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Discussion Starter #1
I’ve received conflicting advice on assembling my 6C2500's driveshaft. One advisor says the engine unit and diff should be exactly lined up so there’s no flex in the couplings. But this would mean a marked downslope for the engine/gearbox and upturn in the diff unit, neither of which seem present in the stock motor mounts or diff mounts. Both the motor and diff seem to sit about level. Since the gearbox output is higher than the diff input, the driveshaft slopes downward and normally there would be flex at both joints. Other advisors say this is the way it’s supposed to be.

Both sides, of course, insist that if it’s not put together the way they say, irreplaceable parts will be immediately destroyed.

Please, can someone give me an opinion on this that’s backed by convincing experience and authority?
 

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6C 2500 Drive Shaft Alignment

Paul,

Firstly let me stress I am no authority on these cars but I have put one together with a fair amount of though, cursing and head scratching for what it is worth.

The 6C Drive shaft centres were designed to be as close to a straight line as possible. The couplings used are pin and bush type and they are not intended to operate with much more than 0.05 of a degree of misalignment. They are designed to cushion the drive and handle a minimum of miss alignment. There were two types used.

The first were quite large in diameter and had 10 bushes and pins. The pins were free to slide inside the rubber bushes. The bushes have grooves around them and are held in the carrier by interference so they are quite flexible. They need to be because of the large diameter of the coupling.

The second type is a smaller diameter and has 6 rubber bushes. The bushes are made like a conventional suspension bush. They have inner and outer steel shells with the rubber molded between them. The bushes are held onto the pins with a centre bolt and washer so they are more rigidly held to the pin carrier. Angular miss-alignment is handled by the rubber flexing between the shells. There is not much flexibility in them.

The Gearbox does not have any mounts supporting it and is cantilevered off the back of the engine. It relies on the four engine mounts for its support and its output shaft alignment.

The engine mounts are very tight in that they are constrained in the chassis by long through bolt that holds the lower rubbers and upper load bearing rubbers together with light compression. It is a very tight system unlike modern gravity only mounts.

The trick to getting the alignment right is in the way it is all shimmed. The large washers (Rondella p/n 47350, - /3) between the upper rubber mount and the engine cradle arms came in 4 different thicknesses; 2 mm, 3 mm, 4 mm & 5 mm. A slight adjustment in any of these washer thicknesses can have a big effect on the height of the gearbox tail coupling height, alignment, the loading on the output shaft bearing and the coupling rubbers. Any decent washers can be used to add height above the large washers that are original to the car. Reducing height means making thinner large washers.

The other area of adjustment in the system is in the prop shaft steady bearing height. The bearing housing is suspended in rubber blocks that are clamped down to the chassis on plates (Spressore p/n 47287). These were available in different thickness from 4 mm to 8mm. So the centre of the prop shaft can be raised or lowered to get the straightest transition towards the diff coupling.

The differential casing is not height adjustable. It relies on 4 tubular rubber bushes mounted on through shafts to isolate it from the chassis. The rear mounts can be shimmed back ward and forward but not up and down. So with fresh rubbers the diff input shaft angle should be consistent from one car to another and align with the prop shaft. It can all be aligned with a stock straight chassis.
 

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Paul,

From what you are saying, it sounds as though your engine is a little higher than it needs to be.

In the factory they had an alignment tool (670.1042 which was a straight tube the full length of the drive shaft. It had three equal length bars attached at right angles to the tube. The tube was laid above the installed drive shafts. The ends of the bars were all supposed to touch the round sections of the shafts adjacent to the coupling splines with equal pressure and no gap. In other words the shaft was intended to be straight from gearbox output to diff input. Unfortunately you have to remove the body or at least most of the floor to use such a tool. However with a little bit of ingenuity you should be able to measure those same points from under the car up through the chassis.

If you remove the small bolts that hold the bushes, from the coupling pins you can measure the thickness of the coupling at different points to see if it is deflected. If the thickness is consistent all the way round then it is perfect.

You might start by dropping your engine as low as it will go on the engine mounts.

Loosen the front coupling spline clamps so it can slide back and forth on the drive shaft. Measure from the top of the chassis cross member to under side of the shaft just behind the front coupling to get the current shaft height.

Drain some coolant from the radiator. Remove the top radiator hose. If you have a long L rubber bottom hose, it should be flexible enough to be left on. The other engine and gearbox linkages should be able to cope with the engine being moved around so long as it is no more than 10-15 mm up or down and not allowed to slip off the mounts.

Drop the four engine mount bolts, gently jack up the engine with wood bearers against the sump and remove the big flat washers. Replace them with thin washers or 1 mm thick strips of steel sheet with 12 mm holes drilled through. Push the bolts back and put the nuts on loose. Or drop some long 7/16 bolts down through the mounts.

Now remeasure the chassis to shaft height and check the angle to the centre bearing. If it is now lower than the centre bearing then you need to shim the engine up off the mounts. The back of the engine is much heavier than the front with the fly wheel and the gearbox so you need thicker shims on the rear mount rubbers than on the front.

Keep shimming the left and right rear mounts or left and right front mounts until you get the front of the shaft the same height off the chassis as it is just ahead of the centre bearing. Bear in mind that the centre bearing may need to be raised or lowered slightly to get it straight all the way through.

Once you get the shaft as straight as you can, take the shims out of the engine mounts mark each set of them. Check the thicknesses against your original mount top big washers and decide if any of them are usable or if you need to make up new ones to match the shim sets. Fit the new washers to the mounts and reassemble the bottom washers, rubbers, bolts, lock washers and nuts.

Unless something is drastically wrong with your chassis or the parts of the drive shaft you should be able to get it all to align with a minimum of deflection to the couplings.

