I've been lacking with updates as seems to be a theme with this write-up, but, thankfully not because no work has been going on!
I've been having (anticipated) trouble with rear axle ratios. When I test drove the car, the gearing was very low, as expected. in 6th at 35-40mph. I tried running on wheels/tyres with a 2 inch larger diameter, but even this didn't make a significant enough difference.
The original W108 axle has a 3.46:1 ratio. There are versions from the 6.3L 300SL that have a 2.82:1 ratio, I think. However, these are rare, and accordingly expensive. Also purchasing one over the internet comes with a lot of risk as they are probably all more than 40 years old at this point. I looked at the idea of just putting in new internals into my existing axle, but failed to find anything that would fit easily. It quickly came down to having to swap out the whole axle for something more readily available with an appropriate ratio.
I already had the rear diff and subframe assembly from the 1999 E300 (W210) car that donated its engine. That has a ratio of 2.88:1, however, some quick measuring tape work established that it wouldn't fit between my chassis rails on the W108. Some further googling uncovered a sketchy report that someone had fitted a subframe from a W203 and although I couldn't find a write-up on the job, the photos looked like it hadn't been too troublesome.
I found a complete W203 sub-frame from a 270CDI for 120Euro. It wasn't in awful condition, there were a couple of joints and bushes that needed replacing along with the brake backing plates and it needed cleaning up, but was structurally sound. Brake discs, callipers, CV boots, etc. all looked good. Most importantly, it had a suitable ratio in the diff of 2.65:1
The original W108 axle was removed, and the bracket attached to the car floor that was used to stabilise it laterally was cut off to make room. It became apparent early-on that 2 things weren't going to work out. The first was that the wheels would be too close together to clear the inner rear wheel arches. Nothing some appropriately sized wheel spacers wouldn't solve. The second issue was that the front-top suspension link was going to foul the chassis rail on each side. After much head scratching, it was decided to just remove these links until the whole thing was fitted and then cut and strengthen the chassis rails by the required amount. It's a pity to have to do this, but there should be great benefits from having the modern multi-link rear suspension rather than the old swinging axle setup.
There were some good points too. The subframe fits quite well into the existing floor pan without any chopping of the car required aside from the chassis rails mentioned above. The shocks looked like they'd line up with the original top mounts well, and there was room for the new spring seats to go in without anything being in their way.
Thinking about how to fix the subframe in place, the rear was relatively straight-forward, just a cross-member to add with two spuds sticking down from it. This was chopped from the W210 donor car. The front, however, was completely devoid of any siutable anchor points, and of space to add anything as the rear seat footwell interfered with anything that could be added. I have a great friend who's been helping out throughout this build and he came up with the idea of butchering the W210 subframe in order to take its front mounting points and add them to the front of this W203 one. That would make teh W203 subframe longer and enable us to fix it to the structural points where the original axle had been fixed.
So, two front "arms" were cut from the W210 subframe and welded to the W203 item with a lot of reinforcement being added to make sure they would stay put!
I've managed to delete all my photos of the process, so I've stolen an internet photo here to try to explain what went on with a picture. I'll upload a proper one when I have the subframe off the car again, but for now, this will have to do. This is not to scale or anything and there's loads of extra reinforcing gone into it, but this gives you the idea:
Two mount points were added to the car for these to bolt up to. An enormous amount of care and attention went into the location and alignment of these fittings so that the subframe would end up in the correct horizontal plane as well as being centred properly in the car at the same time as having the line between the two rear wheels perfectly perpendicular to the car's mid-line.
Once the two front mount points were done, we bolted the rear cross member to the subframe and bolted it up to the front mounts. This gave us positioning for the rear cross member which was duly welded up to the boot floor.
Now that all 4 subframe mounts were sorted, we could turn our attention to shock absorbers and springs. The position of the lower shock mounts meant that they couldn't be mounted up inside the existing shock turrets as we had hoped. They hit the edge at the bottom of the turrets. So, I bought some short shocks (VW Type 2 van itmes) and we made mounts for the tops of them that were fitted inside but near the bottom of the turrets. The bottom mounts just had to be drilled out from 10mm to 12mm. The upper spring seats were already part of the rear cross member section that had been removed from the W210, so that aspect actually worked out well!
As mentioned earlier, the upper front suspension links would hit the chassis rails when the suspension was compressed. So, now that the subframe was in place, we could figure out how bad it was and cut sections from the chassis rails to compensate. The chassis was cut, re-made and plated on both sides of each rail to add back the strength that had been removed. You can see in the photos below how much was chopped and why.
The inside of the chassis rail looked in reasonable nick in this area which was encouraging:
Next we turned our attention to the prop shaft which needed to be shortened a little and be adapted to somehow attach it's 4 hole UJ flange to the 3 hole job on the new rear diff.
The hubs, new backing plates and callipers were all blasted and painted in the hopes of preserving them. They were then re-assembled with new bearings, bottom bushes and handbrake shoes and look like this:
I've struggled to write this update and keep interesting without making the work sound easy. Believe me, this was not a pleasant nor an easy job. We met problems everywhere. The mounting spuds on the W210 rear cross member didn't fit in the W203 subframe mounts and had to be chopped off, turned down on a lathe and re-attached. The outside of the W203 subframe had to be chopped to wihin an inch of its life to fit between the car's chassis rails. The W203 hubs needed their bearings and lower bush replaced. The two front suspension links on the subframe which are used for adjusting rear tracking were seized in place and had to be cut out. The new brake backing plates were too big for my original 14 inch wheels so had to be modified to fit. The drive shaft was too long for our lathe which made alignment a nightmare when welding it back together having shortened it. The drive shaft flange (which was attached to the rear universal joint) didn't match the input flange on the diff, so a custom adaptor job had to be made up. The newly made exhaust had to be re-designed to go around the subframe and the back box shortened by about 1o inches. The handbrake cables were too short and had the wrong fittings on the ends, so we had to modify the hubs to take the original W108 cables. One of the W203 callipers was badly seized so the W210 ones were used but these wouldn't fit inside the wheels so we took the angle grinder to the pad wear sensor mount point on the callipers. This meant using the W210 disks but these rubbed the backing plates so had to be adjusted slightly to fit.