How to install a caster in metal or plactic chair bases
A single drop of any oil wiped onto the stem tip and grip ring is often helpful while installing.
In other words, since the grip ring is larger than the insertion hole, it is often best to place the stem with ring at the angle shown in the drawing. Then push down into the hole slightly while straightening up the stem. This will cause the ring to compress to the hole size immediately below the hole opening. Then the caster may be pushed in and seated.
If more force to seat is needed, place a 1/4" to 1/2" thick piece of wood between the two wheels and use a hammer to strike the wood nearest to the stem end.
The photo's show wood placement, which is always over the stem.