either im reading this thread wrong,or.......If it's that squiggly rubber water pump gasket the only reason to use any sealant would be to hold it in place while you reassembled the items. You'd be just as safe if not safer usng something water soluble to temporarily tack it into place.
Sorry, got caught up in a similar thread the other day and got everything all mixed up.
I'd go with Shep
s suggestion for the mech seal although most any good Permatex product would probably work. I tend to prefer their reddish hi-temp copper gasket compound, it hardens but not to 100% leaving you with some pretty tough rubber and it has ecellent gripping characteristics.
I use it when changing the headlamps on my car. The car is anoddball, came with some really strange sealed beams that cost around $40 each for just a standard old yellow type. What we do is to gut the existing lamp capsule out of them and glue in a modern 9006 capsule. I go a step fuurther and use the 9005 high beam part, 10 more watts and aims the same since that's controlled by the design of the glass envelope itself rather than the bulb itself. You've got to trim a tiny amount of plastic off to make them fit the connectors, they added a tiny tab in the bulb socket in hopes people wouldn't mix them up.I get acceptable light this way if I use Sylvania 9005 Silver Stars, the stock lamps alone really sucked.