Classic Saab gearbox rebuild

nutcase · 16 Aug 2011 at 20:45

Been talking about it for a while on Saab forums, but it's time for me to have a crack at a gearbox rebuild. I'm going to try and do it as close as possible to the book. Thought some of you might be interested in the mini-adventure and see how I get things wrong!

Why am I doing it? I don't need a replacement gearbox yet, and my 99T has and always will have a 4 speed Chillcast as originally fitted. But I decided a late 5 speed from either an Low Pressure Turbo (LPT) or N/A car would be good as 1) shouldn't have been too abused and 2) parts for the later boxes are a little easier to find. What will happen to the box afterwards, assuming a sucessful rebuild, is yet to be decided but most likely it will be looking for a new home.

I bought an unknown GM45614 (Gearbox Manual 5 Speed Type 6 Primary drive revision 14) gearbox from good old ebay and collected it. No history with the box but the only sign of any tampering was the front plate bolts were mostly partly undone. The gearbox did have oil in it and it wasn't fresh. The first thing I did was give the outside a quick wipeover with degreaser to get rid of the worst of the gunge. The case and covers will get a proper degrease later when fully stripped.

nutcase · 16 Aug 2011 at 20:46

I then popped the covers off to see if there was any bad news. All looking good until I took the front cover off. The input shaft nut has already been undone and there is a cable tie across the chain tensioner. Hmmm wonder what's been going on there then! The reverse idler looks remarkably good which is a bonus as they're normally chewed to hell as no synchro.

nutcase · 16 Aug 2011 at 20:47

Now Jim Mesthene (US based Saab guy who has rebuilt an awful lot of boxes) says not to bother measuring the backlash of the box prior to dismantling because it simply has to be set to factory spec on rebuild regardless of what it was before. I can understand that, but as this is my first I decided to measure it anyway, and also figured that it might show up any possible issues. The backlash measured at a smidge over 0.11mm. I haven't read the spec off the crownwheel yet but from what I understand 0.17 +/-0.05 is normal. So looks like it maybe a touch tight (stamped spec pending)?

nutcase · 16 Aug 2011 at 20:48

So onwards with dismantling the box. I've now taken the inner drivers and bearing housings off, removed the differential, and removed the primary drive. As could be seen above, the input shaft nut was already undone, and I then found the circlip holding the upper sprocket was also undone. Curious...

So the speedo drive was removed, then onto the inner drivers:

I have the special puller for the inner driver bearing housings, unfortunately only 2 hands so can't show it in position:

Looks like the backlash should indeed be 0.17mm so it was out of spec. I shall reassmeble into spec so it's a little looser.

Next was to measure the pinion depth:

The tool zeroed - 60.000mm from centreline of inner drivers to pinion head nominal depth.

Again, only two hands so no photo of the tool in position. The measurement was -0.02 it looks like the depth should be +0.07 +/-0.05 so again slightly out of spec. I shall re-shim to as close to the correct figure as possible. It was mentioned in another thread that the tool is only as accurate as it was positioned. What I found was a distinct flat point when rotating the tool that the measurement did not change so I am happy it is an accurate measurement as far as user error is concerned.

nutcase · 16 Aug 2011 at 20:50

Then it was off with the primary drive sprockets, simple enough with the hooked puller

Had a quick toothcount and it's the original type 6 primaries. Was kind of hoping that teeny bit that they'd been changed as type 8's are very rare and valuable. They up the theoritical top speed to well over 150mph.

Meanwhile, one piece of debris has been found so far:

A small piece of hardened steel. Not sure what it might be from, any ideas?

nutcase · 16 Aug 2011 at 20:51

I've released the circlip for the 5th gear coupling and moved it across.

Next step is to release the layshaft, which along with the reverse idler shaft is locked into place by a metal plate near the pinion.

nutcase · 16 Aug 2011 at 20:53

A better pic of the bit of wire

Ok, so I removed the locking plate from the cluster shaft and reverse idler shaft. One manual says withdraw the cluster shaft using the tool to allow 5th gear to be removed, one says use a drift to drive the cluster shaft out from the primary housing end. I had to use a drift for the first bit.

I attached the tool anyway to control it. Not looking forward to getting it lined up properly again.

