I know several folks here count the turns to get proper shank length.
Another way is to measure how much of the bolt shank sticks out the backside of the wheels. I learned this from an Otis tech a few years back, 17mm to 20mm sticking out the backside of the wheel is what they recommended for any Mercedes, (about 3/4"), that is what I have always used as a guide, and it's never been wrong.
With the wheel off the car, hold one of your test bolts firmly in place, and simply measure the shank sticking out the backside of the wheel..
If the measurement is more than 20mm you need a shorter shank to fall into the 17 to 20 range, if shorter that 17, you need a shank longer to fall into the 17-20 zone. foolproof, and simple addition or subtraction gets you to a proper length.
With that measurement method, it's easy math to calculate the correct bolt lengths for any particular wheel, (IE if you measure 30mm sticking out the backside of the wheel, you need to buy bolts 10mm to 13 mm shorter than your test measurement bolt. really easy
Alternately, if you get 4 turns, or 9 turns on the test bolt, how does that accurately translate to the correct length you should buy?
If 6 plus is the right place to be for a minimum length, how many turns is too much? I haven't seen this posted anywhere.
As far as what to do if you don't get 6 plus turns. I
wild guess would be 6 turns = 17mm, or 2,83 mm per turn, apply that to 20mm max length and the calculator says 7 turns is 19.81mm.
Or you could simply measure the backside shank sticking out...
The only place in the world I know of where turns of a screw are used to commonly describe length is in lumber. Old sawmills used 1/4" per full turn of the cutting depth adjustment, which is still used in labeling some lumber today, A "five-quarter", ( 5/4), square trim piece is 1-1/4" square, where every full turn of the cutting depth adjustment on the saw mill moved the blade .25".
I prefer a simple measurement in mm.
Otis sends you a free 6" scale when you purchase bolts from them, it makes this shank length calculation very easy to do.
Just a thought...
