Milling 225 head

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Taz

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Hi everyone: I am working to improve my '72 Duster for better everyday driving. The stock compression on it is 8.2:1. Can I mill the head to reach 9.2:1? How much comes off? Do I need to change out any of the stock valve train? Are there any problems with milling the head?
Thanks,
Taz
 
A quote from BBBCUDA in a different thread. "Well, for a brief moment, I'll try to stick to the origional question. Cheapest way, 1: gears. Slants come with lousy gears for acceleration, 2.94's in the ones I had. If you are really cheap, (like me), a 60's International Scout (model 80) has almost the same track as an A body, the perches are even close enough to work. They come with a 4.27 gear ratio. With the help of a lift, I swapped one in in a little over an hour, and WOW, what a difference. Later I milled .100" off the head, (at least lap the valves to make sure they are sealing), added a 2 barrel, and had a reasonably "snappy" car. The 5.0 Camaro's were popular around here at the time, and I never even came close to getting beat by one. Not saying much, but there it is."

If you have adjustable rockers shouldn't be an issue. Make sure to free up the exhaust flow as best you can.
 
With the original head gasket (.021"), you need to mill around .060" off. But those are getting hard to find. With a later head gasket, like .050" thick, you need to take around .090" off. Suppopsedly, you can got .100" off. But I've never done that much, so I suggest you read the articles at www.slantsix.org, and check the forums there.

As above, the rockers need to take up the adjustment for the head being lower on the pushrods. That should not be any issue. Cam is in the block so the head milling does not effect the cam timing.
 
I really doubt the compression ratio was 8.2.........

You need to take measurements to determine what the actual compression ratio is currently............
 
emsvitil said:
I really doubt the compression ratio was 8.2.........

You need to take measurements to determine what the actual compression ratio is currently............
Click to expand...

If you were lucky, it might be high 7's! After taking the head off, measure how far down in the hole the pistons are. Then do the math to figure the current compression ration. I would deck the block first and then cut the head to get to the compression ratio you want. My block is decked 90 thou with a head clean-up at 10 thou. my compression ratio will be a little above 9.0
 
emsvitil said:
I really doubt the compression ratio was 8.2.........

You need to take measurements to determine what the actual compression ratio is currently............
Click to expand...
I thought the same thing... but looked up the numbers, and it worked out in the low 8's. Check these numbers:
Deck height 10.680"
Stroke: 4.125"
Rod length: 6.700"
Compression height: 1.740" (244NP's)
Head gasket thickness (original shim type): .021"
Combustion chamber volume: 54 cc (from Doug Dutra's worksheet in his 'Stroking' article; I have never measured one myself)

Distance below deck works out to .178" (seems pretty normal for a /6)
With flat top pistons, SCR works out to 8.32:1 .

The one variable I am not 100% sure of is the combustion chamber volume and the pistons in the OP's engine. If the combustion chambers were 60 cc's instead, the SCR before cutting would be 7.83:1. Either way, the OP will be going up almost a point in SCR with cuts of the level discussed. So does he want 1 more point, or exactly 9.2?

Dartman, part of what you are fighting is the newer, thicker head gaskets. If you have one, then your .100" total deck + head cuts works out very closely to my prior computations. It would take .090-.100" total cuts to get to the low 9's with a thicker head gasket.
 
Interesting! I would have thought high 7s but the numbers say otherwise! Most likely I will use a stock steel head gasket on mine.
 
casting#2843169 I cc'd @ 58.6, had.060" off & it came in @ 50. Had same off block w/a
set of stock .060" over slugs, deck over piston .130". With a printo-seal fel-pro @.039" it
worked out to 9.35:1, and actual cc'd w/8yrs. carbon build-up @ 9.469:1 .
Pushrods stock will be OK unless you add a decent lift cam (.470" lift NET & up), then
you may need shorter units.
 
nm9stheham said:
I thought the same thing... but looked up the numbers, and it worked out in the low 8's. Check these numbers:
Deck height 10.680"
Stroke: 4.125"
Rod length: 6.700"
Compression height: 1.740" (244NP's)
Head gasket thickness (original shim type): .021"
Combustion chamber volume: 54 cc (from Doug Dutra's worksheet in his 'Stroking' article; I have never measured one myself)

Distance below deck works out to .178" (seems pretty normal for a /6)
With flat top pistons, SCR works out to 8.32:1 .

The one variable I am not 100% sure of is the combustion chamber volume and the pistons in the OP's engine. If the combustion chambers were 60 cc's instead, the SCR before cutting would be 7.83:1. Either way, the OP will be going up almost a point in SCR with cuts of the level discussed. So does he want 1 more point, or exactly 9.2?

Dartman, part of what you are fighting is the newer, thicker head gaskets. If you have one, then your .100" total deck + head cuts works out very closely to my prior computations. It would take .090-.100" total cuts to get to the low 9's with a thicker head gasket.
Click to expand...

The exact Compression isn't important. I just want to still be able to run 87 octane gas. Thanks for the info.
 
The exact compression isn't important. I just want to still run 87 octane gas. Thanks everyone for the info.
Taz
 
Taz said:
The exact compression isn't important. I just want to still run 87 octane gas. Thanks everyone for the info.
Taz
Click to expand...
Um, then the exact comp. ratio is eminently important, if your only
using regular your limited. I wouldn't take more than .060" off the
head only.
 
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