10. Special selector rod.
    11. Steel selector fingers.
    12. Special selector 3/4th
    13. Strengthened Primary Chains
    14. Universal Joint on selector shaft.
    15. Special 4 chain primary gears with new chains.

    Moving attention to the primary housing, adding a 4th row of gears and chain to the primaries increases strength and power transfer. For this addition the primary housing will need to modified to accommodate an
    extra bearing on the upper primary as per the Works gearboxes

    16. Speedparts 4 speed dog gears (38570Kr)

  • Speedparts manufacture a special 4 speed dog-tooth gearbox with substantially wider gears. This gear set is undoubtedly strong but clearly only suitable for track use – what you gain in strength you comes at the cost of louder gearing. And at significant cost.


161 Posts
Discussion Starter #4
Part 4: Oil


The type of oil you use – and how regularly you change it – can have a profound effect on not just the longevity of your gearbox but also the shifting.

There are three issues which have exacerbated the problems with the SAAB gearbox:

  • The original design was for the 1968 89bhp SAAB 99. Although as we have seen there have been slight changes to the design, this gearbox was used until 1993 including use in the 900 Aero with its stock 175bhp.
  • SAAB never recommended changing the gearbox oil. As such it was never a routine service procedure so these gearboxes which have seen power beyond their expectation have rarely – certainly not in their early life – had routine oil changes.
  • Where SAAB did prescribe an oil it recommended 10W30 engine oil and advised against synthetics. Synthetic oils are not what they were 40 years ago.

Thick oils work better than thin oils and if using synthetic oil then it must be designed for gearboxes. Synthetic engine oil should not be used.

Below are a few oils which have been recommended:

  • Amsoil
  • Quaker State Fully Synthetic (as recommended by Gearbox Guru Jorgen Eriksson)
  • Honda MTF
  • Redline MLT
  • Redline Shockproof – some people claim difficulties in shifting until it reaches operating temperature.
  • Do not use AMS Oil Manual Synchromesh Transmission Fluid. It is too thin and has been proven to not
    work too well in these transmissions.

Filling the gearbox with oil

Ideally, all of the original oil will be drained to avoid contamination of the new oil and the components will be washed in kerosene or similar solvent to remove old oil. If you cannot do this then the next best method is to remove all
gearbox covers to clear as much of the old oil as possible. The gearbox should be out of the car to fully clear all of the oil. If you are unable to open all of the covers and cannot turn the gearbox on its side to drain all of the old oil then up
to a litre of the original oil will stay inside the gearbox. If the gearbox is still in the car then it might be possible to use in oil extractor through the side cover and by sliding into the bottom of the primary chains housing to remove the last
of the old oil.

When filling a recently drained or rebuilt gearbox it is important to prime the Primary Housing. As the gearbox has no oil pump it relies on the kinetics of the moving gears to circulate the oil.

To start with, remove the wire clip that the cover onto the front of the primary housing. Pry off the round disc behind it. Behind this you will see the top primary gear and a small white plastic propeller.

Pour in about 0.3L of oil in between the top primary and the casing. This small amount of oil will sit in the bottom of the primary housing and lubricate the chains and tensioner.


161 Posts
Discussion Starter #5
Part 5: Rebuilding your Gearbox

Rebuilding your own gearbox

1. Research and decide what kind of gearbox you want to build. Standard or modified, 4 or 5 speed, synchros or not, etc, etc, There is some information in this article but also lots of information elsewhere on the Internet.

General engineering principles apply: Strengthening of certain areas will result with a stronger gearbox but a properly rebuilt gearbox will be better than an old worn gearbox in the first place. Obtain and study the Saab workshop manual that describes the rebuilding process.

2. Collect the parts you need to build your gearbox. The relationship between the differential crown wheel and the pinion gear on the end of the pinion shaft must be maintained. If this measurement changes then you increase the
chance of excessive noise and wear and the chance of a gearbox failure. Only new or parts less than 6000 miles old can be assembled to original specifications printed on the gears themselves or in the manuals.

