or also if he has a high lift cam or goes FI...

 

I know the valve stems themselves are a hair longer on the a6's vs. z6's. I've used a6 valves in a z6 head before, the valves aren't alot smaller in diamter than the z6's are, and actually sit pretty well flush with the z6 head once they're seated in. Just had to grind down the tips of the stems to keep them from binding on the z6 springs. I suppose the opposite could be done with them...get an valve angle job and they should fit in nicely. Might even be able to retain the z6 springs in conjuction with a larger cam lobe if you wanted to save on cash. I would imagine you could also have z6 valve seats machined in to make life alot simpler. It all pretty much depends on how much extra cfm you want the head to flow.

 

It's a .5 mm size difference. You can cut the existing seat to accommodate just fine.

 

Or you can do that The machine shop I go to told me they won't cut the seats, only machine out and replace them with the larger ones...they're gay :P

 

Both have merits, depends on what you're doing.