Are there any disadvantages in milling heads to lower the combustion chamber. (I don't mean disadvantages of higher compression ratio--rather take a set of heads for a 350, mill them down so they'll have around 58 cc chambers)

There are a lot of heads out there, that flow pretty decent, that are fairly cheap, but have combustion chambers for 350's. Somebody told me once that milling the heads is "BAD" (talk about vague), but I know that milling Vortec heads to maintain the compression ratio for a 305 is common, and acceptable practice.

 

people have been doing it for many many years. No it's not bad.
Just don't cut so much off that you hit the water jackets.
To keep everything lining up (as in ports and bolt holes) you should mill the same amount (approximately) from your intake manifold mating surface. Then double check for port & bolthole alignment when it's done to see if you need more milling.
no problem.

 

I guess it all depends on your definition of acceptable. Milling the heads thins the area between the water jacket and the outer surface, making it more prone to warping and/or cracking. Usually about .030 is as far as you would want to go for a daily driver, beyond that it gets risky. Ive milled more than that off of some heads before, but I knew the risks I was taking as well. If you angle mill the heads to save the intake alignment (common practice) you introduce a new variable of the water ports lining up. There are some 350 heads (sorry I cant remember which, I think lightweight 350 heads) that will either leak past the head gasket ring where the spark plug coolant passage is, or it will crack there when you mill them too much.
Personally I prefer to leave heads alone and choose appropriate pistons to achieve the desired compression ratio.

 

In small amounts it's OK to mill heads. However, you're talking about a MASSIVE amount of milling, which those heads aren't prepared for. Remember, they are just replacement truck heads not high-performance ones, so the design processs for them didn't necessarily include setting them up to have enough metal to support that kind of work. It's probably not a real good idea. The passages and even the exterior of the casting aren't perpendicular to the deck, so you can end up with a situation where the metal doesn't line up with gasket material any more. Plus, you'd have to get the intake planed too, so by the time you get through with all that, you'll have more money tied up in creating a Frankenstein than you'd have in a set of WP 305 heads.

There are Vortec design heads specifically for 305s also; if you really want to get that particular kind of heads, it would be alot smarter to start out with the right thing in the first place.

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"So many Mustangs, so little time..."
ICON Motorsports

 

I know there are 305 Vortec heads, but they have smaller intake/exhaust valves than the L31's.

Would it be cheaper to have the 305 Vortec heads machined to accept 1.94 intakes or to have a set of L31's milled?

 

milling is a very cheap process and is just an extension of head resurfacing which is a common overhaul practice.
If you angle mill, be sure not to forget to true up the headbolt boss surfaces so that they are parallel to the head mating surface. Otherwise the head of your headbolts will dig into the head at an angle and throw off your torque readings.

 

So if I mill a set of heads, say that have 64cc chambers by .15", that should give me about 58cc, at least that's what they did in an article I read where they milled some L31 vortecs.

Regardless of the amount, provided it is small, do I understand that in order to keep everything matched perfectly, I should have the intake milled as well....is .15" enough to make a difference?

 

I hope you have that decimal point in the wrong place because 0.15 is a lot of material. 0.015 is more like it.