Been down this road a few times I know but there's too many inconsistencies and no easy answer and I hopefully want to clear up especially when the larger diameter spline came in and how big is the larger spline.
Unfortunately the manuals just say -
(http://img166.imageshack.us/img166/6981/466100001smallaq0.jpg) (http://imageshack.us)
and nut sizes
(http://img293.imageshack.us/img293/2654/466100002smallwc0.jpg) (http://imageshack.us)
But we've seen a number of times people with 78 and maybe later 116's who say they have 15mm splines which is the same as my cars and wrecks. I also have a wheel off a w123 which has a 123 part number that fits 15mm, despite what the manual says above and I've seen a w108 wheel fit too.
Styria and others in the past including the person I got my boss adapter for my Italovolanti wheel has said the larger spline is 17mm and is the same as w126 wheels. But when I look up w126 wheels, sellers usually say the spline size is 21mm. Grrrr
Can anyone shed some light?
I would take the key as manufactured starting 1978 as it is possible that a car sold in 1978 (or later?) was manufactured pre-1978. Or at least the steering components were manufactured pre-1978.
As for the size, could it be the official sizes in the manual measure the shaft only and excludes the thread?
Fair point on the build dates. Makes sense.
But on size and that second quote I posted from the manual, I assumed M14 and M18 were the nut sizes but I don't know what they mean exactly. I've never understood nut, bolt and thread nomenclature or made an attempt to decipher them. If M14 means a 14mm nut which fits a 15mm spline then you would think M18 equals 18mm and fits a 19mm spline :-\ which noone has ever said that they have a 19mm spline that I'm aware of. Unfortunately, nowhere can I find in the literature in anything I've got or in the library that specifically states that the early version is 15mm and the late is 17mm or something else. But I do take your point on the spline size excluding the thread or the troughs and peaks of the spline.
Furthermore, I hate giving out misleading info and there's been a few times I've said the later version was 20mm approx based on w126 splines being 20 or 21 approx and that w126 wheels have been fitted to later w116's. It makes me wonder though if w126's had two spline sizes to coincide with the two series of w126. ??? I'll post a similar Q on the NSWMBC forum and see what the likes of Matt and others know. Couldn't find any info on benzworld unfotunately.