The part number of these bolts is/was N908 921 01 and it is an M8 x 28mm (with a multi spline socket head), now, as I said before, there is more than one standard M8 thread, just check the tap against the old bolt, do Halfords or Machine Mart not sell a kit for cleaning up threads - that you would be allowed to check against first?

Standard ISO metric M8 threads are 1.00mm and 1.25mm pitches - that is fine or coarse.

You need to aim as high as the Laser Tools one in Machine Mart before you find an honest statement as to what is in the kit, it is £59 inc VAT. (all the others just give "normal" thread pitches, in the case of M8 that would be 1.25mm pitch)

Screwfix sell a kit for £25 that includes both fine and coarse threads.

Scrub Halfords from that list as they only seem to carry one thread per size.

metric taps come in sets of 3 ... fine standard and course .

m8 fine - 0.75

m8 standard . 1.0

m8 course 1.25

if you know the pitch of the tread you want you can buy a single tap from a local engineering store of ebay etc ....

e.g.

http://www.ebay.co.uk/itm/M8-Tap-and-Die-All-Pitches-M8x0-75-M8x1-0-M8x1-25-/140904448375?pt=LH_DefaultDomain_3&var=&hash=item20ce8f4177

Or to give them the official titles:- coarse, fine, and rarely used extra fine - I left out that third pitch as it would not be used here.

Thanks for all that - very informative and I think its about time I educated myself on threads

I've found that I have a M8 13mm nut and bolt combination about I didn't think was identical at first - looking at the threads side by side....however....looking at it again maybe I was looking at an odd angle and it does look pretty much the same on second viewing

I found the nut works quite easily on the old bolt thread (the old bolt is now back on the car only finger tight however due to the problem with the blockage plus the problem with the spline thread now worn out too!)

The standard M8 bolt I found is not quite as long as the old bolt but I think that is the only major difference between the two, so I agree now I think it is a standard thread

I also tested the bolt on the cars threads (just by finger tightness with a socket to begin with) similar results to the OEM bolt, something is blocking it when its wound in about half way down

That wouldn't go further than whatever is blocking the thead in the car (maybe a bit of muck or something fallen into the threads or old thread lock) and I'm not keen to just try forcing it, although I might wait until better weather so I can get the seats out completely, maybe just lots of lube will help but I think I may well need to look at tap and die to clear out the threads

Hazarding a guess I'd say coarse threads

Good, you would have got no where when screwing the other M8 bolt in if it was FINE thread. BTW, the "13mm" nut you are using will be an M8 nut, the "13mm" will be the AF size (across the flats) of the spanner that fits it.