triumph 500 unit twin

I've recently stripped the top end and found that the valves have
small black rubber 'seals' on the shaft. Neither the manual or parts
book make any reference to this and I can see no reason for them.
Logic tells me that all they can do is impede oil flow to the guides,
but my logic doesn't always work. Any ideas?

 

(mike) struggled to ejaculate:

They wouldn't be valve stem oil seals, designed to prevent oil making
its way down the valve guide, which when they wear out leads to the
telltale puff of blue smoke on starting up - would they?

 

They certainly were when I had a 5TA. Ditto for T140s. IIRC they got
included in later gasket sets.

 

Thanks gentlemen, that makes sense and proabably explains why the
thing was burning oil on the one cylinder where the valve had no seal
plus the guide was loose in the head.

 

Thus spake mike unto the assembled multitudes:

Better I suppose than having no guide in the head at all, making the valve
a bit loose, as happened to my Tiger 650 when an exhaust valve seized,
resulting in ejection of the remains of the guide through the exhaust port.