So far I haven't been able to find a set of 243 or 799 heads locally. My heads will be coming off for a lifter swap, so I wanted to machine them while they're out. I've seen people mill them between .040 to .050 to get the chamber size to the mid 60's. I plan on milling .040 to stay a little on the safe side.
I'm hoping to use the thinnest head gasket I can to help maximize compression ratio. I've seen many people reference that the LQ4 pistons usually stick .007 out of the bore. This is something I'm obviously going to measure, but would I be cutting it too close using the .036 (compressed) Cometic gasket?
When milling this much does it have to be angle milled to maintain proper intake alignment?
Will the standard head bolt set still work properly? Are there any other special consideration when reassembling?

 

I have some 317's milled .040 and the shop milled the same off of the intake side to keep alignment perfect. I don't know for sure but with the .036 gasket you should definitely check the PTV clearance. total thats about .060 less material.

 

Thats cutting it close for the piston to valve clearance if youre running a big cam. What are the specs?

 

The cam I intend on using is either the Vinci Ultra-torque or the Butt Kicker. The only difference between them is the LSA. The Ultra-torque is 114, Butt Kicker is 116

DUR @ .004" 272*/280*
DUR @ .050" 210*/218*
LIFT .551/.551
LSA 116*

I think my PTV will be ok, however I am a little concerned with my quench. I had to order a deck bridge to measure how far out of the hole my pistons are traveling. with the .036 gasket I think I'd be cutting it too close. Unfortunately I haven't found any middle ground between it and the .052 MLS.

It's nice to hear from someone who has milled off this much. I see a lot of theoretical talk, but not much action. Brent, if you were to do it over again, would you still stick with the 317's, or go with a different head?

 

Well, it turns out no engine machine shops can cut the intake side due to the valve cover flange overhang. I haven't found a production shop yet that wants to do it since there's a lot less money in it than the work they're currently busy doing. I'm starting to see why people say that the 317's aren't worth it for an NA application.

I still haven't been able to find an affordable set of 243's or 799's. How much would I be giving up by using a set of 241's? I have a set that I pulled off my Camaro at 19k miles.

EDIT: I should also state that I'm not looking for peak numbers. This is simply a DD I'm trying to wring a little more performance out of. Since this is a heavy Avalanche, plus I'm using it for towing, I'm looking for some good low end torque.