Can I use 12.9 bolts as head bolts?

[bkaesvziank]

So I have a substantial oil leak from my head gasket. The car currently has OEM head bolts which I obviously can't use again. I can buy hardware with a company discount and also next day delivery. With company pricing I can get 12.9 m10x1.5x150 bolts for almost the same price that I can get OEM head bolts. The tensile strength of class 12.9 bolts are about 175k psi which are close to what ARP head studs are but I don't know if there's anything else that I need to worry about.

[ForcedFirebird]

Can you, sure, but it's not ideal. The reason behind TTY bolts is two fold. One, the bolts keep clamping after installation so as the gasket compresses the bolts shrink and keep the clamping force - the other reason is speed of manufacturing. A single machine can tighten all the bolts at once. In fact, the m20 originally came with 12.9 and were revised because of the hexagonal heads tend to break off after use. There was a recall in the 80's and the dealers were supposed to swap out the hardened bolts for TTY. The m30 never got the update, neither did the s14 - so - you have to torque the head bolts to 45ftlb, wait 20min, then 59ftlb, run the engine for 30min, then angle them 45°.

Also, don't use a crap gasket. The m20 tends to leak at the oil return passages where the head meets the block since the oil returns are outside the head bolt pattern. Goetze or OEM BMW is what we use. The Elring/VR etc have anemic silicone, and are almost guaranteed to leak.

[bkaesvziank]

Gotcha. I already have a goetze head gasket so I'm glad you said that those are good to go. The main reason I wanted to go with a bolt was so I could get my car back on the road. I've had shipping take a couple weeks recently and I don't really want my car off the road for that long.

Would there be an issue with just using bolts or socket head cap screws as a stop gap until getting the tty bolts?

[ForcedFirebird]

No, in fact you could leave them, just be weary the head tend to break off.

Most places are still shipping....

[bkaesvziank]

Yeah I understand most places are still shipping it's just that a few things I've ordered recently have been delayed by quite awhile and I don't know if I want to take that chance

[Vincenze]

Can't you get the bolts from the closest dealership?
They are not very expensive: the MSRP is $40.

[bkaesvziank]

The 2 dealers that are close to me do not have any in stock.

[digger]

No, in fact you could leave them, just be weary the head tend to break off.

Most places are still shipping....

the heads break off because of a manufacturing process issue with the grade 12.9 and sometimes 10.9 its not a design issue of hex fasteners. all fasteners that grade are at risk if processes are not followed. Hydrogen embrittlement is a much better understood process, if you avoid zinc plated 12.9 bolts (these are more prone to it) and source from reputable place its very low risk issue these days.

[ForcedFirebird]

Apologies if that came off as a complaint about the driver shape, it was more about the metallurgy.

The old bolts were just standard shoulder-less bolts, the reverse Torx have the extra wide shoulder near the heads. The shape of the driver was probably just to keep all the engines similar for tooling purposes, and BMW fixed the old design that required the cams to be removed to get to the head bolts. The second generation 4v heads can use the special socket to remove the head without removing the cams.

I also wonder how good so many heat cycles over an engine lifetime can be for extremely hard fasteners. Annealing doesn't really need very high temps.

[digger]

250C/480F is normally the limit for Gr12.9 no real chance of seeing that in a headbolt or rods/mains, other grades are capable of higher temps. The heat cycles will kill the composite/squishy gaskets before the bolt. The bolt relaxes slightly due to high contact stresses in threads under the head etc and other asperities so torque in stages waiting between steps. The problem with non torque to yield studs and bolts is the need to re tighten a composite gasket as the gasket movement can lose a big chunk of preload when they get heat cycled. A stretch bolt stretches more and is more flexible/springy so can accommodate the movement much better.

[bkaesvziank]

So I should be fine with the black oxide bolts from fastenal that I'm using?

[digger]

So I should be fine with the black oxide bolts from fastenal that I'm using?

yeah cant see why not, i think they have decent quality. When i used to work for one of the big OE in industry we used get our bolts from fastenal for US build jobs. We still lab tested each batch internally though