I started wonderin' about this when I was having all that noise I thought was valve tapping. So when I changed out the lifters in my UC this last weekend, I pulled one apart to see what was inside. The pix are below.
And here is a pretty cool animation that shows a lifter in operation:
YouTube - hydraulic tappets (hydraulic lifters)
Finally, here is a pretty good explanation of how they work. This is from a VW, so don't pay any attention to the actual numbers and specification provided. The description of the operation of the lifter is what's important.
Hydraulic Lifters
TQ
And here is a pretty cool animation that shows a lifter in operation:
YouTube - hydraulic tappets (hydraulic lifters)
Finally, here is a pretty good explanation of how they work. This is from a VW, so don't pay any attention to the actual numbers and specification provided. The description of the operation of the lifter is what's important.
Hydraulic Lifters
TQ