Is anyone else having trouble viewing pics posted by kwame82 ?
The S500 wheels use M14 bolts wheel and your SL M12. I reckon 10mm of thread going into the hub would be okay, careful if the thread is too long on the back, it can catch on the brake shoes, if this happens grind them down a bit to get clearance.
a) Buy 12mm bolts with 14mm ball seats
b) Buy spacers that bolt onto a hub with 12mm bolts but is tapped for 14mm wheel bolts.
Neither is perfect, but b) is slightly better because you don't have to use extra long bolts with a non std seat.
Mercedes Alloys - Alloy Wheels Direct
The best option is to sell the wheels and buy some that fit the car. Don't use those wheels with M12 bolts, they won't torque up correctly.
I reckon 10mm of thread going into the hub would be okay,
I'm pretty sure you want closer to 15-17mm into the hub, about 3/4" (but I could be wrong). I see others say 12mm is right, as some authorities say at least 8 threads must stick out the back (~12mm).
I can check this number, but at least 4-7 turns into the hub once the bolt starts threading in. I think it's 4+ for the rear and about 6-7 for the front. (It's more dangerous if a front wheel falls off then a rear.). Most of my experience comes from earlier models with less power, like the r107.
All the information are looking for you will find right here, best advise I can offer:I’ve got this company to custom make me rims for my 1990 500SL. These are the dimensions, pls let me know if anyone has experience with fitting these size rims on early R129s and if there will be any issues? I have to wait three months and pay upfront for the rims and can’t return them...
18x8.5 5x112 ET35 66.6