When you are removing the parts from the original gearbox you should measure the depth of the pinion gear. The depth is controlled using shims underneath the housing for the pinion bearings. Measure the distance between the bottom of the openings for the drivers and the top of the pinion gear, At least .5mm accuracy is needed. A special Saab tool is
available to make this measurement. The tool holds a dial-gauge so you can get an accurate reading. A reading could be taken without this special tool if it is unavailable but you need to be able to accurately measure before removal and again during installation.

3. Soak the parts in kerosene or similar solvent such as petrol to remove old oil.

4. If building a new box from various components then it is a good idea to ‘Dry assemble’ the box to begin with to make sure all your chosen components will work with each other. Do not use your new bearings or threadlock and do
not fully torque any fasteners or set any clearances at this stage. Just make sure all the components are compatible. Disassemble and correct any problems.

5. Once you are confident you understand the assembly process then you can begin to build the box following the manual to obtain order of assembly and specifications.

Finding a specialist to rebuild a gearbox for you

1. Recommendations from other Saab owners are probably your safest bet. Anyone can tell you they can rebuild your transmission but it’s one of those things that’s easier said than done. Worst case scenario, your chosen specialist
does not even do the job himself and actually sends your gearbox away to be rebuilt by an unknown builder. Make sure the builder does not simply swap your box for some other box and claim he is providing you with a rebuilt box –
unless you are happy with this solution.

2. Know your specifications and tell the builder what you want. Provide details of preferred primary drive ratio, oil you want to use and any other specifications you feel are important. Also ask for the quote to include all replacement bearings but be aware that a full set of bearings will cost around £300 at retail prices.

3. It is a good idea to agree with the builder on what sort of terms you will work if there is a problem with the rebuilt gearbox. A properly rebuilt gearbox should’t have any issues if the rebuild has been completed properly. Your
gearbox builder might not be prepared to provide any kind of warranty due to the transmission being pushed so far beyond its original specifications. You might be able to agree on some conditional terms. A lot of work is required to
install or remove a gearbox and neither the builder or the owner will want the box in and out of the car more times than necessary.

5,779 Posts
Great post, thanks.

Genty said:
Do not use AMS Oil Manual Synchromesh Transmission Fluid. It is too thin and has been proven to not work too well in these transmissions.[/QUOTE] Curious about this, as I used it for some time until I had a gasket leak and refilled with something else. I had no problems with it over 6-8 months, although I guess long term effects can't really be determined there.

858 Posts
Great job and write up Genty. This start is great.

I have a list of bearing/journal numbers per OEM for all bearings found in a 5spd 88 adapted to a 78 chillcast. I can PM them to you and perhaps augment this write up. Perhaps E. Jenner can add information regarding SAAB special tools. I know that a significant portion of my first time rebuilding a tranny was making and acquiring the tools to do the job "properly". Having access to a hydraulic press helps.

In response to RB, I would add Royal Purple Synchromesh Fluid. It works great in other applications, but not in SAAB 900 trannies. It seems the "thinness" makes it susceptible to heat breakdown.


161 Posts
Discussion Starter #10
A pleasure. As I said it is really so many other peoples knowledge I have just tried to combine into one convenient place.

My 'expertise' (if you can call it that!) is with the V4. I have been helped by so many with my 99 project I'm glad to be able to contribute.

It wasn't entirely altruistic either - it's all prep for my own grand plans!

As for any changes or additions I'll keep going back to it every now and again and updating it. Opinions change and technology develops. I think for a rebuild article (with part numbers and torque settings and clearances etc) that really should be another Sticky Thread. I have a really good rebuild pdf but have not actually been inside one of my c900 boxes but will gladly compile if someone with more experience would like to pick up the gauntlet!

622 Posts
I've also found it to be an excellent thread and, this afternoon, it's helped me to establish that the gearbox that I've just taken off of my little 99 is one of the stronger chillcast types and it's only done 45k from new.

