Just purchased a set of oem wheels and center caps along with wheel nuts. Tire shop had problems with the black plastic wheelnut covers that keep the caps on. Went for a drive and lost 2. It seems there is not enough thread for them to catch onto the wheel nuts. Is there different sizes of wheel nut or plastic caps or do I have the wrong center caps!!???

There are different sized center caps, but the black nuts are all the same. What wheels are you using?

True. There are two different center caps. Gm is very proud of these, if I recall somewhere around $30.00 each. But hey, that's with the black nuts included.

True. There are two different center caps. Gm is very proud of these, if I recall somewhere around $30.00 each. But hey, that's with the black nuts included.

Oh no, my son..Much higher...as in 90 clams a piece...
If you have aluminum wheels, they take a different cap than the steel/chrome wheels do..

Got a set of 4 "like new" chrome PYO caps at the local tire place for $20:D

The wheels are alloys, so I guess they gave me mismatched center caps? They had a few sets there and he just grabbed a set of GM. Guess I'll go back and see if he has some that are not as deep which would cure the problem. Hopefully I won't have to put Chev ones I my GM but who'd notice. Thanks Guys..

Just purchased a set of oem wheels and center caps along with wheel nuts. Tire shop had problems with the black plastic wheelnut covers that keep the caps on. Went for a drive and lost 2. It seems there is not enough thread for them to catch onto the wheel nuts. Is there different sizes of wheel nut or plastic caps or do I have the wrong center caps!!???
I watched the local tire store guys use an impact wrench to put on the plastic nuts. Put them on finger tight or you will strip the threads.

Not saying this is the problem but, tighten center caps by hand. Do not use an impact. There are little detents on the black nuts and the cap that should be tighented until they are seated. This takes a few more minutes, but I have never had a centercap ever fall off.

The wheels are alloys, so I guess they gave me mismatched center caps? They had a few sets there and he just grabbed a set of GM. Guess I'll go back and see if he has some that are not as deep which would cure the problem. Hopefully I won't have to put Chev ones I my GM but who'd notice. Thanks Guys..
The diameter of the cap is your problem I believe. Make sure that the set you get is smaller than the ones you have.
A search for Center caps or H2 center caps should get you some pictures so that you can see the difference.