This post is a bit long, but I thought that sharing what I've learned about installing the distributor or the timing chain might be of interest.
The auxiliary shaft drives the distributor, oil pump, and injection pump. There are only 9 teeth on the distributor drive gear (40 degrees per tooth), so for any given position of the auxiliary shaft sprocket relative to the timing chain, there are only 9 possible rotary positions of the distributor. This is not an issue for later cars – one of the 9 positions will certainly work. But clearances are tighter on the 230SL, with its rearward-facing vacuum advance unit (see the photo on page 00-16/1 of the BBB), particularly if the car has power steering, so the position of the auxiliary shaft sprocket is important.
If one is just reinstalling a new timing chain, the issue does not arise – Dan Caron’s method of replacing the chain is given in the technical articles on this site. But when reassembling an engine from scratch, some attention must be paid to the position of the auxiliary shaft sprocket. It is possible to install the sprocket to achieve any desired distributor position to an accuracy of a single degree (see below). Does anyone know if there is a mark on the sprocket or some other method of achieving the factory position?
If the engine is already together, and the distributor position is unsatisfactory – for example, the vacuum advance unit prevents removal of the power steering reservoir lid, as it did in my case – it is easy to rotate the distributor by 40, 80, 120, 160, etc. degrees. Just pull it out, rotate the drive gear by 1, 2, 3, 4, etc. teeth and reinstall. But if there isn’t clearance to allow for a 40 degree rotation, here are two ways to achieve the intermediate rotations of 20, 60, 100, 140, etc. degrees without removing the timing chain from the crankshaft or the auxiliary sprocket:
(1) To rotate the distributor counterclockwise by 20 degrees, first put the engine at TDC. Then remove the camshaft sprocket and the rocker arms, and dismount the injection pump far enough to disengage it from the shaft. Then rotate the crankshaft 360 degrees and reinstall everything else just as it was. Then take out the distributor, rotate the drive gear 5 teeth (200 degrees) counterclockwise and reinstall, rotating the distributor body 20 degrees counterclockwise in the process to line it up with the rotor. Then of course reset the ignition timing, to get it exact.
The reason for removing the rocker arms is of course to close all valves so the pistons won't hit them as the crank is rotated. (Alternatively, one could remove the head). The 360 degree rotation leaves the crankshaft in the same position as originally, and as the cam and injector pump were disconnected, they also stay in the same position. The only change is that the auxiliary shaft and the distributor rotor have rotated by 180 degrees. Then turning the drive gear and the distributor 200 degrees in the other direction leaves them 20 degrees from where they started.
If you want a 20 degree clockwise rotation, just rotate the drive gear counterclockwise 4 teeth (160 degrees) instead of 5.
(2) This next procedure should be regarded as only a temporary solution until you have the time to apply the one above. To achieve a 20 degree counterclockwise distributor rotation, take the distributor and drive gear out, rotate the drive gear one tooth (40 degrees) clockwise, rotate the distributor body 20 degrees counterclockwise, and reinstall everthing. Then remove the plug wires from the distributor cap and reinstall them one socket in a clockwise direction from where they started. As above, reset the ignition timing.
The initial 40 degree rotation of just the rotor leaves it misaligned with the distributor body -– there are 60 degrees between lobes on the cam and between poles for the plug wires. Rotating the distributor 20 degrees in the other direction realigns it with the rotor. But now the plug wires must be reinstalled because they are offset 60 degrees from where they should be. That is, whereas at TDC the rotor was originally aligned with the slash on the distributor rim, it is now 60 degrees clockwise from that, so the plug wires must be moved so the spark goes to the correct plug. This of course still leaves the slash mark unaligned, so the next time the distributor is installed it would be easy to get it wrong – which is why the procedure is only a stopgap one. But the car will run fine and the distributor interference problem has been solved.
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I mentioned above that the auxiliary shaft sprocket can be positioned relative to the timing chain (that is, to the crankshaft and camshaft) to achieve any desired distributor position to an accuracy of a single degree. There are 40 teeth on the sprocket (9 degrees per tooth), and as mentioned above, 9 teeth on the distributor drive gear (40 degrees per tooth). To rotate the distributor clockwise by one degree, rotate the sprocket clockwise by 9 teeth (81 degrees) relative to the chain, and rotate the drive gear counterclockwise by 2 teeth (80 degrees). 81 minus 80 is one degree.
To get other rotations, just multiply the number of teeth above by the number of degrees desired. For example, to get a 10 degree rotation, rotate the sprocket by 10 x 9 = 90 teeth. The sprocket has only 40 teeth, so this is the same as 10 teeth (that is, 90 mod 40 = 10), or 90 degrees. Rotate the drive gear by 10 x 2 = 20 teeth (the same as 2 teeth), or 80 degrees. 90 minus 80 is ten degrees.
Thus any desired distributor position can be achieved. But I still don’t know how to install the sprocket the way it was done in the factory, and I haven’t seen it written anywhere. Have I just missed it in the documentation?
Many thanks to Joe Alexander for taking the time to discuss this all with me at some length. Any errors or misinterpretations are however mine alone. Thanks also to David Gallon for encouraging me to look into the problem.
Chris Earnest
