[monte55]

What would stronger in aluminum.....1/4 x 20 tpi or 1/4 x 28 tpi ?

[Geof]

You might find disagreement of opinion on this one.

I have done tear out tests on 6061 and found that 1/4"-20 is better than 1/4"-28 by quite a bit.

This was with cutting taps not form taps and I have no idea whether the results would be different for form taps.

I also found that if you have a correctly tapped 1/4"-20 thread using a 13/64" tap drill, which gives about 75% thread, 8 threads engaged means that the bolt is almost certain to break rather than the aluminum strip out with a socket head cap screw that is equivalent to Grade 8. Using Grade 5 bolts 6 threads engaged is enough that the bolt will probably fail first.

There is a caution however: If you constantly tighten, loosen and retighten threads in aluminum it will fatigue the thread and this will result in failure in the aluminum.