I was recently supplied with some new Flywheel Bolts described as such. The supplier inadvertently supplied low-tensile and not high tensile, and he willingly put matters right (so no complaints there). The bolt issue is potentially serious with unpredictable consequences. I may well be the only person to have experience this, but equally I may be the only person to have noticed.Moral is "ALWAYS CHECK THE BOLT HEAD NUMBERS"For Information:Standard (low tensile) bolts have 8.8 embossed on the bolt head.High Tensile have 10.9 embossed on the bolt head.If low tensile bolts are fitted and torqued to the correct level (42Nm) then they will already be in “yield” i.e. stretching beyond their elastic limit and will almost certainly fail in service. You can ignore and additional embossed letters like SB, as they are to indicate “Structural Bolt” under the construction product directive (where both bolt and nut must be of the same manufacture so that the whole fastener is matched.There is an excellent guide on-line at http://www.kamax.com/fileadmin/user_upload/dokumente/pdf/Bolt_and_Screw_Compendium.pdf
Actually 8.8 bolts are often regarded by suppliers as high tensile but, like you, I use 10.9 in all high stress applications. 12.9 and 14.9 bolts are even stronger in tension but they can fail if subjected to repeated bending stresses as they are less ductile.
NAS or AN aircraft spec fasteners are definitely the best available but, as far as I know, they are only available in Imperial sizes and thread forms.
For the OEM-equivalent I simply read the original factory fitment bolt-heads. Mick Walker’s restoration book also lists them as “10k” i.e. high tensile.My concern is that, if torqued as specified the 8.8 bolts would already be overtightened before the engine even fires up. (Hence my inclusion of the bolt and screw guide link).Simple torque tightening of bolts is, in itself, not entirely reliable.I agree with you that 12.9 and 14.9 are overkill and introduce other problems. In this application bending should not be a factor, nor should shear. The bolts are there simply to compress the flywheel against the crankshaft flange.