Harry wrote:
Repairing a fence for my daughter and I had to use Hilti bolts,
drilling into concrete. I've never used the things before and they
don't come with instructions.

What exactly do you mean by Hilti bolts? Do they look like these
http://www.toolstation.com/shop/Scre.../d90/sd/p22455

I figured it's simple enough, drill the holes, remove the nuts, fit
the metal plate, replace the nuts on the protruding threads and tap
the bolts into the holes.

Not sure what you mean by 'metal plate'?

Of four bolts, three failed with stripped threads and were absolute
swines to get back out. What's the secret? Don't whack them in too
hard? Whack them in really hard? Screw the nuts down tight before
tapping them home, leave the nuts on the ends of the threads? What?
Any clues folks, the damn things are expensive and I've four more to
do tomorrow as well as the three I lost today.

If they are sleeve anchors, these will fail if you whack them in too hard -
either the nut distorts or the thread becomes damaged - or both. Net result
is that the sleeve spins as you try to tighten them. Leave the nuts on
when you tap them in to protect the thread, use a wooden block to protect
the thread/nut. Tap rather than whack :-)

Important to drill the hole the right diameter (I assume you have) and the
right depth and clear the dust out.

Sleeve anchors are cheap as chips from places like Toolstation or Screwfix -
M10 x 100 for example are £5+ for a pack of 6 in Wickes, but £2.52 for 10 at
Toolstation.

Even better, forget sleeve anchors & use masonry bolts
http://www.toolstation.com/shop/Scre...ors/d90/sd2040


--
Dave - The Medway Handyman
www.medwayhandyman.co.uk