I think there are two different style o-rings for protec heads . part numbers # 5267T57 and 9464K114 from Mc MAster Carr , the 5267T57's are 1/16" , the 9464t57's are 3/32"
you need to measue what you have to know which one you need, at any rate the mc MASTER website has all the dimensions on the o-rings , you shoulod be able to match up something locally