Technical question on valve train clearance

I've been doing a lot of reading on various threads concerning the different things that I'm gonna end up doing to my '82 XJ1100 project. I've read some very informative threads on valve adjustment and they all say that as the valve train wears on these XJs, the clearances get tighter.

On every single piece of machinery I've worked on to date, (Automobiles, Aircraft, Lawnmowers, etc), as the valve train wears, the valve clearance increases.

I understand the overhead cam setup on these Xjs, but I can't wrap my head around how the clearance gets tighter with wear. As the cam lobes themselves wear, the clearance should increase. If the shims in the buckets experience wear, then the clearance should increase. The only way I can picture the clearances getting tighter is if the valve seat itself wears, causing the valve to sink deeper into the head, therefore the stem will sit higher and therefore making the buckets sit closer to the cam.

Anyhow, I'm just curious about this and I was hoping that someone here has the technical answer to explain this phenomenon. It's burning a hole in my head.

 

I suspect its the valves seating into the valve face.

 

That is the correct answer, you win the internets today!

As the valve (under massive spring retraction pressure) continuously slams closed against the valve seat (inside the combustion chamber), it ruthlessly pounds that seat and both wears it away and, more importantly, compresses it. Thus the valve itself, when closed and resting on this "compressed" seat, has move further "up" into the head, closer to the camshaft lobe, thus REDUCED clearances. It's also why when the engines are new, they require more frequent valve clearance adjustments, as the seat gets more easily compressed, and after the engine gets a few 10's of thousands of miles on it, the clearances change less often, as the seat has gotten almost fully compressed and relatively very little further valve recession occurs.

 

Thank you for the informative explanation! @chacal @JCH
The first obstacle in trying to work on any system or component is understanding how it works.

 

Just to add, the valve train will wear as you expected, increasing the clearances later in life. However, that's much later in life, like tens of thousands of miles down the road. Both wear happens at the same time (and therefore sums), but the valve seat is more significant early in engine life and tapers off to zero, while valve train wear is smaller but consistent over time.

For example, my turbo seca had 6k miles when I bought it. Probably never had its 1.5k mile adjustment, and 7 of 8 valves were tight.

 

Yamaha used toffee for valve seats and then made them narrow, as they "compress" they actually widen, hence reduced need to adjust.
Wear on the valve stem tips, cams, followers is actually minimal, and ultimately only valve tips and followers contribute - unless of course someone set the clearances extra wide, which would pound the seats and tips and followers.