John, make sure the washer and sleeve are in place. The sleeve goes into the mount, evidently to protect the alum. mount. The only other things I could suggest are to make surthe bolt is the correct length, s/be 55mm, or use loctite.
Ken, thanks for the advice but have no sleeve on this unit and it fits very very tight in the mount. I have removed the AC and alternator form the bracket and removed the bracket from the engine to see if the bracket was cracked with no joy as it was in great condition. I have used a lot of loctite but the rear bolt still backs out. I tried to get a nyloc nut on the rear bolt but the space is too close. I have to check it about every 100 miles or so and retighten the rear. This is a brand new ac unit and the threads are great. I have used the right length bolt and a longer bolt to try to get a nut on the back. I am thinking of drilling a hole in the bolt and putting a pin in through it so it can't back out. I have a washer and a lock washer on both bolts front and rear. they do not hold. I checked the torque settings and the rear is 27 flbs any more that that and I think you would strip out the threads. when you rev the engine you can see the ac unit move with the torque which is why I think it backs out.