Hi, I have just seen your post, and I would be interested in a couple of things.
I have Penta's on my 1983 500se, and I used the original wheel bolts, and there was no problem. But I didn't make a note of whether the seat was conical or ball, and I notice that you say your original bolts are conical.
I am guessing that mine are the same, but the reason I ask is because I want to order chromed bolts from McGard and I need to specify whether the seat is conical or ball, and as I am away, working for a couple of weeks, I posted the question on the forum.
I didn't get any replies, except for one from ianrandom [thank you], who said that the original bolts would be conical for the Penta rim.
Did the bolts for the Aero I's turn out to be ball seat ?
I fitted 10mm spacers on my 500, because the 8x16 rim on the rear wheels tend to sit a bit too far in under the wheel arches, and I am positive that the new, 10mm, longer zinc bolts are a conical seat. This is the disadvantage of using the same wheel size all round, on the saloon. The front wheels are further out than the rears, and I think it is why a lot of sets in the States are sold as 2 x 8x16 and 2 x 9x16, for example.
I would be interested to see if the SEC has the same issue, if you fit 4 same size rims, do the rears sit a bit too far under the rear arch, and make the car look a little bit unbalanced. If it does then, in my opinion, fitting 10mm spacers makes a big difference in the stance of the car, without having to go to a different sized rim on the rear.
The reason that I want to change the bolts is that the original bolts look very shabby. I think that they may have been chromed originally but I am not absolutely sure.
Perhaps you could just give a very brief reply to say if your original bolts were chromed, and if you have to use the ball seat bolts for the Aero I.
It would be an interesting "sticky" thing to have, a listing of the bolts lengths, patterns, and seat types for the different original, and option, rims. Although it could be a lot of research to get it accurate.
parkman