[Will]

The ratio of amount milled to cc reduction is a guideline, it's not a hard and fast fact. Look at the various chambers used over the years and the fact they're all dome shaped and it's easy to see why one head might lose slightly more volume than another with the same amount milled off.

Either way, if you mill .030" and get a 5cc or 6cc reduction, does that one cc really matter?

I have a set of 6x-4 heads that were supposedly milled .030 before I bought them. I had to slightly elongate the bolt holes with a round file to get an Edelbrock performer to fit.

[steve25]

Yes, it's not a 100 % cut in stone fact as even the 3 factory casting heads with RA4 type chamber and the different shape that they where machined to have a different CC removal ratio when you Mill those heads, as does the short lived half season 061 casting.

[Skip Fix]

As Steve said when you start looking at chamber diameters there are lots that are different diameters-and that is what determines how many cc/0.010 mill you get off. Even a 6X-4 is smaller diameter than a 6X-8 head.
RAIV 72cc chamber being much larger(and hence shallower) than other 72cc heads.

[78w72]

Originally Posted by Will

The ratio of amount milled to cc reduction is a guideline, it's not a hard and fast fact. Look at the various chambers used over the years and the fact they're all dome shaped and it's easy to see why one head might lose slightly more volume than another with the same amount milled off.

Either way, if you mill .030" and get a 5cc or 6cc reduction, does that one cc really matter?

I have a set of 6x-4 heads that were supposedly milled .030 before I bought them. I had to slightly elongate the bolt holes with a round file to get an Edelbrock performer to fit.

i agree with the amount milled vs how much cc reduction theory. my initial post was an estimate of "about" 6cc for .030 based on what wallaces chart said. not sure why that had to be "corrected" to say it was actually 5cc & wallaces chart was wrong, but now its not "100% cut in stone fact." but regardless what is actually correct, roughly 1cc probably doesnt matter on a low hp build like we are discussing here.

also intake fitment can have many variables, like how much was milled off the block & the brand & model of intake... i recall reading numerous issues over the years with edelbrock intakes not lining up perfectly & needing some bolt elongation on even unmilled heads/blocks. so it all depends on the combo of machine work & parts.

[Skip Fix]

My 6X-4s on the 400 in the 81 TA were 93 to start I smoothed the "eyebrow" ridges and milled them 0.050 and they were still 93cc. I did have to fit the intake and valley pan some though.

[Will]

In my particular case, the engine was together and running. It was an all original '71 400 with the original #96 heads, had never been apart and must have had a lot of miles on it. A head gasket blew and the engine had been using a lot of oil so I threw the 6X heads on it 'cause they were fresh. Rings still weren't that great any more but the engine ran lots better and used a lot less oil after that.

The bolt holes in the manifold didn't need a lot of work, just a little bit to allow the bolts to go in easy.

[Traveler]

A machinist taught me a trick to determine how much to mill a head to get a certain chamber size.

1. Lay the head out combustion side up. Level it out perfectly. I did it using bolts in the bolt holes with adjusting nuts and a small level.

2. Fill the combustion chamber with fluid from a graduated burette to the exact cc's you want the chamber to be. I used windshield washer fluid.

3. Next measure the distance from the head surface to the fluid with a caliper depth rod. This is actually pretty easy to do as when the end of the caliper rod touches the fluid it will ripple.

4. Read the caliper and this is the amount you need to mill to get the required combustion chamber size.

Sounds pretty shade tree but it worked very well. After milling, the chamber size was right on.

Phil

[Will]

Sounds like a great idea to me. Definitely room for some error but as with most things, if you're careful and double check your methods and results it'll probably work fine.

[steve25]

Can anyone post up the milling chart from the old HO racing engine book , as that has the chart I go by and it list all 3 style chambers, 389/ 421, open chamber D port and the round port chambers.

I will dig it out tonight and post it up tomorrow if no one has done so by then!

[Traveler]

Originally Posted by Will

Sounds like a great idea to me. Definitely room for some error but as with most things, if you're careful and double check your methods and results it'll probably work fine.

I found the head charts on the internet to be very rough approximations. I have a set of 1970 #15 heads. The chart says they are supposed to be 87cc. I measured them and they were 93cc.

You don't know what has been done to your heads in a previous life. They could of been surfaced, milled, or the combustion chambers massaged.

Other factors such a valve jobs, valve recession, and installing larger valves or new valve seats can have an effect on chamber size.

Measuring the actual part eliminates these issues. You end up with the actual final combustion chamber size. It beats flying blind.

Getting the head mating surface level is critical. Finding the distance from the mating surface to the fluid is most critical part and prone to the most error. I measured it by dropping the depth rod from the mating surface off the side of the combustion chamber. An easier and more accurate method would be to use a straight edge, such as a steel rule, laid across the chamber. Then subtract the width of the rule from your measurement.

Getting the depth rod to just barely touch the fluid takes some playing around with. The more effort the better the results. I think with care the amount to be milled can be determined to a couple of thousandths.

The thing is, you are measuring the actual part and not using some approximations. Try it. It is easier and more accurate than it sounds.

Also, measure both heads. They might be a mismatched set with different histories. Using milling charts won't tell you this and you can correct for this.

Hope this helps some one.

Phil

[4zpeed]

Hear is a link to a simple DIY way to CC your heads at home, as mentioned one never knows what's been done to them before you get there.

Or after work has been done, takes all the guess work out.

https://www.youtube.com/watch?v=4ToIc6FzPF4

Edit: Although he's doing a runner, he mentions the same method used for chambers.


Frank

[steve25]

Here's the head milling guide from the old HO racing motor build guide.

[Skip Fix]

I have that Steve and after comparing 6X-4s diameter and 6X-8s they would have to be a little different.

The filling chamber to a certain CC then measuring depth is sue an interesting way to do it.

[78w72]

Originally Posted by steve25

Here's the head milling guide from the old HO racing motor build guide.

thats the same chart wallace has on their site & what i was referring to. says .030 is 6cc off 71-76 heads... is HO racing wrong too? from my experience with mid 70's pontiac heads, the wallace & HO chart is pretty darn accurate for estimating the amount mmilled vs cc removed. but yes a final cc check is needed to be 100% accurate.

[steve25]

Yes skip your right !
There are differences in the absolute amount to Mill off between any of the low comp heads that are stamped either -4 or -8 due to the lay back of the upper part of the chamber above the spark plug and also between the D port high comp heads and the high comp and low comp heads that have the RA4 chamber chamber!

[tininjun68]

Did this build commence?

[makilaak]

Finally I can release the outcome. I bought #62 heads, Butler BP6011Sp cam, Speed Pro pistons .030, RA headers, etc.
Got 391.0 hp at 5100 rpm, but max torque of 623.8 at 3600 rpm! I think that went well with pretty original set up. Now enjoying sunny weather and soon with new rear tires...

[78w72]

623 torque?? Is that a typo? What kind of dyno? Seems rather high for a 400 with a mild cam.

[ZeGermanHam]

Originally Posted by 78w72

623 torque?? Is that a typo? What kind of dyno? Seems rather high for a 400 with a mild cam.

This stood out to me as well. Something isn't right.

[Matt Meaney]

624 NM = 460 lb ft