I was wondering how much lift you can run with stock 6X pontiac heads. A friend told me .490 was the limit, but I was thinking I could go up to.517

 

I wouldn't go anywhere near .500 unless you know you're not going to have retainer-to-guide clearance problems. Also, your choice of guide seals will affect how much room you've got in there as well. Obviously, you have to have the springs spec'ed out pretty well, too. Pontiac heads came with MANY different length valve stems and if you have the wrong valves in there, the installed height of the spring will be wrong and, again, you'll bind 'em up well before .500".

Moreover, Pontiac heads were build for low-lift flow. 30* seats, small ports, etc. In stock config they just don't flow any more above ~.450" lift. I wouldn't try to make them do something they weren't built to do unless they have had a thorough performance massaging.

Of the Pontiac heads I've worked on I come down to one basic rule: MEASURE EVERYTHING YOURSELF. There are very few shops around that know their butt from a hole in the ground when it compes to Pontiac iron.