Bring the engine to TDC using the timing marks if you haven't already.
The next step seems counter-intuitive but it's critical for your sanity. Mount the Hall effect trigger in your desired location. I've seen the M115 mounted in four or five different ways depending on year, accessories and body styles. My M115 is mounted in a 1971 250C W114. I am lucky enough to have the front stabilizer plate which makes for a good mount.
Mount it loosely to allow for adjustment.
Slip the crank trigger ring onto the back side of the pulley. Do NOT secure the ring!!! Try to get it as even as you can. Thus is not a final fitment so it isn't critical. I used a bit of melted wax to hold it into place. Putty would work as well.
Here's the confusing part. Mount the pulley/ring assembly to the crankshaft. Just snug the bolts. Don't torque them down. This is an initial fit.
Do your adjustments to the sensor mounting bracket. They call for a 2MM clearance between the gear and sensor. I'd start with 5MM (3/16" apx.) and adjust for final later. Try to get the sensor centered over the gear as close as possible.
You want the sensor to be in the middle of the tooth which is the ninth tooth PAST the missing tooth on the 36-1 ring gear. Undo the three bolts (if you put them all back in place) Locate the missing tooth. Count 9 teeth counter clockwise. Rotate the pulley assembly so the selected tooth is centered on the sensor.
If you're lucky three of the SIX (I told you we'd get back to them) holes in the dampener will line up with the existing THREE holes in the pulley. You've got some wiggle room, half a tooth or so but it's better to be exact. If not, then install the bolts in the closest matching hole and twist the gear to the proper position. MARK that tooth and it's corresponding position on the pulley. I used a scratch awl the SECOND time I moved the pulley. Magic marker doesn't last when you use a solvent to remove the wax (
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Your choice on how to secure the gear. I used epoxy but there's no reason you couldn't tack or spot weld it. I wouldn't run a bead all around because of the chance of warping. Try to evenly distribute the weld around the circumference.
I used washers as spacers to ensure the ring gear was parallel with the pulley. If you've got a turntable or know someone who does this is an excellent way to check the straightness of your gear.
Center the pulley on the turntable and spin 'er up! You'll be able to see any wobble. Using wax allows you to get the placement just right. You could just take it to a machine shop and have them do it.
Here's the bracket alignment with the alternator/water pump belt in place.
Shot from the left side, no belts but all pulleys in place. Gets kind of crowded down there and that's with my use of an electric fan. Imagine adding a fan into the mix and you see why I don't want to use a trigger wheel mounted in front of the pulleys
