Milled heads; how much is too much?

[Gary B.]

I posed a similar question a few weeks ago and I didn't receive much in the way of specific advice, so I thought I would rephrase my question and expose my ignorance yet again. Can someone tell me how much milling is too much when it comes to buying used heads, specifically for my 1966 L79. I've seen used heads for sale that have been milled 10 thousands, 15 thousands, 18 thousands, 30 thousands, etc. Obviously, there is some point when too much material has been removed and I guess heads that haven't been milled would be preferred over heads that have been milled, PROVIDED that the head surfaces are true and in good shape in both cases. So, can someone tell me a rough guideline about when not to consider a specific set of milled heads? For example, is it unwise to consider a set of heads that have been milled, say, 30 thousands, even if the price is right, e.g., free? What about 60 thousands? I don't even know if these numbers are reasonable for 461 and 462 heads, but I'm trying to learn something here, so please forgive my ignorance.

Thanks,

Gary Beaupre

[Clem Z.]

Re: Milled heads; how much is too much?

anything over .020 will require the intake to be milled also. i have milled heads .100 with no problems. i have angle milled heads .150 but that is a different ball of wax. the best way to check how much has been milled is to take a unmilled head and with a pair of vernier caliphers measure across the bottom row of bolt holes spotface to the deck of the head. anything less than this thickness is the amount that has been milled within a few thousands. if this is not clear i can scan you a sketch.

[Gary B.]

Thanks Clem. Exactly what I wanted to know! *NM*

[Gary B.]

Clem, a follow-up question

Clem,

If a head is milled 0.020, how much would this increase the compression ratio,
if the same thickness head gasket was used for the virgin head and for the 0.020 milled head? Might it go from 11.0 to 11.1 or would it increase much more than that for 461/462 heads?

Thanks,

Gary

[Craig S.]

Re: Clem, a follow-up question

Gary - something to consider - if you use a gasket such as a FelPro competition with printoseal around the water ports, they run around 0.038" compressed, versus the 0.022 steel shim GM gaskets. Personally, this is one area I shy away from stock, unless you mill the heads and decks, I don't like to reuse stell shim gaskets. What this nets out to is your 0.020 mill is a wash after factoring in the difference in head gasket thickness. Now, if you do use the steel shim GM gaskets (if they are available), the CR will go up slightly, to figure it out, you can CC the heads etc etc and factor in.....Craig

[Clem Z.]

Re: Clem, a follow-up question

you remove about .15 cc per .001 cut to SBC heads. .020 cut would remove 3 cc and 3 cc would change the CR about .3 of a point.

[Gary B.]

Thanks Clem & Craig *NM*

[Clem Z.]

Re: Clem, a follow-up question

you have to be careful using thicker head gaskets because as you go past .050/.060 deck clearance even though you have lowered the CR you can run into detonation because of the trapped unburned fuel in that space. lowering CR with thicker gaskets is not a good idea.

[Craig S.]

Re: Clem, a follow-up question

Clem - I don't know of any FelPro gaskets less than 0.038" compressed, seems like that's the norm for 1017, 1027, 1037 series for BB engines. The steel Shim GM gaskets were 0.022" I think. Are you saying using FelPro gaskets will get you into detonation trouble? Thx, Craig

[Clem Z.]

Re: Clem, a follow-up question

yes using a thicker gasket and increasing the distance from the piston flat to the head at TDC to a larger distance will lead to detonation. a lot of raw fuel get trapped in that area and does not burn correctly. the closer you keep this distance to .035/.040 the less chance of detonation. this may not be a problem with a 9:1 compression engine but if you are above 9:1 it will be a problem.

[Craig S.]

Re: Clem, a follow-up question

Clem -thx, what you are saying is there is too much space in the quench area for detonation to occur. My 858 heads have had a 0.0.010 clean up cut since there was some pitting etc, my original decks are intact on the 942 block, I didn't want to mill off the numbers. I did have it torque plate honed using a Felpro 1027 gasket (0.038") compressed. I am using Speed Pro L2268F 0.030" over pistons, and a Speed Pro blueprint grind of the orignal L72 cam. So, it is all stock build back except the overbore and gasket choice. At this stage, am I better off using the FelPro competition series gasket I chose or trying to use steel shim gaskets even though the torque plate work was done with the FelPro gasket. I am at the point of assembly where the choice must be made, I figured I am already in the high octane special blend territory anyway....thx for your thoughts....Craig

[Clem Z.]

Re: Clem, a follow-up question

i would stick with the gaskets used to deck plate hone the block and see hat happen as far as detonation goes.

[Craig S.]

Thx Clem for the advice! *NM*