If I remember correctly there is no over all thickness limit for machining the head. (which in the extreme situation is a bit daft!) There is, however, limit between the depth of the valve seats...

...before you get any machining done get hold of a copy of chapter 01-418 from www.startekinfo.com (from the free FSM for W123 for example) - print it out and make sure the machinist knows what is required.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!