I put the O ring seal a la sportster on my tappet blocks, but they didn't have the hole to flywheel cavity. I put the O ring grove about where the hole exits the side of the guide. I've O ringed my covers at valve guide and sliding joint as well. Quick tip for using these covers as follows:
When screwing the covers back down onto the tappet guide follow this routine.
Screw upper and lower cover together. Then ease the combo down onto the threads on the tappet guide. When the lower cover feels firmly on the threads, stop. Then, applying slight downward pressure on the upper cover, (I use the late u shaped tool for the late UL covers), begin unscrewing the lower cover from the upper cover. There's a slight taper leadin thread on the tappet guide, and if you have the lower cover pretty closely lined up, the downward motion of unscrewing the 2 covers apart will gently force the lower cover correctly on the tappet guide threads. Watch closely and don't totally unscrew the 2 covers apart. If necessary, keep some the downward pressure on the upper cover and screw it back on the lower cover to repeat the process until you've gotten the lower cover well started on the tappet guide. Then screw the pair all the way down using the octagonal flats on the upper cover to screw them home. The lower cover almost never cross threads on the tappet guide, whereas the joint between covers is easily crossthreaded. Just my 2 cents worth.