20" softline lock cover o-ring seals

[Drew]

Hi, was looking at my wheels today, to see why one of the lock covers was so tight a fit. Recently, on one wheel only, it has needed quite a push to get the lock cover fitted, and a tug to get it out again
What has happened is the o-ring seal has perished and started to split, and is partly coming out of its fitting channel, so jamming the lock cover

Does anyone know the size of the o-ring seal? It's quite a thin one