It's a problem I've encountered many times -- take apart some piece of pool equipment with a BIG O-ring, go to put it back together and somehow, the O-ring is too long.

I don't have a 'solution', but here are some tips I've used:

1. Sometimes a new O-ring won't be stretched out, and will fit better. (But not always).

2. Many O-rings are stretchy enough, so that HOW you put them in the groove matters. If you fill the groove with pool compatible lube (Jacks Magic, various silicone lubes, NOT Vaseline!!) and 'stick' the O-ring in place, and then work it in, 'cramping' the O-ring into place by pushing it into the groove, away from your point of insertion, you can sometime sorta' 'un-stretch' it into place.

(In my experience, it would often pop back out, just as I had ALMOST fit the two haves together. If this occurred in the morning, it wasn't too bad. If it occurred at the end of the day, the usual result was some decidedly un-family-friendly remarks!)

3. It's tricky, but if you have an O-ring kit, you can cut out 1/16" of a O-ring, and reglue the ends, giving you a new O-ring that fits nicely. Very satisfying when it works. But, you need to know where the nearest industrial O-ring supply house is, so you can find a replacement, when it doesn't.