get the holes the o-ring goes into very very clean. easy to do if your pulling a head anyway. but a bit more tricking if you replacing them on the car.
if on the car, do a steam clean before you start, then pull the tubes and seals then with a bright flash light and mirror, carefully inspect for zero dirt ALL AWAY AROUND the seal bore, THIS IS CRITICAL!!!!! make sure your tubes seal area is also perfect clean and still round also!!! the bores on teh engine case are hard to see up in there with engine in car, hence teh flashlihgt and mirror are needed. o-rings will not seal well if installed over dirt. you should be surgiacally clean when doing this
o-rings like to be lubed when installed, it prevents them from twisting and tearing, either which can make a BIG leak. I lube them with engine oil, but automotive greases should work fine too.
Once installed, you need to do the whole mirror flashlight routine again, this time makeing sure that the oring did not get pinched. You must look 360 degree all around the seal at teh engine case if you see ANY o-ring pertruding out over the ridge on the tube, you need to repair it. hence the light and mirror are needed.
oft one or more of the of the rings will get pinched. examine the seals as you push the tube in, if you see it starting to pinch and poke out, back off a little bit and with a blunt wooden or plastic stick tuck it back in, then contiune to push in the tube untill seated, then re-insepct. do not use metal pokers like a screw driver as they can easily damge the o-ring.
dont use glue or sealant
use of VITON tyep rubber is hgihly recommended, Viton is a high temp polymer that will out last the older style stock seals. it cost a bit more, as it is a hightech specail rubber, but it is well worth the price as it helps prevent doing it again soon!!!!!!!!
take your time, inspect , be supper clean, lube up, install tubes with slow even pressure, inspecting for potruding o-rigns as you push the tube in, and re-inspect when done, you only want to do this job ONCE!!!!!!
then you get to do a valve adjust!!!!!!! inspect your valve adjustment screw ends for pits and waer at this point, same for the valve stem ends. if youfind an adjsuter screw with a step worn in it, get a new one, elve your valve wont stay adjusted, and valve rotation may not happen. note for correct valve stem/adjuster screw contact, you want ot have good roations, by having them slightly offset, with nice smooth contact surfaces!!!!!