I read people have this problem all the time. At least the miata people do. Mainly because they're doing it wrong.
Best to use a stud and nut, but bolts work too. I used bolts. You want the fastener to thread into the manifold 2 to 2.5 times it's diameter. The more, the better. And the bigger the diameter, the better. When I got my manifold, it was 8mm holes. I drilled and taped it for 3/8 (about 10mm, but 3/8 tap was 20 bucks cheaper than 10mm tap
). Rather than tapping the holes 1/2 an inch deep or so, I tapped them as far as possible. On my manifold, this gave me about 1.25" of threads. I C-clamped a 3/8" nut to the table, screwed turbo bolts in it, and then drilled the heads of them with a 1/8" bit. Got everything installed, torqued the bolts, and they haven't been touched since.
But for baller status, today I went ahead and wired the bolts together.
Go to Lowes. Buy:
1' of 1/16" braided cable........ $0.25
Pack of 1/16" ferrels...............$1.50
Put wire in bolt heads like shown. This way if any bolt tries to turn out, it pulls on the two surrounding bolts, trying to tighten them. This way as long as all the bolts are tight when the cable's installed, none can ever loosen. Pull the cable tight with pliers, then crimp a ferrel to hold the wire. I put the ferrel hanging off the side so I could get some pliers in their to crimp the ferrel.
Works great.