This guy struck gold

[Icetech]

Well I`ll be the first dummy to ask, how does that even work?
I assume the mushroom part is bigger than the body and would have to be installed from the bottom?

I don't think it was mentioned other than installing from the bottom but that larger platform for the cam to ride on allows for some pretty radical lobe profiles for a flat tappet cam, coming close to roller.... You never used to hear bout mushroom lifters cept from really serious guys...

 

[Dale Davies]

Well I`ll be the first dummy to ask, how does that even work?
I assume the mushroom part is bigger than the body and would have to be installed from the bottom?

Correct. In some applications at close to max lift the mushroom could hit the bottom of the lifter bore. In those cases a bit of relief had to be machined.
There was some engines built that had small diameter lifters of about 1/2" and required the mushroom.
The mushroom allows a more aggressive cam profile without the edge of the lifter "digging" into the cam ramp.