This is an identical reproduction of the valve guide bottom collar used on the inlet valve of Norton Internationals, and is used to hold the hairpin valve springs.
This item is cast from a Lost Wax casting - which provides a higher quality finish than normal sand cast type, and even includes the correct cast in numbers. It is CNC machined to a high standard of finish. Note: I have had made exhaust valve guides in the same style as the inlet guides, so this same collar can be used in conjunction with the Exhaust guide, rather than the original single piece exhaust guide\collar - see the Exhaust Guide description for more information.
They are a light tap fit over our valve guides (which helps heat dissipation from the guide - which I think may be why the exhaust guides/spring holders were originally made as a single casting.
Note: valve guide sold seperately. Update Note 3/10/14: We have just made a new batch of these and have incorporated a useful modification - over the years there seems to have been various diameter pegs fitted into head to stop this collar rotating (normally on the left of the valve guide). Therefore, we have had this batch made with two different diameter peg hole/slots on its bottom face, to account for two common sizes, therefore you can select the slot most appropriate to your head's peg (or just file the slot/peg accordingly!) - see 3rd photograph which shows the underneath of the collar in the lower example, with different slot/hole sizes