Originally Posted by sydney
Wrong concept.Some 27" rims have no hook to retain the bead and are intended only for relatively low pressure.
If you get the tire seated right you can run them at 90psi. Actually it sometimes take 90-100psi to get these in place just right. It seems that the pressure from the tube holds the tire in place with friction between the rim and tire, so the more pressure you get they tend to fit better.
Does it pop off right away or does it sometimes take awhile?
It seems that there is no true standard for 27" tires and rims, some tires have a small bead, some have no bead, and some 27" rims have what appears to be a regular bead. I've always been able to get the tires to work. Sometimes they take awhile. Every 27" tire have seen is an ISO 630, so they fit, just not right.
Another thing, make sure the rim sidewall isn't bent or damaged. Make sure it's true also, that helps alot.
When you get the tire to 30psi make sure the markings on the tire are sticking out from the rim evenly all the way around, inflate slowly and wear earplugs.