The VW air cooled engine requires periodical head work. Guides and springs wear and exhaust valves erode. We have found that the stock VW engine, when driven normally, requires checking at the following recommendations. Sedan 70,000, Type III 60,000, Type II 40 to 45,000 miles. Now, if you have a higher lift, faster ramp camshaft, you must check for guide wear sooner (to prevent burned and/or broken valves). If the valve does not come down straight on the seat due to guide wear it takes longer to cool. If bad enough, it will try to bend the head of the valve when it hits the seat crooked, it rotates and hits in a new spot, bending it in a new direction. When bent back and forth enough it breaks. The guides may be checked with the engine together by moving the spring/retainer/valve up and down when in the adjusting position. Valves must be checked and set regularly, about every 3000 miles as recommended by VW. They get tighter, not looser, and will burn prematurely if this is not done.
Note: When the head is properly ported and polished it will hold the guide in correctly when cut for inner and outer/triple springs. We have street engines with over 100,000 miles with this done and not one guide has come loose.