Phil
I don't seem to have a v shaped piece of nylon fixed in the after end of the slot, just the usual four nylon 'runners'or 'sliders' down the sides of the front and back edges of the keel itself. See the third photo (to the right) in my article .....
http://www.parkerseal.or...es/modsmaint/sskeel.aspx
When I bought Elsa in 1982 I immediately asked Bill Parker about keel clonk. It seems that these 'runners' may not be quite thick enough and that the clonking is more likely to be the result of rolling not pitching.
Bill suggested fitting penny washers (which he sent) round the bolts between the 'runners' and the keel to pad them out a bit but warned about making them too wide with the result of the keel jamming lower down.
It appears that it is worth checking that the slot is exactly parallel all the way down. Mine is ... but I seem to remember one comment many years ago which suggested that there might be a very slight taper in some boats.I chickened out on the washers and fitted some strips of neoprene between the 'runners' and the keel instead. This worked a treat for many years but they have squashed and rotted in the 29 years I have had the boat and it clunks again.
If your problem is the result of pitching rather than rolling then this wont be of much use.
Chris. Elsa S/S 26 103