Now the manual states clearly not to lose the bearing rollers from 5th gear. So what did I do? Promptly dropped them all over the floor! Ah well, wiped them down and bagged them, along with the inner race and the dowel from 5th selector fork (the manual doesn't state how to remove that - turns out it's a slide in fit).

Excuse the dust on the end of the gear, it's been cleaned off and everything will be properly cleaned before refitting. The bearing surface looks good to go. They look ok too.

Then out came the input shaft bearing housing. This was the first point where I don't have the special tool which is an adapter to use the slide hammer to withdraw the shaft with housing. Fortunately for me, it slid out without any real force.

Next step is removing 5th gear selector fork (had in mind that it comes out then removing the primary housing but just checked and it comes out now), then remove reverse idler shaft and idler.

Will · 16 Aug 2011 at 20:54

Interesting thread, gearboxes are one area of cars where I know nothing, to me its just a box of cogs so learning more is always good!

What exactly is 'backlash', and why does it matter?

Also surprised to see those chains in the gearbox, I didn't realise car boxes had such things, I figured they'd just be various cogs and gears, is this common or a peculiarity of Saabs/this particular family of gearboxes?

nutcase · 16 Aug 2011 at 20:54

Tiny bit more progress. I've been held up again through tool shortage. I can find my 32mm impact socket for the lower sprocket nut which I didn't need, but can't find my 24mm for the pinion shaft nut. The trials of moving home!

So anyway, I've removed the reverse gear lever, shaft and idler

And by pushing the control rod into the main case, also removed the 5th gear selector fork and muff.

Next step is the above mentioned pinion shaft nut

When that's done I can remove the primary housing, the cluster gear, and start stripping the gear stack. Then I should get an idea of just what needs doing to this gearbox to get it into decent condition. I'll photograph all the various sub-assemblies later, but the reverse idler whilst not missing any teeth is showing some hammering to the teeth. Looks reusable at a glance however.

nutcase · 16 Aug 2011 at 21:01

Will said:
Interesting thread, gearboxes are one area of cars where I know nothing, to me its just a box of cogs so learning more is always good!

What exactly is 'backlash', and why does it matter?

Also surprised to see those chains in the gearbox, I didn't realise car boxes had such things, I figured they'd just be various cogs and gears, is this common or a peculiarity of Saabs/this particular family of gearboxes?

Me too - it's one of the few areas of older cars I have no real experiance of, hence jumping in at the deepend with this little project

The backlash is the amount of play in the teeth. Too tight and they'll bind together causing a lot of excessive wear, too loose and the teeth will chatter causing a lot of noise and probably break teeth off.

Saab 99/900 gearboxes are the only ones I'm aware of with this arrangement of primary drive. Early boxes used transfer gears, very similar to that used on the original Mini, and Allegros, Metros etc. The chain arrangement is stronger however. Because the engine is mounted directly on top of the gearbox, there needs to be some way of transferring the output from the engine crank to the gearbox.

The splined shaft is driven by the flywheel/clutch assembly. The top of the gearbox is where the engine bolts to and the gearbox case serves as the sump. Unlike the Mini etc. the oil supplies are seperate.

Will · 16 Aug 2011 at 21:18

nutcase_1uk said:
Me too - it's one of the few areas of older cars I have no real experiance of, hence jumping in at the deepend with this little project

The backlash is the amount of play in the teeth. Too tight and they'll bind together causing a lot of excessive wear, too loose and the teeth will chatter causing a lot of noise and probably break teeth off.

Saab 99/900 gearboxes are the only ones I'm aware of with this arrangement of primary drive. Early boxes used transfer gears, very similar to that used on the original Mini, and Allegros, Metros etc. The chain arrangement is stronger however. Because the engine is mounted directly on top of the gearbox, there needs to be some way of transferring the output from the engine crank to the gearbox.

Thanks for that, makes sense when you think about it, have to say there is a lot of things about gearboxes that are a black art to me despite knowing a bit about basic mechanics. When people start talking about diff preload my eyes start to glaze over.

It does seem a quirky gearbox design just to look at (sump built into the gearbox casing?), typically strange as one might expect from a Saab!

nutcase · 16 Aug 2011 at 21:28

I've got a (unused for 10 years!) degree in mechanical engineering and agree 100% - there's some kind of black magic involved in gearboxes. Even after reading and re-reading various books on them, without actually looking at one of these things in bits I couldn't figure out how they work!

