Valves

Run at 6 thou, run at 4 thou, run at 0 lash, but whatever is right for your engine - check valve clearance at least every 10,000km (6,000mi.)

If you, or your mechanic, do not do this religiously your engine will die young!  If an engine is rebuilt, the valve clearance needs to be checked after the first start, after the first 20 minutes, then at 800 1,500 2,500 and 5,000km services (you do all those, don't you?)

All old pushrod/flat tappet engines needed their valve clearance checked and adjusted regularly, our VW just needed it a bit more often. Here's Why:

1. Air cooling. I am a big fan of air cooled engines (pun intended,) but they do tend to run hot - particularly where cooling is needed most, the heads.  The hotter things get, the more they change size and the more prone metal is to stretching or fatigue.
2. Pushrods - the aluminium in the pushrods can easily be pounded if lash is allowed to get too loose, this will generally cause the tips to recede into the rod, making it shorter, which in turn creates more lash, which then...
3. Crankcase. Magnesium is wonderfully light and strong, but itself is rather soft, and alloying it to make it harder tends to make it brittle. Extreme cases of lash can make the lifter bores go out of round, causing noise, further wear, loss of oil pressure in the heads.
4. Camshaft. Hopefully the cam developed a nice hard surface during the run-in, but a loose valvetrain can effectively knock the tops of the lobes, creating another spiralling wear pattern.
5. Rockers. Stock rockers have a very small area of contact with the top of the valve stem, any excessive lash will increase their tendancy to hammer against it. "swivel foot" type adjustors can also be damaged by such hammering, in extreme cases rotating out of position and breaking.
6. Valves. This is what it's all about - if the lash is too great, the top of the valve stem is being pounded by the rocker, if the lash is too tight, the valve will not be closing properly. The valves are also one of the reasons the lash changes - valves get mushroomed stems, they stretch, they pound themselves into seats and they wear. they are also often what causes the catastrophic failure of your engine if valve lash is not taken care of religiously.Specifically, it is usually No. 3 exhaust - the head breaks off, jams the piston, which then breaks the rod, etc.  And it all starts with someone figuring checking and adjusting the valves is too much hard work!

So check them, if in doubt, check them again. The correct lash for any aluminium pushrod is .006" and chromoly should be set at .002"
You can't check too often - some check weekly. If you keep this up, your engine will live a  long life, if you don't it won't. Simple.