1966 427/ 390

[Duke W.]

Re: 1966 427/ 390

Kevin-------


Your experience with the forged pistons is EXACTLY the reason that I do not recommend their use for a street engine. I would recommend that you use Keith Black hypereutectic pistons. KB360 will result in a final CR of about 9.5:1; KB176 will result in a final CR of about 10.5:1. I HIGHLY recommend that you go with the KB360 and the 9.5:1 CR. I feel that 9.5:1 is the absolute highest CR I'd use with a big block and I'd prefer even less.

A catalog can only give a ballpark CR value. It's highly dependent on deck clearance, which should be measured prior to disassembling the block. Then different piston and head gasket combinations can be tried to achieve close to the target CR, and one should always pay attention to piston compression height as replacement pistons may be different than OE, which will alter the value of measured deck clearance at disassembly by the difference in compression height.

The attached paper explains the procedure.

Duke

[Richard F.]

Re: 1966 427/ 390

Joe: This was 30 years ago, and it is possible that I have misremembered my reason for going to positive seals. I had the umbrella type on hand, and maybe got them with the springs. In fact, I may have simply removed the seals and used the retainers. The point was that it is possible to use the 627 springs with positive seals.

[Joe L.]

Re: 1966 427/ 390

Joe: This was 30 years ago, and it is possible that I have misremembered my reason for going to positive seals. I had the umbrella type on hand, and maybe got them with the springs. In fact, I may have simply removed the seals and used the retainers. The point was that it is possible to use the 627 springs with positive seals.

Richard-------

Yes, the 3970627 springs can be used with positive seals. However, the seals have to have an OD of less than 0.81". I would not feel comfortable with a seal OD of greater than 0.75". There are some positive seals that would meet this but all require valve guide machining.

[Joe L.]

Re: 1966 427/ 390

A catalog can only give a ballpark CR value. It's highly dependent on deck clearance, which should be measured prior to disassembling the block. Then different piston and head gasket combinations can be tried to achieve close to the target CR, and one should always pay attention to piston compression height as replacement pistons may be different than OE, which will alter the value of measured deck clearance at disassembly by the difference in compression height.

The attached paper explains the procedure.

Duke

Duke-------


Yes, the CR's that I mentioned are approximate. That's why I used the term "about". Still, there are only a limited number of pistons available, especially for 427 cid engines. So, regardless of what the other factors affecting final CR turn out to be, one has to use one of the available pistons. For a 427, I recommend the KB360 since it produces the lowest compression ratio of the available Keith Black pistons. I think that the Sealed Power hypereutectic offerings are about the same but I have not checked them lately.

[Richard F.]

Re: 1966 427/ 390

Joe: I checked my paperwork and found the reason why I used positive seals. Machining the heads - $16; PC seals - $24. Of course, maybe that was real money back in Feb of 1986. But it sure looks like a bargain now.