[blykins]

Oh duh...after reading Sharpe's reply, I guess you're talking about valve-train.

I suppose you're asking about mechanical/hydraulic instead of flat tappet/roller tappet though, right?

You can buy either style in either hydraulic/mechanical. The only real drawback is that hydraulic lifters have to be pumped up. Not that this causes a problem, but when you're doing your engine build you will have to check piston-valve clearance and cam degree. This is a lot easier with a mechanical lifter, as a hydraulic lifter will collapse as you put pressure on it and give you false values...i.e. your cam will show up as .400" lift instead of a .480" , etc.

Does this help?