Last but not least you will need to shim the radiator mounting rubbers so the hose connections are aligned horizontally and not touching the top front of the head.

Let us know how it goes.
 

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Discussion Starter #4
Thanks very much for the advice.

One key fact I omitted: I have a short-chassis SS, with a one-piece driveshaft. But your points apply just the same: the engine mounts must be shimmed to align the driveshaft as well as possible.

The fact that the diff unit’s position isn’t adjustable I think tells me I shouldn’t worry about it. That coupling is smaller anyway, so should be more compliant.

I’ll be lucky to install the engine within six months, so I’ve just got the gearbox sitting on its mount, with the rear part in a temporary sling. See attached picture. The engine will squash the rubber mount down somewhat, and I’d forgotten that since the rear, with overhanging gearbox and flywheel, is heavier than the front, it’s going to droop towards the rear naturally. My immediate problem is making a tunnel and gearbox cover that will fit the components reasonably closely, which is hard to do when their correct position isn’t known.

When my gearbox is exactly aligned with my driveshaft, the angle between the front face of the mount (ring that goes around the bellhousing) and the top of the frame at that point is 88.7°. That is, the frame slopes upward toward the front a bit more than the engine does. If you put a digital level on, say, your cam covers, then on the frame next to the engine, is this about what you get?

Thanks for your help.
 

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Discussion Starter #5
Thanks very much for the advice.

One key fact I omitted: I have a short-chassis SS, with a one-piece driveshaft. But your points apply just the same: the engine mounts must be shimmed to align the driveshaft as well as possible.

The fact that the diff unit’s position isn’t adjustable I think tells me I shouldn’t worry about it. That coupling is smaller anyway, so should be more compliant.

I’ll be lucky to install the engine within six months, so I’ve just got the gearbox sitting on its mount, with the rear part in a temporary sling. See attached picture. The engine will squash the rubber mount down somewhat, and I’d forgotten that since the rear, with overhanging gearbox and flywheel, is heavier than the front, it’s going to droop towards the rear naturally. My immediate problem is making a tunnel and gearbox cover that will fit the components reasonably closely, which is hard to do when their correct position isn’t known.

When my gearbox is exactly aligned with my driveshaft, the angle between the front face of the mount (ring that goes around the bellhousing) and the top of the frame at that point is 88.7°. That is, the frame slopes upward toward the front a bit more than the engine does. If you put a digital level on, say, your cam covers, then on the frame next to the engine, is this about what you get?

Thanks for your help.
 

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Discussion Starter #7
Driveshaft photos

Sorry. I can't get it to work. I go to "manage attachments," browse, click on the photos, tell it to upload. It works on something for a while, but when I preview the post, no pictures.

I am a better mechanic than computer whiz, please believe me.
 

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6C 2500 Drive Shaft Alignment

Paul,

In your trial fit up the gearbox looks way high in the chassis. On the back of a fully assembled engine it will sit much lower in the chassis and has only an inch to inch and a quarter under the drive shaft. The gearbox and the whole drive shaft was originally covered by a tunnel in the floor as you probably know.

You must have gone to a lot of trouble to move the pedal shaft back to where you have it now. If I am not mistaken there is a short cross member around there somewhere with the brake master cylinder attached. I assume you will move the steering box and column back with them or do you have very long arms. It should give you enough room for the blower.

The top of chassis should be flat from the rear end upturn all the way to the front under the radiator. It is only the bottom of the chassis rails which rise up towards the front where it tappers to a shallower depth.

The gearbox coupling is yet another variation on the ones I have seen before in that it seems to have the small bolts and washers holding the rubber inserts in like the 6 pin type I described above.

Another factory tool I learned about was a thin walled light straight tube with a dummy half coupling on one end. It had no rubbers in it and fixed onto the gearbox out put coupling. There was a centred pointer on the other end. They mounted a "target" plate on the diff input shaft with concentric marks on it to help shim the engines and get a straight line to the centre of the rear coupling.
 

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Discussion Starter #12
Hi Pete,

Very useful information. Most helpful is that the driveshaft should be only a bit over 1" above the floor just behind the gearbox. This is what I need to know in order to make a tunnel that’s not overly large.

Yes, moving the pedals was quite a job. And with the supercharger the steering box has to be relocated on the outside of the frame rail, with a jointed column. I’m just now working on one of the biggest challenges, which is to make a shift linkage from a stick convenient to the driver’s left hand forward to select gears from a column-shift gearbox. I think I’ve got it worked out, but can’t be sure till it’s all in place and tested.

My advisor who insisted on aligning the driveshaft built a modern version of the factory tool you describe. It’s a laser pointer that mounts on the gearbox coupling; you then adjust the shims under the engine so the red dot is centered on the diff input.

Again, thanks for your help.

Paul
 

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6C 2500 Drive Shaft Height

Hi Paul,

I have never introduced myself on the forum but name is Jim.

I realized it could be important to check the actual height of the tail shaft in my car more accurately so I got under and measured. It is 42 mm which is 1.6 inches or a little more than I guessed above.

Since your guy has developed a mount for a laser beam it might be too hard to make a mount for the beam on the rear coupling. That would give you the current angle of the diff pinion shaft.

It would be a good idea to check the diff casing has not sagged. The original rubbers have not been available for many years. It is very likely they are not as fresh and fully supporting as they were originally. If the through shafts are no longer centred in the tunnels through the castings it will indicate some wear and sag in the rubbers. Putting the laser on that end will give you a good idea of whether the gearbox pinion can be got into the right position to get the straight line alignment of all three shafts.
 

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Discussion Starter #14
Hi Jim,

Again, many thanks. I'll check everything out. The final adjustment has to wait till the engine is done and in, of course, and that will be a while.

Paul
 
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