What causes sunken valve/seats?

[northernopposition]

I just tore down my old stock single port 1600 engine as it had a few signs of needing a rebuild, one of which was a couple sunken exhaust valves. I’ve done some searching and there’s discussion of people having SVs but not much information on the cause. Mainly just curious but also want to know to prevent it from happening on my new engine. Thanks.

[Dusty1]

Heat and hammering. It's difficult to explain because we call both the metal ring inlaid into the head and the sealing surface on said metal ring "valve seats".

It's easier to see on old cast iron V8s manufactured to run on leaded gas. The tetraethyl lead forms an excellent lubricant on hot surfaces yielding reasonable valve seat durability. Still, V8s in the leaded gas era got around 80k out of a valve job. Then it was a trip down to the machine shop to grind the valves and seats, replace the lifters, the riming chain and sometimes the cam. 80k and she's all done! If a timely valve job was neglected we would end up with a wheezy engine that used lots of oil.

I find it remarkable modern engines go 200k, even 300k or more before major overhaul or really with only minimal service aside from oil changes.

And timing belts.

Except for Subarus. Subaru heads try to turn themselves inside- out with alarming regularity.

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[modok]

To me
"sunk" means the valve seat is extremely worn.
Not usually a problem in old VW heads.
Means Valve seat material not right for application

Has a lot to do with the chemistry of combustion and what residue it leaves behind.
The thin layer of oxide on the metal surfaces,
How hot the valve is, and hot hardness of the metals.

IMO aircooled heads the main problem is the guides wearing out, or the seats falling out.

[Dusty1]

A few more things that will sink exhaust valves:

Tight valve lash. They're cooler when the valve is on the seat not out in the stream of hot exhaust gas.

Dirt. Even those super duper "race approved" K&N filters will let enough dirt through to pave your driveway in a dusty environment.

Or:

Your stock oil bath air cleaner needs oil to catch dirt.

Or:

Plain old wear and tear as found in a 50 year old engine.

[northernopposition]

I do have a very dusty mile to drive to and from my home and it is a double cab. I always have oil in my filter but it is quite a dusty mess in my engine compartment. The heads were “overhauled” 18 years ago at a local machine shop that hadn’t done much with air cooled VW heads (I do live in the subarctic) so who knows what they did. I don’t remember well but I think the engine I got them from threw a rod so perhaps they overheated in the episode.

So it sounds like I possibly had a few things that could have caused the problem. Thanks for your smarts.

[Dusty1]

18 years out of a set of heads that were used to begin with is pretty good. I gotta get me some of those.

Six of one...

Dusty roads in a double cab might qualify as severe duty.

Half dozen of the other...

We're in serious trouble if Alaska gets as hot as Arizona.

I would be treating my double cab to new heads right about now.

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[Alexander_Monday]

[northernopposition]

I should add that 18 years was about 14k miles ago. They sat for several years and I only put about 1000 miles on every summer. They were probably tired ass old heads to start with. They went to the scrap heap.
The double cab has a newly rebuilt 1600 DP with all kinds of new goodies including heads so hopefully it’ll last the rest of my life at the rate I drive it.

Thanks for the info, y’all.

[SGKent]

overheating that softens the aluminum

hammering of the valves - meaning that the valves slam shut too hard, or they don't shut hard enough so they bounce

Poorly cast and annealed heads when made

soft valve seats

incorrect geometry that spins the valves too fast and wears into the seat. Not technically a sunken seat but may look lower than it should
_________________
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Skills@EuroCarsPlus:
"never time to do it right but always time to do it twice"

[mikedjames]

Running on LPG is a sure fire way of eating valve seats. No protective additives.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy