milling an '83 head for my '66 block

[Anonymous]

I got a big log '83 head off a 250ci motor and am putting it on my '66 200. How much do I need to mill the head to get the appropriate compression? Thanks, Parker

 

[CZLN6]

Howdy Back:

There are two issue, you must consider, IF you want to maintain a stock ratio of about 9:1.

1st the chambers on an '83 head is about 62 CCs in volume, the '66 about 52. The '83 chambers will have to be reduced by aproximately .050". That's as a result of estimating by using a formula of 2 CCs = .010" cut.

The other issue is the difference in thickness of the stock steel shim gasket of .025" and the likely composite, replacement gasket of .037" (Corteco) to FelPro at .050".

Be sure to use washers under the head bolts on reassembly.

Mike at FSPP has Corteco. NAPA has Victor. FelPro are available everywhere.

So, a mill cut of .062" to .075" will maintain your original CR. These heads can safely be milled to .090", but it is best to save a little for the future. It is also best to measure twice and cut once.

Reread the sticky at the top of this Forum on dead swaps.

Adios, David

 

[Anonymous]

Generally, when milled this much it is best to take down the block's deck height rather than to take it all of the head. The reason is that when milling this much off the head, you change the shape of the combustion chamber and affect the quench area of the flame front at spark ignition.

The compression ratio calculator at this site is a nice tool to work with in determining what your final compression ratio will be, taking into account the compressed thickness of the head gasket, whether you have dished or flat top pistons, and the size of the combustion chamber after milling.

When looking into the Corteco head gasket, I was told that it was the performance version of the gasket that compressed to .038. There is no Corteco performance head gasket listed for the 200/250, only the 240/300. Whether the Corteco factory replacement head gasket does or does not compress to .037 is something I have not been able to determine. Determining the compressed thickness of the headgasket is critical to to BEFORE milling the head, as it will tell you how much to mill off to meet your desired compression ratio. I read that the compressed thickness of a (non steel insert) head gasket is about 80% of its uninstalled thickness.

good luck.

 

[CZLN6]

Howdy back All:

While I can't disagree with 65Fastback on the recommend to deck to block, I see no problem with milling a head .075". It will not effect the quench dimension (Deck Clearance) or effect, as it is a product of chamber-to-bore ratio, and deck clearance- the space betweem the top of the piston and the deck surface of the head. Deck clearance includes head gasket thickness and deck height, the distance from the top of the piston to the deck surface of the block. On a '66 OEM 200 that is typically .025" for the steel shim head gasket and .025" deck height for a deck clearance of aprox .050". .050" is on the large side for effective quench, but not bad with a 9:1 CR. If you have to go with a .050" thick composite head gasket it will balloon up to .075", dropping CR by a half a point, for very lazy combustion turbulence. Short of finding a NOS steel shim head gasket milling is the best compromise. Raising the CR improves the quench equation. The higher the CR the more mixture is squeezed out of the quench space.

Poor quench and low compression = not good combustion efficiency.
Poor quench and high compression = better combustion efficiency.
optimum quench & higher compression = best CE.

Milling the head has several positive effects in addition to raising CR. It does not require a complete engine tear down, it move the spark plug closer to the piston dish, it allows the intake valve to drop deeper into the cylinder, in effect unshrouding it. And it trues the mating surface of the head.

The only downside to milling this much is the possibility of bottoming out the head bolts in their holes. That's one reason for the washers recommend

Decking the block is the most effective way to improve quench- assuming you can find a gasket that will allow a .035" to .040" deck clearance. The NAPA Victor 3910VC head gasket specs out to a compressed thickness of .044", which may be the best available, known choice.

I'm in the process of examining the Corteco head gasket for the 200/250. I just received two from AzMike. They mic out at aprox .050". They have no compressed specs stated in the packaging. Neither does their website.

The 80% of uncompressed thickness generality would depend greatly on the composition and construction of the gasket and the torque applied to it.

that's my two cents, for what it's worth.

Adios, David

 

[Anonymous]

Well said! I was under the impression that shape, as well as volume, of the combustion chamber affected the flame front and quench area.

Thanks for the part number on the head gasket. Whoever rebuilt the motor on my stang before I bought it milled the head and neither put washers under the head bolts nor put thread sealer on the end passenger side head bolts. I found this out when, after 500 miles of post-purchase driving, there was a catastrophic failure of the head gasket. There was a massive accumlation of water/oil mixture on top of the engine and I observed air bubbling up from underneath the caps of the head bolts. I am now using a new set of ARP headbolts with their washers. I used ARP threadsealant (which is a lot thicker than the permatex thread sealer - more like plumbers thread sealer) and also ARP assembly lube on the threads of the remaining head bolts and also on the face of the washers. Haven't started the engine yet so I don't know how it all is going to turn out.

 

[CZLN6]

Howdy back 65 Fastback:

Chamber shape can be a major contributor to inducing swirl, and turbulence, enhancing cylinder filling and combustion efficiency. Unfortunately, the shape of the chamber is not a positive factor. Milling .075" will not alter the chamber shape/

Thanks for sharing your adventure. As always, the devil is in the details.

We've kinda hijacked Parkers topic here. Hey Parker- What did you decide to do?

Adios, David