Bolt torque discussion

Duane said

I saw a video by one of the major head gasket manufacturers and they were saying they had made a custom
Machine that monitors how much a cylinder head lifted off the block as the piston went through its cycles. Yes thats right lifted.
Although the head appears to be stationary during engine operation, the head actually lifts
A very small amount unloading the head gasket. If this unloading is excessive, the gasket will fail. The elasticity of the bolts allows this to happen
The sbm with its 4 bolts only per cylinder was particularly prone to this at high compression ratios.
The reason the Mls style head gaskets were a big help to the troublesome sbm is the spring steel can move with the head but still retain a seal and spring steel is noted for its ability to retain its shape.
So a certain amount of bolt stretch during operation is considered normal.

Click to expand...

Yes, the combustion pressure against the head is taken by the bolts so the idea that the external forces don't end up on the bolts is incorrect. With the extra peak force on the head from combustion, each head bolt around a cylinder will see peak tension increase by a few thousand pounds, and then go back to just the tension from torquing. That increase in tension will stretch the bolts a tiny bit each combustion cycle before they relax back to the original length.

The solution to getting less movement each combustion cycle is to use a thicker bolt/stud, or one with a higher Young's modulus, also known as the modulus of elasticity. That is what ARP is doing with different ('stronger') stud/bolt materials.

Even then, you need more clamping force because the head is going to bow between the fasteners each cycle. More torque on a stock bolt might trap the gasket better, but can you an only get so much improvement before it reaches its limit in other ways.