Driveshaft bolt size
 

So my project is slowly coming closer to something that resembles completion, but over the (too long) gestation period of the thing some bolts and miscellaneous objects have disappeared as an offering to the car gods. So what I'm wondering is, what are the drive shaft bolt sizes for the fb?

They're 12mm socket head bolts, with a split-washer and a 12mm nut on the other end. All 4 are the same, and hopefully you marked your driveshaft for how it lines up with the rear end - extreme cases can result in imbalances. HTH,

They are also grade 8

Thanks. Any idea on length or the diameter of the shaft of the bolt?

I would say your best bet is buying OEM bolts, nuts, and lock washers. They aren't too expensive and can be found on practically any Mazda OEM parts website.

Doesn't matter, but it's a good thought for keeping gear wear patterns the same when rebuilding transmissions.

The driveshaft is balanced independently, and the yolk for the tranny and the flange for the rear end are kept within a certain tolerance. Otherwise you could never put in a new rear end, driveshaft, or transmission without changing all of them and balancing them all at once.

For the bolts I'd recommend OEM, but really any grade 8+ shoulder bolt with lock collar will work. Just make sure the shoulder on the bolt is damn close to the same size as the hole in the driveshaft and rear end flanges.

If they're the same bolts and/or compatible with 2nd gen then you can use miata bolts. That's what I ended up doing with my FC. A much cheaper option and easier to find in junk yards than buying new old stock. I mention this as I recall reading somewhere in my search for bolts that Mazda has used the same driveshaft bolts for a at least a several decade spread of cars.

I'm necro threading this. Just for everyone's reference, the bolts are M8 and 25mm in length. The head from the factory is a 12mm however I found a much nicer and tighter fitting with a 13mm head since I needed to replace them when the 12 heads started stripping out and spinning and I'd lost 1 already. If you do insist on using the stock bolt then I'd recommend buying from either Atkins or Mazdatrix since, as of this writing, they are priced at $5 per bolt from Atkins and $5.06 per bolt from Mazdatrix and the dealer priced them out at $8.64 per bolt to me this morning.

I didn't see where you stated the year of your car.

The earlier FBs had a different - smaller pinion flange on the rear end. Later cars (83 and later?) have a larger pinion flange.

The significant difference is that the earlier small pinion flange has threaded bolt holes and use a bolt and lock washer. The later, large pinion flange does not have threaded holes and requires bolts and nuts. In both cases you should use only stock replacement bolts or 10.9 bolts that have the same shoulder length.

I use the an early drive shaft with the small pinion flange in my race car to save weight. I replaced my factory bolts with 10.9 bolts that have the same shoulder length but with a longer thread. I use a flat washer under the head of the bolt, blue lock-tite on the treads and then put a lock nut on the portion of the thread that goes through the pinion flange. I do this because these fasteners shake loose on my car and I see the lock nuts as insurance.