[250 Al head] - Milling 55 cc head to raise compression - is there a rule of thumb, max limit?

62Ranchero200

Famous Member
Greetings Ford Six Fans,

Background: Have a '74 250 with "early" 300 rods and custom RaceTek forged pistons, .005" deck height, Clay Smith 274/274/108 hydraulic cam, 1.6:1 adjustable stamped rockers, Smith Brothers custom pushrods, CI Al head (base valve size, no porting), highly modified Holley 2300/4412 "500 CFM" 2 BBL carb, CI headers with true dual exhaust, "X" crossover and FlowMaster Series 40 mufflers, custom curved DS II with MSD-6AL. Very solidly built, with ARP main and head studs, ARP rod bolts, statically and dynamically balanced and blueprinted.

Now that I've replaced my C-4 auto trans with a T-5z and an 8" 3.80 Traction Loc is being built for me, I'd like to warm the engine up a bit. I'm not in a hurry to do this, but I want to start planning for this now. My intended approach is a longer-duration, higher-lift cam (I'm thinking the Clay Smith 280/280/108 hydraulic, but that's a subject for a different thread) and more compression (would like to add no more than a point of static compression). I will probably try a 4 BBL carb as well, after the cam and compression are sorted out.

With the custom curved DS II (I run 20 degrees of initial advance but limited to 16 degrees of added mechanical advance; probably don't develop enough vacuum to get much vacuum advance) and running 93 octane gas, the Ranchero has never detonated, so I'm thinking I could go with 1/2 point higher compression with the cam I have, 1 point higher compression with a longer duration, higher lift cam (since that will reduce dynamic compression).

Having just installed the T-5, I'm not eager to pull it back out immediately to work on the short block ... so I'm hoping that it would be possible and advisable to gain some compression by milling the head. The head will need to come off to change the cam, so that would be a great time to mill the head. My CI Al head has 55 cc combustion chambers. Is there a rule of thumb for the shape of the small six head that indicates how much must be milled off of the head to reduce combustion chamber size by 1 cc? Is there a maximum limit to how much can safely be milled off of the small six head before overheating or the probability of cracking become too likely? Are these considerations any different on the AL head than they would be on an iron head? I want to be able to continue to drive the Ranchero on the street, even on long drives such as to regionals in other states, and also in Houston traffic on hot (100 degree) Houston days. More than anything, that AL head is not only expensive but precious (in that only ~100 or so of them exist at the moment), so I don't want to mill too much and damage the head in any way.

What issues am I likely to encounter if milling the head for compression? Hopefully my adjustable rockers will be able to compensate for the altered valve train geometry, if not I may have to order shorter push rods. What else?

I searched for these topics on the forum and found many posts about milling the head to increase CR, but I couldn't find a rule of thumb that correlates thousands of an inch milled from the head with cc reduction of combustion chamber, or a maximum limit. They are probably buried somewhere in the 100's of posts about milling heads for compression.

Thanks,
Bob the Builder

P.S. I put this post here, rather than in the Al head forum, since I don't know if the guidelines for milling a head for compression are any different for the Al head than for an iron head; and also, more people will see it here.
 
i BELIEVE THERE IS SOME INFO ON milling the CI head in the archives. Depending on how much is milled in the past with factory heads i would make spacers to fit between the rocker shaft stands and the head.
 
turbo2256b":82nivj1r said:
i BELIEVE THERE IS SOME INFO ON milling the CI head in the archives. Depending on how much is milled in the past with factory heads i would make spacers to fit between the rocker shaft stands and the head.

@turbo2256b,

Thank you for replying. I looked in the General Tech - Read Only section (very bottom of the board index), Small Six, Small Six Tech Sticky's section, Small Six Head Swap topic, and found the following:

.010" head mill is about equal to 2 cc's reduction in combustion chamber
.090" is about the limit for milling the head

I would be interested to see if some of the more experienced members of the forum agree with this, but I will work up some CR calcs based on these guidelines (one, to gain 1/2 point of SC, another to gain 1 point of SC).

The Sticky also mentions the need to put washers under the head bolts to compensate for the milling; I don't think that would be necessary with head studs (as I have).

Thanks again,
Bob
 
This is on a log head, so take it with a grain of salt.

I had my head cut 0.070". I measured the chambers at 61 cc prior to the cut. The machine shop measured after the cut and came up with 48 cc. So that is a 13 cc reduction. This is in the neighborhood of the 0.010" to 2 cc number you quoted.

Again, I don't know if it's apples to apples with the aluminum head nor how thick the casting on that thing is to say what is safe.
 
CC and Surface Mill - Out of the box combustion chamber sizes vary from 48-50cc's, with the average size being 50cc's. For best results, cc the cylinder head so that all chambers are the same size or within one cc, which translates to roughly one tenth of a point in C/R on a 200ci motor, less on a 250ci. Then surface mill the cylinder head so that the desired chamber size (compression ratio) is achieved. This will vary depending on deck height, head gasket thickness, cylinder and gasket bore size, and combustion chamber size. We recommend a C/R of 9:1 to 9.5:1 for street applications. Race builds may utilize higher compression ratios, but caution must be observed to prevent detonation and/or pre-ignition. Extreme compression ratios may require the use of water injection or other means of preventing pre-ignition and/or detonation. Warning: High C/R's may lead to pre-ignition and/or detonation, which may results in severe engine damage or failure (see warranty).
NOTE: To reduce chamber size 1cc, mill the head approximately .007 thousands.

tHIS IS LOCATED IN THE ARCHIVES UNDER THE CI aluminum cylinder head specifications. Might need to read some more on info for max milling specks.

I was speaking of spacers under the rocker arm stands if .060 was removed from the head .060 spacers would need to be used to raise the rocker shaft upwards to keep rocker/ pushrod geometry correct. The spacer under the very rear rocker shaft stand needs special work as it is were the oil comes from to oil the rocker assembly

Also need to check piston to valve clearance
 
Greetings Ford Six Fans:

Continuing this line of thought (milling the CI AL head to raise compression):

Current have a 55 cc combustion chamber, .020 over bore (3.700"), stock stroke (3.910"), a 15 cc (0.9152 cubic inch) piston dish, a 0.050" thick gasket, and 0.005" deck height. My current SCR is 9.65.

Changing the combustion chamber size to 51 cc (a 4 cc reduction) and keeping everything else the same raises SCR to 10.1, according to my calculations:



Changing the combustion chamber size to 47 cc (an 8 cc reduction) and keeping everything else the same raises SCR to 10.6:



Please check my math.

Thanks
Bob
 
Your math is correct if you assume the head gasket bore is the same as the cylinder bore.
If the 274, 108 is set at spec which is 2 degrees advanced the dynamic compression ratio is 7.75 which is good for pump gas without detonation.

If you raise the compression to 10.1 and change to the 280, 108 cam the dynamic compression ratio is close to the same @ 7.79
If you raise the compression ratio to 10.6 the dynamic compression ratio goes to 8.17 which is still ok for pump gas with an aluminum head but it narrows the range of acceptable pump gas octane.
If you are always using premium grade pump gas then it's probably OK.

May I suggest going to the 280 cam with 110* LSA if you increase the compression to 10.6
The dynamic compression will be 8.00 and the idle will be close to what it is now with just a small increase in roughness.
 
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