I'm sure the struts on the 911 was exactly the same for a long time. Various hubs bolted on the strut would give you various back spacings, etc. What I'm curious about is if someone had say, four 911 struts, what measurements are pertinent to decide what is compatible and what is not?

1. Caliper Bolt Spacing is an obvious first measurement.

Beyond that, are there different wheel bearing diameters? Different spindle lengths? What about determining which shocks will work and which wont?

Caliper bolt spacing actually doesn't always work, as M struts have the same 3" spacing as 914 struts. Spindle diameter is different, however. I *think* the 911 spindles are larger in diameter than 914 spindles, but I don't know the actual figures.

Pre-69 911 struts are also very different from 69 and later, as the balljoint attaches in a completely different way (not just the pinch bolt v. wedge pin difference that appeared in 72-73).

Spindles are fatter on 911 struts.

Check the ball joint attachment at the base. That will give you a clue to the year. Slot is pre-71 on a 911.

3" will be T struts (in general) and will be Boge. They will have a coarse thread pattern for the inserts. Brad's given the run-down on what fits in there (everything basically).

3.5" Koni struts will generally be orange but could have been repainted etc. They will have a finer thread and only Koni inserts fit.

3.5" Bilstein will generally be green and will fit a Bilstein insert only.

Early struts or more over early "hubs" are designed with a different offset (about 5mm in) that holds solid rotors. They won't line up properly on vented calipers. Look out for that.

What else??

Bilstien have a roll pin on the bottom that hold the strut

Boge/koni have treaded collars on top

early struts have fine threads for the clamp nuts, later struts are coarse thread

early early struts (all) have a different attachment method for the ball joints

the bearing diameter are the same for all

the offset is different

among other things