N/A fatter O-rings more prone to leaking air ?

[airgunfans]

This goes againts my intuition ..... I am replacing the O-rings of my Walther LP400 pistol. One of the O-rings with spec-size CS 1.5 OD 15.5 ( unit is mm ) is for isolating the ambient air and the inner chamber of the regulator. I put in a CS 1.8 OD 16 ( over sized ) and it sealed for about an hour before suddenly leaked with loud hissing sound. Then I replaced it with a CS 1.5 OD 15 ( under sized ) and it is still sealing after a few hours. This is kind of puzzling to me , I thought over-sized O-rings should be a safer choice ? can anyone shed some light on this ?

 

[nervoustrig]

Your intuition is correct. The larger O-ring did not leak for the singular reason it is larger. It must have been compromised in some way, potentially damaged during installation, or it is a softer elastomer (lower durometer) and it soon extruded into the gap and failed.

 

[Gerry52]

As said by nervoustrig, reason for failure is probably not attributed to size only. Most of us have learned to have multiples of each ring needed as sh*t happens. I'm willing to bet that undersized ring that's sealing NOW won't be for long (not as long as the mfg specified size would). Your title would have been more accurate if it said "non-specified o-ring more prone to leaking" There's a reason mfg's specify sizes.

 

[Motorhead]

Your intuition is correct. The larger O-ring did not leak for the singular reason it is larger. It must have been compromised in some way, potentially damaged during installation, or it is a softer elastomer (lower durometer) and it soon extruded into the gap and failed.

Yup ... shaved a sliver of the cord most likely being forced into position.
Better bet if NEEDING to substitute ... Is, use a smaller O.D. / I.D. o-ring than you did and stretch it on. As stretching on a slightly smaller o-ring REDUCES it's cross section ( Cord Diameter ) slightly and can be a fudge substitute when you don't have the exact size you need.

Don't ask how I know