That makes sense - thinned down head similar to seat belt bolts and most probably 3/8" UNF. Additionally, the nice man from Caterham confirmed this morning that they were 3/8" UNF. The reason the heads are thinned is that the steel Sierra hub will contact standard head bolts, hence the suggestion that socket head bolts may work. They don't work on mine BUT they are very close, so I will either "fettle" the socket head diameter or the hub OD. The tight fit when I first tried the 3/8" must be due to residual loctite - I have re-tapped most of the threads but I have a really tight one where I would like to "dissolve" the residual loctite - has anyone got any "good recipes" for dissolving loctite? Cheers Brian