Shouldn’t head bolts all be the same?

[digger]

I actually went and looked at an OEM hex head bolt before posting. I have a box of them that have accumulated over the years. Clearly have ”12.9" stamped on them.

Correct. With ARP it any other rigid fastener, graphite/composite gaskets require retorque. This applies to the m30, s14, and many early m20's. It wasn't until about 1990 iirc that the bolts changed to torque to yield.

Yes, penny pinching and assembly time are a big factor when you are making 2 million cars. TTY bolts make the assembly process automated. A machine can spin all the head bolts at the same time 180° (90°+90°) vs having to heat cycle and retorque.

Also, yes, MLS gaskets won't compress like a fire ring made of sheet metal rolled around graphite.

yeah i meant the OE torx which is what people are not using by going to the metric blues hence why people say upgrade. they probably went with 10.9 rather than what was originally on there as its slightly less susceptible to hydrogen embrittlement