GM O/C bbc heads

DOTracer · Post by **DOTracer** » Sun Mar 13, 2005 2:20 pm

Does anyone here know how many thousands are milled off to equal 1cc for stock GM open chamber bbc heads? This would be for a flat mill. How about angle mill?

I've finished measuring the chambers and need to measure the dome cc yet to calculate the static compression ratio. Looks like the compression is gonna be much lower than expected.

I was told by my machinist when the heads were built they spec'd out at 115cc. Over this winter I had the 2.19 valves replaced with 2.25" valves. I also had to do a small amount of grinding of the chamber near the exhaust valve to clear the piston dome...very minimal material was removed in a limited area. I measured all 8 chambers, some more than once if the volume measured was questionable. The average of all the cylinders was right around 120.3cc, with the smallest being 119.5cc and the largest at 121 cc...far cry from 115cc.

Any idea how much it's safe to flat mill off these decks? The heads are GM 781 casting large oval port open chamber heads.

My target compression ratio was 13.5 to 1 with a bare minimum of 13 to 1.

There's also a discrepancy on the piston dome volume which is why it's next to be measured. my JE (not SRP) catalog shows the domes as 50cc as does JE's web site. The tech I spoke with at JE claims that's impossible that 46cc is the largest dome they offer for a 454 based combination (4.280" bore). Again, 4cc difference is a big compression difference as well.

Once I know the dome volume, I can calculate how many cc's smaller the chambers need to be to reach the target compression ratio assuming that's a safe amount to mill off the decks.

thanks for any and all input
Todd

Post by **ou812** » Sun Mar 13, 2005 2:40 pm

.oo65 per 1 cc and I think .009 for angle milling.
Brian

bill jones · Post by **bill jones** » Sun Mar 13, 2005 3:19 pm

-I measure the profile of the chamber with 1/4" graph paper to get an exact square inch area number, and that is likely to be somewhere around 65% of the bore diameter.
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-A guesstimate example is a 4.250" bore which is 14.186 square inch area, where estimating 65% of that would be 9.22 square inches of chamber.
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-I take the 9.22, times that by .001" mill, equals .00922 cubic inches.
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-then .00922 times 16.387 equals the cc's per thousandths at .151
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-so if you milled the head 10 thou you remove 1.51cc, 20 thou equals 3.02cc's, 30 thou equals 4.5 , 40 thou is 6.04 etc.
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-You don't use any decimals when figuring the thousandths of mill because it's already figured in so you just use 10, 20 , 30 times the 1.51.
-(I think this is right)

idunno · Post by **idunno** » Tue Mar 15, 2005 8:28 pm

open chamber bbc heads take .005 per cc. and a sbc will usually take .007 per cc.The larger the chamber bore the less it takes to remove a cc.

bill jones · Post by **bill jones** » Wed Mar 16, 2005 6:28 pm

-today I measured up a pair of 428 Ford Edelbrock heads that the guy wants 68cc's
-I measured the chambers with graph paper and found that 10.0 square inch area is real close.
-so I did the math this way:

#1-took that 10.0 square inch area and times that by 16.387 which is 163.87.
-This is the square inch area converted to centimeters.

#2-multiplied that 163.87 times .001" and I get .16387 which is the amount of CC's I will remove with each .001" of milling.

#3-one heads was 74.3 and I want 68 so---74.3 minus 68 equals 6.3 cc's that I need to remove.

#4-I then took 6.3 and divided that by .16387 and this is an answer of 38.44 which in all reality is .0384" that has to be milled off
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-To double check: take 10 square inchs of area times the .0384" milling cut equals .384 cubic inches of volume that you'd mill away with a .0384" cut.
-To convert this small portion of one cubic inch to CC's, take the .384cubic inches times 16.387, equals 6.29CC's.
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RESULTS:
-I milled both heads to attempt to get 68 CC's and the one head I calculated had to be milled .0384" and the other head had to be milled .0410" to get 68.0 CC's.
-I milled the one head .0385" and one head I milled .041" after just lightly touching down to get a witness mark on the heads with the cutter to establish my starting point.
-Both heads were already nearly dead flat and dial indicated within .0010" everywhere.
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-When it was all said and done I got 67.6 on one head and 67.5 on the other so I missed the desired number by .4 to .5CC's so I cut about 6% to far or cut about .002" too much.

Post by **Dave Koehler** » Wed Mar 16, 2005 6:52 pm

Bill, your method is definitely interesting. I don't think I would have ever hit on that idea.

Gosh, I feel bad. I haven't been doing the math correctly. Actually I haven't been doing the math at all.
I just fill the chamber to my target CC, without the plate. I then take a depth mike and slowly move the thimble down until the fluid just "jumps" to the rod. Read the depth and mill away.

Later
Dave Koehler
www.koehlerinjection.com

k-star · Post by **k-star** » Thu Mar 17, 2005 7:34 am

I have used the Bill Jones graph paper deal on several sets of heads and it has been spot on for me... The only thing i did different is used millimeter graph paper. It just saves the one conversion. I keep all the graphs with my engine records just so i can reference that head chamber size.

Dave i'll have to give your idea a shot sounds easy......

Keith