Correct way to measure piston depth?

[qwk dust]

Is this the correct way to determine how deep in the bore a piston is? This is a stock '74 318 and I was trying to get an idea just how deep they are. We are planning to use KB167 pistons in the rebuild with 302 casting heads and I am trying to get an idea of how to get to a 10 to 1 or close compression ratio. Mill the heads? Deck the block? We have good 93 octane here. As always thanks for the help!

 

[BJR Racing]

Thats the best way. And square decking the block will get you close to 10:1 without the KB 167 pistons. A cheaper piston to use would be the 84-90 pistons as they are .025 taller in compression distance than the older stock pistons, and they fit the LA rods pin wise.
The KB 167 pistons with the block decked will give you closer to 11:1 compression ratio.

 

[rumblefish360]

That's .070 in the hole? Yikes, that's deep, could be worse like my '78 400. I can get an echo out of that one. (Wheres my damn glasses!)

 

[qwk dust]

Yeah, those puppies are hiding down in the hole aren't they. What do you guys think an acceptable compression ratio is for 93 pump gas with iron heads?

Bobby, am I correct that with just a square decked block with the KB167s and the 302 heads without being milled at all should give close to 11 to 1? Wow, I really didn't expect that. I thought that basically zero deck with the KBs and no milling of the heads I would be in the 9.5 to 10 area.

 

[BJR Racing]

The KB's would be close to 0 deck without milling and then if you have to take .020 to get the block to be square then the compression go's up in a hurry. Then if you do a clean up cut on the heads and they are around 58-60 cc's and a .055 gasket you'll have .030 for a quench area clearence. This makes for a very tight squeeze. In my 322 I'm at 11.24:1 with 59 cc heads and a .055 gasket with the pistons being .025 out of the hole. I'm pulling 190+ cylinder pressure. KB is showing the compression ratio to be 10.3:1 with a 57 cc head, but now if your out of the hole .025 then you can add about .8 of compression to that.
You'll have to cc the heads to find out what compression ratio that you'll really have. I also run the 1.60 exhaust valves in the heads and this closes the chamber down a couple more cc's.

 

[qwk dust]

thanks again Bobby. You are just a wealth of information and advise! I will proceed with caution for the time being. These 302's that I have are a newly recondtioned set (by autozone..yea I know) that I got for $100 off craigslist. They look perfect although I am going to try my hand at gasket matching and pocket porting (I will practice on my stock heads first). How do I determine the cc of the heads? Pour water in the chamber until level then pour it out and measure? Inexperience question I bet. Thanks again!!

 

[qwk dust]

Right again BRJ! :notworth: I had the heads cc'ed today. 59cc's I am going to follow your advise and go with the '84-'90 pistons and machine the block properly go get real close to that 10 to 1.
This weekend I am going to take my '74 318 heads and practice gasket matching and a little porting and polishing.:tongue5:
The machine shop looked over the freshly reconditioned 302 castings I got off Craiglist for $100 and confirmed that (believe it or not...) Autozone's vendor did a good job with the valve job and reconditioning. They did say that if I wanted to get a little more out of them I could use some stainless steel valves.... Do you guys have any suggestions for some good ss valves that would help performance...Nailhead as opposed to tulip design I imangine...Right? Any suggestions on the best place to get them.. 1.78/1.50 Thanks

 

[BJR Racing]

qd,
Stay with the stock 1.78 on the intake and use the 67-69 340 exhaust valves as they will flow more than any other that I tried. Backcut the intake valves and the exhaust valves and do a 75* cut on the intake and exh. sides in of the bowls. Then radius the short turn on the exh. side and remove the sharp edeges from the rest of the bowl and you'll be done.