The Bosch distributor on the 1800′s is part number 0 231 115 048 and is made of cast iron. They typically remain in benign neglect since they apparently continue to work with no issues. However the precision of your timing will be impacted by the play caused by wear in the moving parts, which is caused caused by all the dirt and grit.
There is a rebuild kit sold under the Bosch part number 1 237 010 007 for “2002 distributors”. I purchased mine from 2002 AD since I could actually see a photo of the kit and its components. I suspect this is the same kit sold under a BMW part number through BMW. The kit is made to work with many different types of distributors therefore there is a large number of parts included not all of which are needed for a single distributor. As well, many of the washers which appear to be the same are actually of different thicknesses since they act as shims to take axial play out of the rotating center shaft.
Here’s what the typical distributor looks like after years (decades…?) of duty:
Some distributors have c-clips inside the hollow cam shaft retaining it in place; not the 048. You remove 3 of the 4 visible screws and the assembly pulls up: The screw holding down the points, the one retaining the points wire to the body and the screw holding down the vacuum advance shaft. Remove the two screws holding the vacuum advance diaphragm housing to the distributor body and it pulls off. You can now pull out the camshaft and the two plates sitting on top of it. Those two plates are still held together by the black metal retainer under which there is a small steel ball. This retainer is held to a vertical threaded eyelet with a screw. Although my photo shows this screw off while the assembly is still in the distributor, it is much easier to undo this once the assembly is out. Photos below will help visualize all of this:
Note that I have already removed the drive gear off the main shaft. This gear is help in place by a 4 mm pin mushroomed at the ends. It can be seen in the photo below. The pin needs to be drilled out. Be ginger when driving it out since you don’t want to be banging too hard on the main shaft and ruin the bushings it runs on. This one had no radial play and I wanted to keep it that way.
The shaft, cleaned and de-greased with new shim and fiber washers. On top of the shaft you can see the plate where the weights slide in and out. The thin fiber shim provides a low friction, durable surface for the weights to slide. Mine was not too worn out so I reused it but you can easily cut one of out of Teflon or some such modern, low friction material.
One of the weights (right on in the photo) has a longer shaft underneath that fits in an opening on the shaft plate. This prescribes how much the maximum advance will be.
There is a rectangular piece of oil-retaining felt between the main shaft bushings set in place such that it forms a cylinder wrapping around the main shaft. You can somewhat see it in the picture below. It comes out easily and is surprisingly resilient as mine was in perfect condition and sturdy enough to be cleaned with dish washing soap and water and after dry, re-oiled with engine oil and put back in place.
After the main shaft is in place comes the task of placing the drive gear back in place and re-pinning it. The drive pin is spec’ed out at 4 mm x 20 mm. You can find a roll pin this size at a good hardware store. It will stick out about a mm on each end but it won’t hit anything. If you hammer the new pin in you risk damaging the bushings and or shaft and introduce radial play in the main shaft. This will be manifested as a difference in timing cylinder to cylinder (your timing mark jumps around when setting timing with a timing light).
Now the camshaft can go back in. Those little springs are the key to accurate mechanical centrifugal advance. If they get lost or overstretched, I have no idea where they could be sourced so be very careful with them. Unless you have a vintage distributor testing machine the only way to test they are performing properly is with a timing light once it is installed in the vehicle.