SAAB 900 (classic): Drive shaft tunnel repair rebuild, DIY
Posted: Mon May 04, 2009 2:45 am
Well my 19 year old c900 failed its MOT on a few things but lets talk about the rusty drive shaft tunnel and floor pan, I was quoted £900 
I spent all week under the car cutting out the rusted metal with an angle grinder, I went home each night looking like a Chimney sweep 
lucky I have a good half face respirator and ear defenders, you just can't cut rusty metal without these two items unless you want your lungs filled with iron and ear ringing like a bell, I wear glasses but a pair of goggles would have been handy, no required.
So the floor pan under the driver seat had water coming in from the sun roof drain pipes, the owner before me had the c900 slip off the jack and it hit the floor pan which crimped it, I beat the floor pan down with a lump hammer and cut out the cancer, then ground off all the under seal and paint, I welded stainless from the inside but the bottom side needed one large plate covering it to making it strong.
The floor pan metal is rather thin so welding is tricky and takes time, I used stainless 1.6mm rods with 35 -40 amps, any more would just blow a hole in the original body metal, the plate I welded in was 1.6mm, there is no point in using anything thicker as it just harder to cut and beat into shape which you need to be able to do.
The other thing is to only weld a spot or two at a time, the amount if heat generated could start a fire, so I had a basin of water and sponge at hand to cool the welds down
next my rear sill had rust because of a leaky drain pipe which goes to the side vents, the Ariel has a drain pipe that just empties into the spare wheel area! I will add a hole to poke that drain through soon.
So its behind the mud flup area which supports the rear bumper skirt, I also welding in some SS from the inside but I needed to make it stronger as the pan rod is around that area.
This picture is directly under it, were the rear pipe exhaust would be, once again grind it back and panel beat the area into a smooth shape ready for a new matching plate.
You have to again use spot welds and cool the area, once I tack welded the corners on and I was happy with the shape, I beat it down to be as flush as possible with the body.
once that is done apply loads of Hammerite paint or some other under seal coating, 4 thin layers are better than one heavy one as always.
Now the drive shaft tunnel, remove the brake disc, calliper, hub and lower A arm. Start by opening the four 70mm ball joint bolt (17mm nuts) and the steering tie rod needs popping out (19mm nut) remove the steering rod first, place a jack under the ball joint and compress the spring to support the hub, you want the blow force to hit the tie rod hip, the force will be absorbed by the suspension if you don't brace it some how; do not hit the threads, just unscrew the nut over the edge of the threads, then hit it accurately with a hammer, if your getting nowhere apply a WD40 or 3 in 1 silicone, I find Tar Spot removers that are paraffin based do a better job than WD40, once that is out you need to support the top A arm, I just used the brake pads in the body gap, you can get the correct tool but most guys use a strip of wood, you just don't want the A arm squashing the body.
Once your happy with the support on the top A arm remove the jack which was supporting the lower ball joint slowly, now you can hit out the ball joint bolts and pull the hub away, if your car has the ABS sensor I strongly recommend you unplug the wire from inside the engine bay and pull the wire away with the hub, these sensors are almost impossible to remove without breaking them as they are ancient! and costly to replace.
I won't mention the brakes because those are straight forward and as simple as they looks, just cable tie the calliper to the spring out the way.
The shock absorber only has one nut on it, once you pull of the shock replace the nut on the lower A Arm, the same for the Tie rod as its easy to mix these up later and strip the threads.
Just put all your nuts and bolts in a tray or bag out the way so you don't kick them around and lose one.
This is also a good time to replace a worn CV joint like on mines or just to replace an old CV boot with a fresh one.
It should look like this, now to remove the lower A arm, its very simple, just six nuts, you will need a deep 19mm sock or a high quality spanner, remove them and give the arm a few taps and it will just fall off.
Next you have to cut out the good metal with the bad, there is no point in trying to do a patch up job here, the original tunnels are not the strong to be honest, this is your chance to make them stronger if you do it right, please support the car with a decent axle stand, I used two stands, a wheel and jack, safety cost nothing here.
The tower was in very good condition thankfully, the dust from the main cutting caked everything it in rusty powder! this is a dirty job.
I have seen drive shaft tunnel templates on the net that you can print out but I didn't like the shape of them, they are made to simplify the job, they are not made for great strength, I just made it up as I went along as I do, I did my off side tunnel last year which was overly elaborate but just over kill.
