I have a '66 Starfire with a 425/TH400. I will be boring it .030 to clean it up and rather than trying to grind the crank, (two journals were stuck) I am looking at just getting a 455 crank. Now, pistons: SpeedPro has a .030 Stock-type dish for $40. I understand that increasing the size of the bore increases the volume of the combustion chamber and lowers the compression ratio from the factory 10.25/1 to 9.32/1 with the 80 CC heads. However, it states that the compression goes up to 10.83/1 with 62.5 CC heads...but I can't find anything about 62.5 CC heads! Mondello even lists 64, 70, 72 and 75 CC heads in addition to the 62.5 and 80. Can someone please enlighten me as to the whereabouts of these other flows or how they are achieved?

 

Just in case you weren't aware of this, if you are going to use a 455 crank you will need the 455 rods along with 455 pistons. The 455 rods are shorter (6.735" vs. 7.00") and the 455 piston compression height is taller (1.735" vs. 1.615").

As far as the combustion chamber size, most BBO have around 80-85cc. Those smaller numbers are just there for information on compression ratios and you would have to do some significant milling on factory heads to get anywhere close to those numbers.

 

Thank you for the input. After I posted, it occurred to me that milling the head might be the solution. I found that an open chambered BB Chevy, at least, requires .0005 of an inch of material removed per CC of desired reduction in volume, i.e., (80 - 62.5) * .0005 = .00875 Granted, I don't know how much material can be removed, but, in theory, it can be done. This requires more research.

 

Hmmmm....
Decimal point location and them zero's matter....
0.0005" is not at all the same as 0.0050" per cc
So the 8.7 thousandths inch calculated cut is more like 87 thousandths inch. Also known as way over 1/16" and closing in on 1/8", which leads to all manner of other issues.

CC is not flow....

if you need a crank, just get a 425 crank, because, as set forth above, if you use a 455 crank then you need mating rods and pistons, and you now have a 455, not a 425....

"increasing the size of the bore increases the volume of the combustion chamber and lowers the compression ratio" is not how that works....


Not to bust yer ***** too much, but you need to do a bit of learning here, starting with how compression ratio is calculated. Not quite up to speed there.

So, evidently you are doing a complete rebuild here.

May I suggest saving a ton of money and time by just buying one half done?

I can inspect if need be, it's near:

[dangit, $1100 rebuilt 425 near Lansing on CL ad now gone]

this guy may have a 425 crank though

https://lansing.craigslist.org/pts/5443251889.html

why not just refurbish your parts?

 

You're right, I don't know as much as as I would like and it seems I got my terms mixed up. I was meaning that I thought increasing the size of the bore increases the overall volume where combustion occurs, therefore reducing the compression ratio. I have connections in Florida with my dad-in-law who used to run Division 2 NHRA down there. He'd probably kick my *** for mixing stuff up, too. I haven't mentioned the extreme milling to him yet today but I am sure he has something else to say about that, too.

I am aware that it would require swapping rods, would be getting a different cam/lifters, aluminum intake, blah-dee blah dee blah...

Thanks for not treating me like a newb....you could have brushed me off as being ignorant about engines and not even responded. I appreciate that. I will get on that Compression ratio 101 right away.