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Head bolts/studs are a very discussed topic, and not just in the BMW FI world. In most cases, many people seem to go with X because its what someone else did. I started asking "why", and that lead me on quite an interesting journey in the motorsport world.
Some may remember the M54 bock and heads that I cut into slices last year. Some of that was to evaluate head stud options, and some of it was to look at matting surface stiffness. I sent measurements and pictures to quite a few motorsport companies to get their feedback on various concerns. The result kind of surprised me.
Over 80% of the "load" on a fastener is held on the first 4 threads of engagement on like materials. A good example of this would be the nut and a bolt. On an engine block, we often thread the head bolt or stud 15-20 threads deep. What's up with that? Un-like materials.
The greater the difference in tensil strength between materials, the more thread engagement is required. There comes a point however where the thread engagement holding capacity exceeds the tensil strength of the fastener outside of the block. When this happens, it won't matter how many threads are engaged on the block...the fastener will stretch without pulling out of the block under load.
Installing a bolt on an engine block applies a twisting, as well as pulling force on the block threads. To distribute this force so that the fastener and the block doesn't get damaged, BMW uses a lot of threads. If BMW used studs instead of bolts to hold down the cylinder head, you'd find a lot less thread depth in their blocks. So yes, the thread depth is over-kill when using studs on most production blocks.
So if more thread depth is unproductive, how do we increase cylinder head clamping ability?
Increasing the tensil strength of the fasteners, and/or increasing the size of the fastener.
Before that is done though, one of the simplest ways of increasing the cylinder head clamping ability is to convert from a bolt to a stud. A stud applies next to zero twisting force on engine block threads when being torqued down, which increases the amount of load you can apply to the engine block threads in one direction.
Increasing to a larger cylinder head stud, from 10mm to 11mm for example, gives a significant increase in clamping force potential. The gains come both from the strength of the larger diameter fastener, and the increased surface area of the thread matting surface in the block. Thread depth? Not really a concern.
So for those looking to hold their cylinder head down with big power: Pursue a stronger fastener material, preferably a stud, and a larger diameter fastener. Forget about increasing thread depth on the block.
Some may remember the M54 bock and heads that I cut into slices last year. Some of that was to evaluate head stud options, and some of it was to look at matting surface stiffness. I sent measurements and pictures to quite a few motorsport companies to get their feedback on various concerns. The result kind of surprised me.
Over 80% of the "load" on a fastener is held on the first 4 threads of engagement on like materials. A good example of this would be the nut and a bolt. On an engine block, we often thread the head bolt or stud 15-20 threads deep. What's up with that? Un-like materials.
The greater the difference in tensil strength between materials, the more thread engagement is required. There comes a point however where the thread engagement holding capacity exceeds the tensil strength of the fastener outside of the block. When this happens, it won't matter how many threads are engaged on the block...the fastener will stretch without pulling out of the block under load.
Installing a bolt on an engine block applies a twisting, as well as pulling force on the block threads. To distribute this force so that the fastener and the block doesn't get damaged, BMW uses a lot of threads. If BMW used studs instead of bolts to hold down the cylinder head, you'd find a lot less thread depth in their blocks. So yes, the thread depth is over-kill when using studs on most production blocks.
So if more thread depth is unproductive, how do we increase cylinder head clamping ability?
Increasing the tensil strength of the fasteners, and/or increasing the size of the fastener.
Before that is done though, one of the simplest ways of increasing the cylinder head clamping ability is to convert from a bolt to a stud. A stud applies next to zero twisting force on engine block threads when being torqued down, which increases the amount of load you can apply to the engine block threads in one direction.
Increasing to a larger cylinder head stud, from 10mm to 11mm for example, gives a significant increase in clamping force potential. The gains come both from the strength of the larger diameter fastener, and the increased surface area of the thread matting surface in the block. Thread depth? Not really a concern.
So for those looking to hold their cylinder head down with big power: Pursue a stronger fastener material, preferably a stud, and a larger diameter fastener. Forget about increasing thread depth on the block.