This is a 2mm mild steel sheet less than £10 from my local metal super market, its easy to shape and will have a good bond with the original body work, again it very easy to blow holes if you use sheets that are too thick, (example 2.5mm is to thick, you would need 80 amp to weld that on correctly) the body is less than 1.6mm so its not a good match, the strength comes from the shape not the thickness in the tunnel.
I didn't have time to drill the holes for the lower A Arm studs, so I used 180 amps of current to blow a hole in the sheet, its like a ruff plasma cutter and is very quick, You just have to dress the edges down, the holes need to be a little larger so you have some play for adjustment.
This part takes time but its worth taking your time , I used this I beam I found in a skip as an anvil, work and fold the metal into the shape you need, you will need to check each adjustment you make on the tunnel, its lots of going back and forth, always make small adjustments is the best advice here. You may notice I made a rolled lip on the shaft opening, this makes it strong, I welded a bead along the lip to add some more metal on it.
So once its snug and tight you have to press it against the body, I don't have lots of welding clamps, a jack and brick do just fine, again tac welding the corners only, make sure its in the correct position and hugs the body.
The black stuff on the welds is slag, it pops off once its cooled down, You can dress this with an angle grinder but to me I'm just taking metal away and making it weaker so why bother, plus it shows the MOT tester its been welded and not glue on
I should say you need to use more current when welding the plate on to the inner leg as it can take much more current, to little and you have a high bead with a weak bite or bond, choosing the right current is vital for strong welds.
Once that is done, coat it with layers of hammer paint or under seal, you will have to spray it into the tunnel, I just poured the tin in and slushed it around with a brush
This job seems like a mission but it's fairly easy as this is my second one, its just time consuming. Does this make a difference to the handling? hard to say really, I think it feels better and stiffer but I hadn't driven it in 6 days, replacing the worn wheel bearing made a difference, the whole front end feels tight but I have replaced all the ball joints, tie rod ends and bushes, now the tunnels as well.
Anyway I know a few people have scraped their c900 because of rusty drive shaft tunnels and general chassis rust, its a crying shame, its not as if wheels have fallen of them, buying a welder and taking a course is pretty cheap these days, you could save a classic car and some money in the process of learning a new skill, I hope this help somebody
I spent all week under the car cutting out the rusted metal with an angle grinder, I went home each night looking like a Chimney sweep lucky I have a good half face respirator and ear defenders, you just can't cut rusty metal without these two items unless you want your lungs filled with iron and ear ringing like a bell, I wear glasses but a pair of goggles would have been handy, no required.So the floor pan under the driver seat had water coming in from the sun roof drain pipes, the owner before me had the c900 slip off the jack and it hit the floor pan which crimped it, I beat the floor pan down with a lump hammer and cut out the cancer, then ground off all the under seal and paint, I welded stainless from the inside but the bottom side needed one large plate covering it to making it strong.
The floor pan metal is rather thin so welding is tricky and takes time, I used stainless 1.6mm rods with 35 -40 amps, any more would just blow a hole in the original body metal, the plate I welded in was 1.6mm, there is no point in using anything thicker as it just harder to cut and beat into shape which you need to be able to do.
The other thing is to only weld a spot or two at a time, the amount if heat generated could start a fire, so I had a basin of water and sponge at hand to cool the welds down
next my rear sill had rust because of a leaky drain pipe which goes to the side vents, the Ariel has a drain pipe that just empties into the spare wheel area! I will add a hole to poke that drain through soon.
So its behind the mud flup area which supports the rear bumper skirt, I also welding in some SS from the inside but I needed to make it stronger as the pan rod is around that area.
This picture is directly under it, were the rear pipe exhaust would be, once again grind it back and panel beat the area into a smooth shape ready for a new matching plate.
You have to again use spot welds and cool the area, once I tack welded the corners on and I was happy with the shape, I beat it down to be as flush as possible with the body.
once that is done apply loads of Hammerite paint or some other under seal coating, 4 thin layers are better than one heavy one as always.
Now the drive shaft tunnel, remove the brake disc, calliper, hub and lower A arm. Start by opening the four 70mm ball joint bolt (17mm nuts) and the steering tie rod needs popping out (19mm nut) remove the steering rod first, place a jack under the ball joint and compress the spring to support the hub, you want the blow force to hit the tie rod hip, the force will be absorbed by the suspension if you don't brace it some how; do not hit the threads, just unscrew the nut over the edge of the threads, then hit it accurately with a hammer, if your getting nowhere apply a WD40 or 3 in 1 silicone, I find Tar Spot removers that are paraffin based do a better job than WD40, once that is out you need to support the top A arm, I just used the brake pads in the body gap, you can get the correct tool but most guys use a strip of wood, you just don't want the A arm squashing the body.
