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The Definitive Gearbox

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#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 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.

 
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#57 ·
Hyperlube?

I am the new owner of a 1990 Saab 900 and have been researching gearbox oil recommendations. A fellow I know who has owned many old high mileage Saabs recommended I fill the gearbox with a quart of Hyperlube and the rest 10W30. I was wondering what you guys thought of that idea? He swears by it and has had great success with many Saab manual gearboxes running well past 300k but a part of me is skeptical. The Redline MTL seems like a better option but his experience is solid.
 
#58 ·
Two things. My experience changes as I see and work on more and more of these gearboxes and it is still a hard decision to make.

1. If anything is broken or severely worn in your existing gearbox then changing the oil won't make it better. If it crunches going into any gears or if you can hear the pinion bearings screaming in the 4th and 5th gears while cruising then changing the oil won't help.

2. Saab recommend not changing the oil because new oil damages the syncros. I think they were right and I think my attitidue towards this has softened over the years. If the gearbox is low then top it up with something similar to what should be in there. The manual recommends a gear oil with a particular EP rating. If you are rebuilding the gearbox then use whatever you want to refill after the rebuild in line with recomendations above.

I think what people should avoid doing is taking a smooth good gearbox, draining the oil and refilling with some specialst fluid. After not too long the syncro on 3/4 gears will start to crunch and not long after that it will become very difficult to engage those gears. Leave well alone. For high performance applications you need a gearbox which has been properly rebuilt and filled with decent fluid.
 
#59 ·
I think what people should avoid doing is taking a smooth good gearbox, draining the oil and refilling with some specialst fluid. After not too long the syncro on 3/4 gears will start to crunch and not long after that it will become very difficult to engage those gears. Leave well alone.
When I got my 900, probably 40-50k miles ago, I drained and refilled the gearbox with fresh full-synth engine oil. Exact same stuff I poured in the engine.

The change improved dramatically, and has remained good. The 'box is still quiet. Total's probably about 240k now.
 
#61 ·
Hi! Sorry bout answering this quite old thread. This is really great topic, and just what I needed, so thats why. I've planned to rebuild as definitive classic 900 gearbox as possible with quite low budget, so I'm going to ask maybe more later. Now there is few questions, which I didn't find answers yet - maybe there is, but I weren't able to find those because my English skills (Finnish is my mother tongue, so), or then there is no answers for those yet.

To the business. Is there anything to find out do a gearbox have chillcast pinion housing, or is only way to find out to take the gearbox of and check? My friend have two cars, MY '84 both, with 5-speed boxes and 8-valve normally aspirated injection engines, so is it possible that I could get those parts from other of those cars?

Other thing I've wondered is about fitting 5-speed parts to 4-speed chillcast gearbox case, that I already have from a Saab 99. I've planned to get MY '91 or younger box to have those sligtly wider gears, which it's told to have. But which years models could possibly fit without modifications to chillcast case? Is it still a reasonable plan to get another box from MY 89-90 and fit that stronger shift fork and maybe rear pinion bearing?

Or is it going to be a real mess, if I try to make four different boxes up to one?:cheesy: I'm not a car mechanic, but few friends of mine are and they will help me, after I've hunt all the parts I need. Problem is to find out what I'm going to need, after that to find those parts is maybe a bit easier here in Finland.
 
#62 ·
There is no way to know for sure which gearbox will have the chillcast pinion housing but there are some which are more likley to have it. The more likely ones are the ones produced around the mid period 84, 85, 86, 87... I suppose if you got the part number from the chillcast pinion housing shown earlier in this thread you could look on the Saab EPC (electronic parts catalogue) and find out exactly which model years had that kind of pinion housing.

As for fitting stuff together from different boxes, provided that you can measure all the clearances and everything measures correctly, all the right parts are included and the completed box can be turned by hand without any major issues + you can select all the gears and still turn by hand then the box should be working when you install it. Follow the rebuild manual carefully. Bearing tensions need to be set correctly.

Depending on which parts you are trying to install you may find machining work is required to the casing to get the parts to fit.
 
