I have posted a lot of questions here lately and I apologize. I bought a whole set of Mastercool orings and it has a huge selection of rings. Many of them are extremely close in size and thickness but some are the same diameter but much thicker. How do you tell which ones to actually use? Is there some calculation or magic or do you just put one on that fits snug and call it good and pray that it does not leak? Some are whole numbers like 13x2 (Cannot remember exact numbers) and some are in fractions. Would I be correct in assuming that the whole numbers are metric and for (mostly) foreign cars and that the fractional ones are Standard and for American cars? I know that so many things are metric these days that it doesn't really mean anything anymore.

 

Oh boy, that's tough. With a vehicle-specific O-ring kit, there are always some more O-rings than you need, as the kit will cover a range of vehicles, but at least it's not crazy with infinite variety.

A big non-vehicle specific assortment could have a lot of O-rings, heh, I don't need to tell ya that!

Is there a same-type/size of fitting that still has the old O-rings on it that you can at least narrow it down to maybe two choices?

The Ford spring-lock O-rings were standardized to size 6, 8, 10, etc., but I think that was pre-metric.