UJ's with grease nipples - do they stay in after assembly ?

["Foster, Stan" <stan.foster@hp.com>]

I'm replacing all the UJ's while I have the back end apart. The new UJ's
(Hardy Spicer) came with grease nipples. The originals did not. Do these
nipples stay in permanently or are they only inserted for lubrication, and if
they don't stay in all the time what is inserted in their place since these
UJ's didn't come with any kind of blanking device ?.

Thoughts ?

Stan