Hey guys. I have a set of 243 heads I will be getting ported and polished. I wanted to replace the valves and got a bit bigger. Stock are 2.00 intake/1.55 exhaust. I wanted to know how big can I go without having to replace the valve seats and without the valves having interference issues. I was thinking about going to a 2.20/int, and 1.57/exh. Please let me know what you think! Thanks!

 

The LS3 valve size is 2.16", and you need at LEAST a 4" bore to use it. 2.20" is way too big for the 243 head.
Even when ported, you will never see more than 2.02" intakes on a 243 head.

 

Biggest a production seat will hold is 2.04”

 

You also have to be careful of shrouding. Sometimes too big is worse.

 

That's probably a typo ....... OP must mean 2.02 go for it.

 

When the ls6/zo6 was the bomb there was a vette road race class restricted to production heads, no porting, factory valve seats. 2.04 intake and 1.6 exhaust was the limit. Manley H series valves were used in an attempt to help the springs live a little longer.