Was away from PC, just read through all this great information. Thanks everyone for sharing, very interesting points...
After reading all the posts it's clear that original bolts are the way to go, I'll call Larry. For the sake of completeness and in case Larry ever runs out of bolts and nuts I'd like to add few points and more questions. Just trying to understand what goes into proper fastener selection, torque wrench use and thread locking on critical applications like these. I'm at the point on my mechanical restoration where I started jetting carbs, adjusting valve lashes, aligning suspension, engine driveline, correcting mistakes (some mines and others') etc and this is all very helpful.
Ed, I hope you did not get hurt
Do you know what caused flywheel bolts to shear (besides the extra power and high rpms)? This is 4th time I'm reading about something like that, scary. Did this happened on your spider, what kind of damage did this cause?
First a Correction!!! I assumed that all 4 yokes were the same 6mm thickness...WRONG!
Yokes on the rear driveshaft are 6mm, yokes on the back of the front driveshaft and on the differential are around 7.5mm thick (My previous post edited) I will measure the shoulder length on Alfa bolts when I get them and will post.
Meanwhile, per Alfajay's suggestion I found some M8 zinc coated bolts at local machine shop. These are class 10.9 with longer than 25mm shoulders/shanks and they can cut and thread them 1.0. (will need replating or some vaseline after assembly to protect from corrosion) Bolts have flange heads and nuts are normal hex (same 10.9).
-I noticed a taper on the edge of the shoulder on most of the bolts that I looked at. It's where the thread begins, how important is to have this taper?
-What do you all think about the possibility of a metal fatigue/ microscopic cracks on these 40 old bolts?
-Torque spec that I found online for M8x1.0 dry threads is 28-30 ft/lb (correct?), so this is pretty close to Alfa's shop manual's torque specs (25.3-28.9 ft/lb).
-I'm planning on using split lock washer and adding medium strength blue thread locker after testing for vibration. Loosening one nut at a time and applying it.
- Great point about properly locating and aligning holes. Now that I think about it, as I was taking out 5/16-18 bolts they felt bit loose in there but dragged on something and was difficult to remove. Maybe holes vere misaligned...During the suspension rebuilt driveshaft was disconnected from the differential and put back together by a "Reputable" shop, the same one that assured me my U-joints were all good. Hmmm
I thinks this is all, for now...