It has been noted that sometimes after inserting the dust cap the housing is staked to hold the cap in place. This makes it nearly impossible to insert another dust cap. This was the case on my car. I used a die grinder to remove the staked section--no more than 1/4" of circumference. I also found that new caps are a hair too big or too thick to fit well. I have used a narrow cold chisel as a drift to set them, but have also just re-used the old caps.