They shouldn't have to be stretched excessively. Just a light stretch through your fingers with a light coating of grease. Drop in the groove and run your finger around a few times, they will then stay put with no issues. Drop the head on square and rotate slightly back and forth, it should feel smooth. I agree with the above post, if its more trouble than that you may have the wrong o-rings, it has happened to me.
o-ring specs/grooves have a lot to do with the material, pressure, temp, and if its a static or dynamic seal. I'm no expert but have some experience in both medical and industrial applications. Published guidelines are available. The GG head /cyl design has never caused a problem if assembled correctly. I have seen o-rings pinched causing a leak. Thats why I rotate the head slightly on install, if an o-ring jumped a groove you will feel it.
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