seats rails ?
 

I bought a used seat on the Internet that included seat rails for a 911.
This was some months ago, but last weekend I was trying to install it and it didn't fit.

The seat rails was to long and to narrow to fit my car.

Has the seat rails changed over the years?

This has the mounting holes 41 cm (16.14 in) apart length wise and 34 cm (13.38 in) in width.

http://forums.pelicanparts.com/uploa...1147691304.jpg

The early cars had different seat rail dimensions. I don't know what they are but it looks like you got the wrong ones. No problem. Either post a wanted ad here or call a dismantling yard for some used ones.
I think starting in 74 they are the same through at least 1989.

Well, I'm thinking of making some adapters from sheet aluminum, 3-4 mm (1/8) thick anyway. I think I need to gain some clearance for my helmet.

But I'm curious to know what car its from :)

I'm not sure that 1/8" aluminum is going to be srong enough to hold you and that seat in place in an accident. The bolts holding down the seat to the rails and the rails to the car are grade 5 (actually in the metric they are grade 8.8) hardened hardware with a very high shear strength. Thin sheet aluminum will probably rip right off of those bolts way before the bolt would have failed.

If you have existing seats, you can just use the factory seat rails.

Seat rails are different depending on how the inside seat belt receptor is located. 85 and earlier 911s the seat belt receptor is on the tunnel. 86+ seat belt receptor mounts to a bracket on the seat rail.
If your seat belt receptor mounts to the tunnel, I'd say you can mount your seat however you want. The belts should be holding you in place rather than the seat.

Sparco and OMP seat rails are $50 per set. Might be less hassle to just buy a set of seat rails rather than fab up something.

As Don said, the seat belt takes the impact in a crash.
I would not bet my life on 4 M6 8.8 bolts holding the seat...