Since I'm in the midst of a t16 conversion on this car, the 'box will be ideal apart from one thing: the primaries are gear driven (I think that all <76 99 'boxes were). Can the later chain drive and extra 5th gear be added to this box without making internal mods?


5,779 Posts
idiot_saabvant said:
In response to RB, I would add Royal Purple Synchromesh Fluid. It works great in other applications, but not in SAAB 900 trannies. It seems the "thinness" makes it susceptible to heat breakdown.
Ok, I have seen no evidence of this in regard to the amsoil synchromesh stuff, I used it for a while and am not now simply because I don't want to bother with ordering fluid. I know one other member (philb?) uses it and likes it too.

2,901 Posts
Yeah, I've been using the amsoil MTF with no issues. Its basically synth engine oil with syncromesh additives of some sort. I'm not sure about the "thin" thing - one of the reasons Saab recommend engine oil is because it has the right viscosity to flow properly through the narrow channels in the gearbox.
Great guide though, I'll be referring to it in the coming months for my gearbox rebuild :)

811 Posts
Great thread,
And as i think i might have just lost transmission number 4 (either final drive or diff) it is about to come in very handy.


779 Posts
Just to add a few things I've seen on my works dogbox:

On 900's the factory would replace the AC evaporator with an oil cooler and vent the output through the floor. The cabin fan is left in place so as to provide additional cooling if needs be. This would only be suitable for a track car- even then you loose the ability to defrost the windshield!

The oil pump is wired to run whenever the motor is on.
The side covers are steel as well.
The bearing seats for the diff are steel.
The inner drivers are sleeved with steel tubing.

Shotpeening specs:

280 shot
12-14 almen

Mask any bearing surface or internal surfaces where the shaft or muff contacts. Shotpeen the gear teeth only.

Maybe one day I'll get around to posting photos of the dogbox.

1,275 Posts
I'd like to see those photos of the Works gearbox. Always wondered how they did this, not so much because I will ever be able to afford one, but because it is so cool! Like window shopping...

Also why isn't this thread a sticky? It is the most comprehensive guide to gearbox modification I have seen anywhere. I know as an N/A owner I don't need any of this, and should have no interest, but I know I'm going to put together a gearbox with at least some of these modifications in mind when mine (whining at the moment) decides to go.

406 Posts

i think that it might be a bit much, but has anyone ever thought of making an entirely steel case? i think it would solve most of the problems, although how you would do it, i have no idea.

also, you could possibly make more space for larger internals.

i dunno, i was just thinking...

i just realized that if you were gonna go that far, you may as well not make it FWD, and just do a full on RWD conversion

405 Posts
Great Summary. Why isn't this a sticky? It seems the gearbox gives a lot of people problems and this seems to be a thread giving real solutions. Sticky

1,275 Posts
Donkehote: There was another gearbox thread where people discussed the pros and cons of making a cast iron gearbox case. I'd do it if I had money! Sounds like it would make the thing indestructible, when mated with timken bearings and a good set of gears. The shell, in aluminum, doesn't weigh much, so doubling its weight by making it in cast iron wouldn't do any more harm than offsetting my A/C delete. And the reason you don't make it rear wheel drive is because once you start doing things like that, it isn't really a 900 anymore. I'd never think of such an atrocity.

Also, if this thread becomes a sticky, the spelling of the word "definitive" in the title has to be changed.

406 Posts
euro, i do realize that its not a c900 if its not FWD. its just that if your gonna go so far to add that much power that even chillcast wont cope, then RWD is a more reasonlabe solution. you can USE that power at further ranges, not just halfway down the straight. it also makes more sense to do RWD for me anyway, where il be racing mostly RWD vehicles, and i wont be bale to keep up for all the wheelspin :p

im in the same boat as you tho, i dont have the money or the time to get/make a cast iron case, or even to do a RWD conversion.
1 - 20 of 73 Posts