Diff preload (and pinion bearing preload) I understand - you wait til you see how it's measured. It involves a piece of string and a spring balance

Quirky but very effective. It means an inline engine for front wheel drive in a very compact arrangement. The problem with the boxes is their strength. The original design was produced in the mid-60s when 60hp was a reasonably powerful car. They stopped selling cars with this basic design in 1994! In the mean time they'd made everything smaller to squeeze a 5th gear in and upped power to 185bhp in some cases... So every now and then they tend to grenade :mad:

kaiowas · 16 Aug 2011 at 22:03

I need to get around to doing a gearbox properly (replaced a bearing in the Galant last year which meant more or less complete disassembly), got a couple here surplus to requirements that need a rebuild. Just need to clear enough space and get a couple more tools (need an in/lb torque wrench for setting diff preload at the very least)

From the pics my best guess at your bit of debris is that it may be a slightly mangled needle from a roller bearing, does that sound feasible?

nutcase · 17 Aug 2011 at 17:05

I *think* the needle bearings have larger diameter pins. Can't remember and not got that far yet

Found out yesterday it's magnetic :confused:

breadturbo · 17 Aug 2011 at 19:19

That has to be the strangest gearbox i've ever seen o.O, is it lubricated by the engine oil?

Kei · 17 Aug 2011 at 19:29

The engine oil and gearbox oil are separate, yet the gearbox case has the engine sump in the top. The layout of the engine and box has its advantages, like truly equal length drive shafts and a clutch change that can be done in less than an hour.

nutcase · 17 Aug 2011 at 21:29

Indeed

So, as I said above, the next part was to undo the pinion shaft end nut and start stripping the gear stack. Never found my impact socket but found an old (and expendable!) 24mm normal socket. First thing to do was lock the pinion shaft. Again, a special tool has been made to do just that and reverse gear is locked

Then in comes the trusty electric impact gun. I was rather sceptical of these, but now I'm well and truly sold! It took the nut off with the tiny-est pulse of the trigger

This enabled the removal of 5th gear synchro and then the primary housing could come off. You have drive a couple of dowels into the main gearbox casting to allow the housing to come off easily.

And then I saw some real damage. Looks like 4th gear has a problem, maybe the piece of metal has come from this assembly

This shows the now loose cluster gear

And a close up of the damaged 4th gear guide ring. Looks like it's been making some contact with the cogs

Off comes 5th gear spacers before removing the cluster gear, else they may fall off the pinion shaft

nutcase · 17 Aug 2011 at 21:29

Out comes the cluster shaft now, and the gear cluster

There's some definite wear to the end of the shaft

Here's the needle bearing, inner race and thrust washer from the pinion end

Now it's time to turn full attention to the gear stack.

nutcase · 17 Aug 2011 at 21:30

Here's the other side of 4th gear and its muff.

This leaves 3/4 selector fork and a light press fit bushing preventing further dismantling. The manual says to press out the pinion shaft at this point using a larger threaded press that is attached to the casing with 4 spacers. I have the press but not the spacers. I've got some copies on order but they've not arrived yet. I tried Mmoe's method with a mallet and succeeded. He managed to remove 3/4 selector fork and shaft but for whatever reason mine were stuck I had to remove the pinion shaft to get the selector fork out the way, and then remove the rest of the stack.

I replaced the pinion shaft end nut to prevent damage to the thread when tapping the shaft out. It did not take much force. Remove the 4 pinion bearing housing bolts before pressing it out.

The bearing housing came this far before the bushing holding everything on came loose

Carefully remove the pinion shaft and bearing housing.

Here's an idea of how much hangs off the pinion shaft!

All that's left dismantling wise is the individual gear assemblies, differential, removal of the selector shafts and the good old pinion bearings. In the background of the above picture is the mounting tool for holding the pinion shaft upright for fitting bearings. There's a different holder and a hoofing great spanner for removing and replacing the big nut.

nutcase · 18 Aug 2011 at 19:37

Amazingly, the 4th gear spring and guide ring are still available new so have ordered those. Need to set up a vice in the garage or take the various sub-assemblies to my parents and use their vice to get much further. I tried mounting the pinion shaft in a workmate but unsuprisingly it can't grip enough to undo the main nut. I did find that the selector fork shaft does just slide out so that's done.