Once your happy with the support on the top A arm remove the jack which was supporting the lower ball joint slowly, now you can hit out the ball joint bolts and pull the hub away, if your car has the ABS sensor I strongly recommend you unplug the wire from inside the engine bay and pull the wire away with the hub, these sensors are almost impossible to remove without breaking them as they are ancient! and costly to replace.
I won't mention the brakes because those are straight forward and as simple as they looks, just cable tie the calliper to the spring out the way.
The shock absorber only has one nut on it, once you pull of the shock replace the nut on the lower A Arm, the same for the Tie rod as its easy to mix these up later and strip the threads.
Just put all your nuts and bolts in a tray or bag out the way so you don't kick them around and lose one.
This is also a good time to replace a worn CV joint like on mines or just to replace an old CV boot with a fresh one.
It should look like this, now to remove the lower A arm, its very simple, just six nuts, you will need a deep 19mm sock or a high quality spanner, remove them and give the arm a few taps and it will just fall off.
Next you have to cut out the good metal with the bad, there is no point in trying to do a patch up job here, the original tunnels are not the strong to be honest, this is your chance to make them stronger if you do it right, please support the car with a decent axle stand, I used two stands, a wheel and jack, safety cost nothing here.
The tower was in very good condition thankfully, the dust from the main cutting caked everything it in rusty powder! this is a dirty job.
I have seen drive shaft tunnel templates on the net that you can print out but I didn't like the shape of them, they are made to simplify the job, they are not made for great strength, I just made it up as I went along as I do, I did my off side tunnel last year which was overly elaborate but just over kill.
This is a 2mm mild steel sheet less than £10 from my local metal super market, its easy to shape and will have a good bond with the original body work, again it very easy to blow holes if you use sheets that are too thick, (example 2.5mm is to thick, you would need 80 amp to weld that on correctly) the body is less than 1.6mm so its not a good match, the strength comes from the shape not the thickness in the tunnel.
I didn't have time to drill the holes for the lower A Arm studs, so I used 180 amps of current to blow a hole in the sheet, its like a ruff plasma cutter and is very quick, You just have to dress the edges down, the holes need to be a little larger so you have some play for adjustment.
This part takes time but its worth taking your time , I used this I beam I found in a skip as an anvil, work and fold the metal into the shape you need, you will need to check each adjustment you make on the tunnel, its lots of going back and forth, always make small adjustments is the best advice here. You may notice I made a rolled lip on the shaft opening, this makes it strong, I welded a bead along the lip to add some more metal on it.
So once its snug and tight you have to press it against the body, I don't have lots of welding clamps, a jack and brick do just fine, again tac welding the corners only, make sure its in the correct position and hugs the body.
The black stuff on the welds is slag, it pops off once its cooled down, You can dress this with an angle grinder but to me I'm just taking metal away and making it weaker so why bother, plus it shows the MOT tester its been welded and not glue on
I should say you need to use more current when welding the plate on to the inner leg as it can take much more current, to little and you have a high bead with a weak bite or bond, choosing the right current is vital for strong welds.
Once that is done, coat it with layers of hammer paint or under seal, you will have to spray it into the tunnel, I just poured the tin in and slushed it around with a brush
This job seems like a mission but it's fairly easy as this is my second one, its just time consuming. Does this make a difference to the handling? hard to say really, I think it feels better and stiffer but I hadn't driven it in 6 days, replacing the worn wheel bearing made a difference, the whole front end feels tight but I have replaced all the ball joints, tie rod ends and bushes, now the tunnels as well.
Anyway I know a few people have scraped their c900 because of rusty drive shaft tunnels and general chassis rust, its a crying shame, its not as if wheels have fallen of them, buying a welder and taking a course is pretty cheap these days, you could save a classic car and some money in the process of learning a new skill, I hope this help somebody
, Some people weld as a hobby because its fun once your get a hang of it and its very soothing, you start looking for things to weld 
you will need a long thick pipe to slide over your breaker bar, I mean like 3 foot long! the longer the better in fact, both wheels should be on the ground, the engine in 5th gear, get somebody to help if you have any back problems, I pushed down on it instead of lifting it 
