Milling vs. decking

[mustangsilly]

I am putting 62 cc heads on my 66 200 cid block. I will have the block out for re-ring and bearings anyway. Is there any advantage to decking vs. milling the head to reduce combustion chamber volume (i.e. increase CR). My goal is 9.5 - 1 (I live at 5800 ft.) I know the Falcon book says milling improves quench, but wouldn't decking the block do the same thng? I appreciate your comments.

 

[CoupeBoy]

If you look at the cylinder head there is a 'deep' part and a shallow part. Milling the head will decrease the size of the shallow portion forcing more combustion to happen in the deep, cup shaped part. The closer your dish shape is to matching the shape of the cylinder head cup the better off you are.

Decking the block decreases the total combustion area but retains the same shape.

-ron

 

[MustangSix]

Milling the head doesn't affect quench distance. Lowering the block surface by decking the block has the effect of moving the piston closer to the cylinder head surface and does affect quench.

 

[66 E100 Pickup]

if you are looking for that kind of CR, you will be better off decking as much as you can, as you will get more bang for the same cut than you will off the head.

Also, you keep the integrity of your head intact. While opinions vary, many say anything over 60 thousandths is a huge cut and risks finding a deep casting pocket (bye-bye to the head).

The downside is you have to set up your crank and at least one piston twice, and clean the block twice. Figure another $200-$400 in labor and machining, depending on how you calculate your time involvement.

The general rule is zero deck the block and then take whatever else you need off the head to get the CR. If you have a deep cut needed to get the CR, you are better off picking a different piston shape to get the CR and limit your milling to about 10 thousandths (just enough to square up the head and knock down the size of the coolant erosion areas).

John

 

[mach1 mark]

I told the guy at the machine shop to zero my block he looked at me oddly and said "cost more", so I decided just to deck the block 20 thousands. He said "Oh, we do that for free" (included in price). The block was almost zero by taking 20 off. So My suggestion is to take 20 off the block. Do your calculation, and make sure enough room for valve movement.

 

[mustangsilly]

Thanks for the help. I guess I'll be doing a combination of decking and milling to get to where I want to be compression-wise.

 

[jamyers]

I told the guy at the machine shop to zero my block he looked at me oddly and said "cost more", so I decided just to deck the block 20 thousands. He said "Oh, we do that for free" (included in price). The block was almost zero by taking 20 off. So My suggestion is to take 20 off the block. Do your calculation, and make sure enough room for valve movement.

huh? I've always understood "zero-decking" to mean that the block deck is cut down so that the piston tops are at the same height as the deck, hence the 0-deck. And a 20-thou deck would be where the pistons are 20-thou below the deck. Either way, the machining setup is the same: install crank and piston, measure height, disassemble and cut.

Sounds like you're saying that your machinist just automaticlaly cuts 20-thou off the block without the setup and measuring - which may well be ok, but I'd wonder...

 

[jamyers]

mustangsilly, CoupeBoy is onto something you might consider:

If you look at the cylinder head there is a 'deep' part and a shallow part. Milling the head will decrease the size of the shallow portion forcing more combustion to happen in the deep, cup shaped part. The closer your dish shape is to matching the shape of the cylinder head cup the better off you are.

Decking the block decreases the total combustion area but retains the same shape.

-ron

I don't recall what our combustion chambers look like, but typically if you can get the top of the piston to right around .035" of the shallow portion of the head, you'll gain a lot from increasing the piston-to-head "squish", sometimes called "quench". The idea is that the piston squishes the mixture from one side of the chamber to the other, increasing turbulence, making it more detonation-resistant, better combustion.

On many engines, the only way to get the piston nearer to that part of the head is to deck the block, since cutting the head also cuts away that part of the chamber.

 

[CoupeBoy]

That is what he is saying, a safer way would be to measure it first, like you said

the machining setup is the same: install crank and piston, measure height, disassemble and cut.

-ron

 

[mustangsilly]

I'm thinking I'll take .050 off the head (= to gasket thickness) and .00 deck the block. Hopefully that will get me at about 9.2/1 and still have plenty of valve clearance. Anyone used flat top pistons with that type of setup? I know that would bring the CR up pretty high, but I might skip the decking.