Serp Alternator Bolt Size?

[petepetrolhead]

Anyone know the size of the long bolt that fixes the alternator to the mounting bracket? Having rounded the head I need to find a new one. I don't think it's metric series (the shaft diameter is 7.75mm or 0.305" and the thread is too coarse to be metric). The head is 12.5mm across the flats. May not be the original bolt of course, but seems a good fit.

[portzi]

petepetrolhead said:

Anyone know the size of the long bolt that fixes the alternator to the mounting bracket? Having rounded the head I need to find a new one. I don't think it's metric series (the shaft diameter is 7.75mm or 0.305" and the thread is too coarse to be metric). The head is 12.5mm across the flats. May not be the original bolt of course, but seems a good fit.

Have you not got a split bush that fits into the alternator location holes so then automatically tighten down to the diameter of the bolt?

failing that all you need is a bolt thats long enough and close to, or just under 0.3 of an inch . 5/16 is too big, and M7 bolt may be a little slack, but I would try M7, high tensile bolts then may tighted up ok? I am away from my car so cannot get a vernier on mine.

[phillpot]

From your measurements and description I would suggest it is 5/16" unc, (18TPI), 1/2" AF spanner size.


M7 does exist but I've never come across it in the automotive trade.

[petepetrolhead]

Thanks for your help guys. I checked the thread pitch (hadn't thought of that!) and yes, it's 18 TPI so looks like UNC. It seems to fall between 19/64 and 5/16 so I'm guessing 5/16 is correct as it's a much more common size. It'll easily go through the lugs on the alternator, it's the bore of the long hole through the bracket that could be a tight fit. I'll give it a go anyway. Cheers.

[Sardonicus]

Its actually an M8 bolt originally with a long shank, the 2 bolts for that serp style alternator are identical top and bottom

[petepetrolhead]

Thanks everyone for your help. For anyone looking at this post in the future, the answer to the question is that an M8 x 120mm bolt and a 5/16 x 4.5" UNC bolt are virtually identical in size and both fit nicely. The only significant difference is the pitch of the thread (UNC is a bit coarser). Either bolt will work fine - I've bought both and tried them!