#63 ·
From what I understand (and mmoe may want to comment on this), 5-speed gearboxes from 85' and 86' cars are most likely to contain a chillcast pinion bearing housing. However, in one of the earlier posts (and referenced in one of my earlier replies), it seems that eeuroparts in the US sells a pinion bearing housing with the same part number as the chillcast housing; I'm not certain if it's the same piece, but it's easy to determine by looking at it (and perhaps someone who is interested might request a better photo of it from eeuroparts).

As for the chillcast rebuild, I've taken an 83' 5-speed box and fitted it into a 99 4-speed chillcast case without any modifications. The biggest thing is you need the transfer case/5th gear housing from the 5-speed fit onto the 4-speed chillcast, everything in the main casing will fit. However, I think this only works for cases before a certain year (89?) after which the pinion bearing housing was enlarged to accommodate a larger pinion bearing on one side and the shifter forks were beefed up. Thus, if you're using parts from later gearboxes, in order to fit a chillcast case you will need to make modifications. Or rather, the reason why gearsets and parts from earlier boxes fit the chillcast case is because their overall fitment remained unchanged since the chillcast case was designed (ie. nothing beefed up, sizewise).

li Arc
 
#64 ·
To fit '89 and newer gears in to a chillcast, 2 things must happen. First, you need to have clearance machined for the cluster gear. The newer cluster gear is longer due to the addition of an inner race with a "T" shape to it. That must be removed from the case. Then, the pinion housing must be made to fit into the case. This can be a modification to the case by enlarging the hole and lengthening the mounting holes/threads to match the late model cases, or you could have a custom pinion housing made that maintains the older outer diameters and accommodates the larger bearings of the '89+. The advantage of a custom pinion housing is that you could also have it made for the older pinion/ring ratio if desired. There is a larger bearing available that will also work with that older pinion/ring which was never a Saab part, so there is no bearing housing that currently exists which provides the larger bearing on the older pinion/ring. A custom housing obviously allows for that sort of change. The later pinion/ring ratio is 9:35 while the earlier ring/pinion ratio is 9:33. You must just decide what your preference is.

I also believe now that the best approach to building a custom box is to decide what is important to you in terms of how the transmission operates, moreso than just which parts are "stronger". Also, condition of the gears is a bigger factor than their size. IMHO, I don't see a huge problem with using the older gearsets ('88 and earlier) provided that they are in excellent condition. If you bought brand new gears and had them cryotreated and shot-peened, I would imagine they could take a considerable amount of abuse for quite some time in a chillcast build.

As for pinion housings, I find the chillcast variety in '85-'88 models and as far and I know that is all they produced during those years. I've never come across a pinon housing in those years that was anything other than chillcast, but perhaps that's just a U.S. delivery thing if ejenner has seen them over there. Personally, I would have a custom housing made (if you can do it or afford it) to allow for enlarged bearings without modification to the case. I think part of what makes the chillcast strong would be disturbed by enlarging the hole for the later pinion housings.
 
#65 ·
Ok and thanks for extremely quick answers!

My goal is to build nice street machine gearbox, not a racing one. But it should be able to handle maybe a quarter mile drag or a top speed mile sometimes, without fear of broke it easily, so i think that it should be customized a bit, if my target horsepowers is somewhere between 250 and 300 horsepowers in somewhere future. For now I'll have something like 200hp and a MY '90 stock gearbox, mileage over 300 000 km, so with that box it's not going to be too very smart to get lot more power out of engine.

It sounds like a plan to have older 5-speed gears to my chillcast case, if it's so much easier. Have anyone any idea, how much power and/or torque is that kind of box able to handle? With oil cooler and differential lock, of course, those are next thing to get after I've decided the part set up.
 
#73 ·
dw about draining the whole thing, its not like you're going to get all the contaminants out.

I have pulled down transmissions with clean oil, but the amount of swarf and crap in the recesses of the gearbox is ridiculous.

The only way to get a worthwhile result from a total fluid change is to strip clean and rebuild the gearbox lol but seriously -

just change the oil and dw about the stuff in the primary case.
 
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