I've been told by a machinist that tapped holes in aluminum are
>> weak.  Any truth to that?

A poorly planned hole with threads of any type can be "weak" What is weak? Compared to steel?

 

If I was terrified of a hole in aluminum I would go to a stud. Then you are not stressing the threads from screwing a bolt in and out of the hole. You would use a course thread on the studs aluminum end.

 

You would drill the hole undersized and have near 100% thread depth. You could custom make your studs, and have the aluminum end bigger than the steel nut end for much better strength. You could make the stud oversized, and heat the plate and chill the stud in dry ice before installation.

 

You could intersect the junction of the stud and aluminum, with a small blind hole. Press in a length of roll pin to prevent the stud from turning. Drive the locking pin below flush, and peen the hole shut.

 

Or, you could pick the largest diameter bolt that will fit through the TB, and get that bolt or better yet a course threaded socket head cap screw for more wrenching room. When you tap the plate, drill undersized holes as above. Clamp the plate to the bed of your drill press. Insert the tap in the drill press chuck.

Start the tap into the undersized hole. Clamp down the bed and the plate. Using the end of the chuck key as a lever in the key holes in the chuck, turn the chuck while firmly pulling down on the feed lever, until the hole is well under way. Then you will have 4 perfectly plumb threaded holes.

 

When you install the screws, mark them with numbers 1 through 4. Could be engraved numbers, stamped numbers or just dots put on with a punch. Mark the holes in the plate the same way. Install the bolts to snug only. None of that working range or recommended torque stuff. Just snug is about 6 foot pounds if you are starting to panic. Then mark each bolt for two safety wire (032") holes. Wire them to each other in turn. Then drill the edge of the plate next to each screw. Wire each screw to the plate.  On final assembly, use Nickel anti-seize on the threads. When the bolts go back into the same numbered holes, the safety wire holes always line up perfectly.

 

Lynn E. Hanover