The upper covers are held by the valve springs(not under the guides like most) The lowers slide up over the upper covers. The lower covers don't have threads, neither do the tappet blocks. Each block is cast and machined with an o-ring groove - the lower cover snaps over the o-ring and seals. A small set screw threads into the tappet block, securing the lower cover. I use 26" bicycle inner tubes, cut to sections to seal between the covers (old school trick a racer told me).
Tappet blocks are all angled weird due to WRs having the valves at angles to the bores in the cylinders. Tappet bore is tipped in tappet block to match the valve angle. Each tappet guide is specific to each valve - (ie Front Intake, Rear Exhaust, Front Exhaust, Rear Intake)
Don't mix 'em up ! The cams are straight in the cases, the cam lobes are straight - but the "feet" on the flat tappet are angled as they ride on the cam lobes. The tappets are also site-specific. The angled valves were carried over to the K-Model design. However, the K design has the cam lobes themselves at angles, bores in case at angles, and everything else (including the tappet blocks) straight to match the tipped valves.
My head hurts.