Chally Tatum, This is how I break down your question. The 7230 Comp valve spring spec are 145 lbs. at 1.900 installed hgt. spring rate is 519 lbs. per inch, the kit comes with a 0.135 spring seat, the factory 6.4 springs are 114.7 lbs. on the seat and 337.2 lbs. at max lift, with a 389 lbs. per inch rate. The 5.7 Eagle has a 1.99 installed hgt. with a 1mm shim spring seat (0.0394), now subtract the shim this comes out to 2.0294 spring hgt. add in the 0.135 Comp seat and this brings the new installed hgt. to 1.8944 with a seat pressure of 147.9 lbs.. Now assuming the 6.4 has the same shim for a spring seat to make up its 2.051 installed hgt. subtract the 0.0394 shim and you end up with 2.0904 hgt. add in the Comp seat 0.135 you have 1.9954 this puts the seat pressure at 116.25 lbs. If you leave the factory shim on the head and add the Comp seat to it , it comes out to 1.9160 installed hgt. with a 136.7 lbs. seat pressure. Add in another 0.015 shim and it puts you at 144.48 lbs. seat pressure. So that's what I am thinking they mean by adding a shim for the 6.4, but even with out the 0.015 shim your still in the ball park for seat pressure. Now the over the nose pressure at max lift (.632 lift) for the 6.4 at the least with 116.25 lbs. seat pressure would be 444.25 lbs. and at the highest 144.48. seat pressure would be 472.49 lbs. and the 5.7 would be 475.9 lbs. I am wondering if the retainers were not strong enough to stand up to the shock of the spring pressures when the valve slams closed. I am using these same 761-16 retainers with PSI springs with 120 lbs. seat pressure and 347 lbs. at max lift with a 384 lbs. rate with no problems for 7 years, but this is a big difference in spring rates.