I had the same horrid feeling after rebuilding hte top end of hte Triumph motor. Found a rocker shaft shim on the floor. These things are an absolute devil to get into place, with springs, spacers, shims etc. all under tension in various directions as you try and get the shaft through it all in the confines of the rocker box. I tore it all apart before I realised it was a spare off a shaft set I had been using as a reference for the part order.
