Another milling-decking-compression question

[Alabama65stang]

: I've got my '65 200 apart and ready to take some measurements before it goes to the machine shop. Got a 77 head from the salvage yard also. My question is whether is would be better to mill the head only(.075 approx) to take care of the combustion chamber size difference AND a thicker head gasket OR would it be better to deck the block .025 to take care of the gasket thickness and mill the head .050(approx) to reduce the combustion chamber volume. I plan to CC the heads tomorrow to make sure about how much combustion chamber I need to lose so my numbers aren't exact at this point. The old pistons had deck heights varying form .023 to .030. I plan to go back pretty much stock and would like to just maintain the original compression ratio. My real question is whether to mill the head only, or mill the head and deck the block.

Thanks for any advice.

 

[Kinky6]

Hey, 'Bama Stang, what part of the state are you from? I'm in Montgomery.

It would be good to at least surface the block, so that you know it is square to the crank journals. I know a lot of builders like to find the thin, high compression gaskets, but with the thicker ones being so common, it seems that shaving the head, and surfacing the block together, taking the extra thickness into account, is a good way to maintain your stock C.R. If you ever have to pull the head, replacement gaskets would be easier to locate.

Just my $0.02 worth. Kinky6 8)

 

[DaveLS]

AL65Stang,

We am currently thinking about the same issue.

Conclusion, Deck the block to set deck clearance and "squench," Bore the cyl to clean them up, Mill the head/select piston type (dish size to flat) to set compression. Check to make sure pistons do not meet valves or head after engine is warm and spinning. Set compression based on gas type and cam.

Things you have to know in advance. 1) The type and thickness of head gasket to be used. 2) The measurements of the replacement pistons as they will not all produce the same deck height.

Do now, Get and study The Ford Falcon Six Cylinder Performance Handbook.

Hope this helped.

Dave

 

[mugsy]

I'm in the middle of my rebuild, its a '66 block and a '77 head. I took the block decking route first. I did this because the hole in the block is larger than the combustion chamber, so a .030 cut on the block versus the head will lower the overall volume more. I would rather take 0.030 off the block to get the compression I want than say 0.045 (or whatever) off the head to reduce the volume the same amount. I have also heard that the higher up the piston is in the bore (without touching the head obviously) the better the "quench". Now, I do not have personal experience that better quench makes more power, but according to everything I have read that is the way to go.

tanx,
Mugsy

 

[CZLN6]

Howdy "Bama65stang:

You're getting good info here. It is best to machine both block and head to arrive at you goal CR. Decking the block not only raises CR faster than milling, but it also reduces the quench band, improving turbulence and combustion efficiency.

Definitely do the block machining 1st; determine how much you'll need to bore to get a good straight, wall, select the piston you're going to use and do a mock assemble with new bearings and the new pistons in place. Most aftermarket pistons come with a minus dimension in deck height. In other words, they will be down an additional amount. If stock pin height on our 200/250 piston is 1.5", new, replacement pistons will be down .005" or more, for a pin height of 1.45" or less. This is what you need to measure to be able to arrive at a true zero deck height.

Once this is done, select the head gasket you plan to use. Know that NAPA carries the Victor brand here. It measures .045" compressed thickness and is composite type for good seal. FelPro are vailable at most auto parts stores and measure .055" compressed thickness. We prefer the Victors for obvious reasons.

Now is the time to determine the size of the combustion chambers for your goal CR. Be sure to use hardened washers under the head bolts on re-assembly.

Adios, David