Are valve guide umbrellas really needed?

[Carl Spangler]

I'm replacing the (O-ring on the valve stem) valve guide seals on my 1971 250 and wondering if umbrella (or perfect fit) seals are really needed (in addition to the standard o-ring). Let's assume the valve guides are in good shape........... at least the first one I checked is great.

Since umbrellas weren't fitted on these engines originally how do I ask for them at the local auto parts store. I've seen comments online that Ford small blocks used them. I would still need more specific info for a parts counter person.

The valve stem diameter measure 0.340" (probably 11/32" nominal) on this engine and the valve guide boss' measure 0.750" diameter.

How tight do the umbrellas have to be on the valve shaft? When looking at the specs on some aftermarket umbrellas they give the I.D. and most of them are 0.342 or thereabouts.