964 valve spring check?
 

What is the values for checking valve spring on the 964?
I can only find it for up to -89 (compressed lengt vs force)

I believe the 964 spring prescribed to the same forces as the earlier springs. When the heads are off I recommend replacing them anyway. Dougherty cams or Cgarr have a great set available now that won't break the bank (compared to Aasco $$$$).

The springs are normally OK, but the retainers are known to crack (if you spend time above 6500) with catastrophic results. So those should be replaced.