Hello Erick,
The Vertical timing gear is also referred to as the "driveshaft for the oil pump and tachometer" in the BBB. The specified clearance for endplay is very small (0.1mm - 0.25mm) or (0.004" to 0.010").
The gear drives the oil pump and the distributor and is driven from the horizontal timing gear which drives the fuel injected pump.
The thrust piece or "aluiminum/bronze bushing" wears and allows this vertical timing gear to move upward during engine operation. In severe cases, the gear moves up enough to allow the oil pump to dis-engage and stop turning. The engine continues to run fine with no warning until the oil starvation causes mechanical failure!
Checking this endplay or wear is fairly simple. Remove the tachometer cable and insert a screw gently fingertight and move the vertical timing gear up and down to measure "endplay". As specified 0.1mm - 0.25mm (0.004" - 0.010") is ideal. Excessive wear will eventually cause premature gear wear or failure.
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If you have excessive endplay the "thrust piece" (alum/bronze) bushing should be replaced and/or shimmed.
The factory recommends shimming between the bronze and aluminum part of the thrust piece. However copper injector seals can be used as shims between the gear and the thrust piece and seem to work fine. Often times the bronze portion of the thrust piece is worn uneven and must be smoothed out before shimming. Replacement of the thrust piece is best.
Removal of the thrust piece is done by removing the 22mm fitting and the 10mm hex head set screw. The thrust piece can be coaxed upward and will narrowly clear the cylinder head. The timing gear can now be removed and inspected for wear at the helicentric cut gear.
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Some owners have experienced problems installing the new thrust piece. Light wet sanding of the outer circumference and freezing the thrust piece prior to installation will help if problems arise. Normally they will install without these measures.
Endplay should be checked again after installation of the 10mm hex head set screw. Shim if needed.
The 22mm fitting has a rubber O ring beneath it along with a seal ring inside it. There is also the infamous "felt washer"!!! which is new to my experience also. We need a part number for this if it does really exist. The seal in the fitting is the most difficult part of the job. Removal from the fitting can be a challenge. Some listers have removed them by drilling them out! If you do not have a oil leak at the tach drive you may elect to leave the original seal in place. You can always replace it later.
You may want to have seals on hand before you begin. Here is a breakdown photo of the gearing with part numbers. Mike Hughes has provided updated part numbers.
Michael Hughes wrote;
" I, to, am having an oil leak at the tach drive on my 230SL with 75k and thought a little preventive maintenance might be in order as it sounds like a leak here might indicate wear below. I went across the street to the M-B dealer and ordered the thrust piece, rubber o-ring, aluminum coverplate ring (just in case, it was under $4!), and seal. The thrust piece is about $91 and the total for all parts just over $100. The parts guys really appreciated the photo detail as it made their search much easier! The part number for the thrust piece remains the same but all the rest of the part numbers have been superseded: 621 050 05 06 is now 007 997 73 46, 126 990 01 04 is now 126 990 01 40, and 120 261 00 80 became 115 274 00 60 and is now 621 997 00 40. Replacing these items looks to be fairly straightforward."
I will add photos soon to this thread so stay tuned!
Joe Alexander
Blacklick, Ohio
