my 1990 camaro has lb9 heads on it from an 87 305 IROC it they are all stock, i am going to be replacing my valve seals and summit gives me 2 options on size is valve stem diameter .341 or .344 inches how do i know what one i need. Heres the link .

http://www.summitracing.com/search/P...rd=valve+seals

 

.341" & .344" are close enough that it doesn't matter... about a sheet of copy paper...

The one is the dia of the hole in the guide, the other is the stem dia.

The "actual" stem measurement is 11/32", which is .34375".

 

What Sofa said. They're 11/32" valve stems in small blocks since forever. Also there are two different valve guide sizes (that the seals press on over). I don't reacall them off the top of my head but they're like .500" and .530". You need the bigger size for factory heads.

I would also mention that the best guide seals on the market are Viton seals. Used by the factory on Vortec heads, they can also be used on earlier heads (guide size and velve stem size are the same). They have a metal casing around the lower part that presses over the valve guide and a rubber-looking seal with a little spring wrapped around it where the seal actually wipes against the vavle stem.