High Compression

[steammech50]

Why can't a stock model-a head be machined too up compression,save money on a new head ?

[lindy williams]

Two reasons : The head will become too thin and the recess over the piston will be machined away and the piston may hit the head since most pistons come above the block at TDC.

[George Miller]

Originally Posted by steammech50

Why can't a stock model-a head be machined too up compression,save money on a new head ?

I have in the past taken .125 off the stock head. It did not help that much.

If you have to pay to have it milled, it might cost more than a new 5.5 head, which is much better than a milled stock head.

[newshirt]

The shape of the combustion chamber is different than the new HC head, and may not provide the same performance, even with identical compression ratios.

[steammech50]

Thanks a lot guys, guess I'll spring for a 5.5 !

[V4F]

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.060 off the 5.5 will give you 6-1

[Purdy Swoft]

Why not just get the Snyder six to one?

[CarlG]

Before Snyder came out with the 6:1, Sacramento Vintage would mill the 5.5:1 head down so that it became 6:1. About the time I bought mine (4-years ago), they quit doing that because of problems they were having with them. They didn't elaborate, just said they quit doing it.

[Terry,NJ]

I have an engine that the head was milled on, about .090 ( I think ) and it runs very nice (It's bored .125 too) I have another engine that the head was cut .040 and bored .040 it ran strong in my coupe for a short while. It cost me $40 to cut the head (vs $290 + shipping for a HC head) So, the stock head will take up to .125 milled off which really doesn't improve the performance as much as a 5.5. but costs less Because of the Combustion Chamber design on the original, you just can't take enough off to make a big difference.
Terry

[Terry,NJ]

Hey guys, If anyone has the tables on this, could you look up my CR on the engine that was bored .125 and the head was cut .090. I'm curious what it is.
Terry

[Skibb]

Have any of you measured the compression with your HC Head and/or shaved head and block's deck. It would be an interesting comparison.

[wensum]

I bought a 7:1 Winfield head from Reds Headers I think, but it then required machining to clear the pistons which was an added expense and the combustion chambers were pretty rough and required polishing.
Haven't started the motor yet so have no idea how much the performance will be improved, but then there are so many other mods that it will be impossible the effect of any one
Keith

[Skibb]

For our engine, Just tested cylinder compression from number 1 to 4. Readings are 86, 90, 90 and 87. The block's deck and the HC headed were both "shaved". Now to break it in gently.

[tiquer]

You should notice an increase in gas milage with the high comp head too

[James Rogers]

Originally Posted by lindy williams

Two reasons : The head will become too thin and the recess over the piston will be machined away and the piston may hit the head since most pistons come above the block at TDC.

Wrong on both counts.

[Ron/IA]

James - please explain further. My "common sense" tells when removing material, the object becomes thinner. Now I know heads can be milled without problem, but when would it too much?

Also, what if you have an engine where the pistons go above the deck that what would be considered normal?

Thanks,

[Joe K]

Originally Posted by Ron/IA

James - please explain further. My "common sense" tells when removing material, the object becomes thinner. Now I know heads can be milled without problem, but when would it too much?

Also, what if you have an engine where the pistons go above the deck that what would be considered normal?

Thanks,

I think intuitively you know the problem. You remove material, either at the edge or by flycutting the perimeter of the combustion chamber and you run a risk of making the area so thin that it won't contain the pressure - your combustion chamber "blows out."

Detecting this - or even knowing how far to go is the problem. Detecting can be done with ultrasonic thickness testers - but their measurement is not absolute - working around corners leaves the device prone to error.

Knowing how far to go perhaps the greater problem. You might measure the thinnest area on the combustion chamber interior and resolve to not go thinner than that? Even so, your thinnest area might be at the center top of the combustion chamber. Duplicate this same thickness at the perimeter and you have the entire combustion chamber interior head area to be supported by the perimeter - and the chamber head may decide to head for parts unknown?

Some - the old timers - may have done this to failure. This back when a head was $2 at the junkyard. Cut one - try it. If it doesn't fail then try to cut it a bit more. Until one finally reaches the point where failure occurs - then back up one step and hope for the best on another head.

I hope Mr. Rogers is among those of that era. I am not.

As to above the deck, time and rebuilders and WEAR have done a lot of different things to the Model A engine. Each engine has to be viewed as something unique - that is unless one is well shod and can afford to "blueprint" their engine with original parts and dimensioning.

Joe K

[Charlie Stephens]

Originally Posted by steammech50

Why can't a stock model-a head be machined too up compression,save money on a new head ?

Although the head is thick there are areas of that are not. The attached photos show what is probably one of the worst area. Note that the head shown is an over milled A6050B (the "Police" head) but the same applies to a stock head. As others have noted there is also a problem when the relief over the pistons is removed and your pistons extend over the deck of the block as many do from blocks being surfaced over the years. The small amount that can safely be removed is probably not worth the trouble.

Charlie Stephens

[James Rogers]

Anything can be overdone. I have milled many heads till the counterbore was gone without any adverse effects. I also, have a couple of aluminum racing heads that were made that way and only have a combustion chamber big enough for the valves and little more. Both ran fine in racing applications. The cracks Charlie shows are common and are most often found on stock heads that were never cut. The combustion chamber top is approx. 1/4" thick at the edge of the counterbore. You would have to cut probably .375 to get into a problem. That would be expensive.

As for the deck, the protrusion needs to be measured before cutting the head. A stock or close engine will not have the pistons hit if the protrusion is .040 or less. The head gasket is .065 when compressed, leaving plenty. I have run many engines like this with heads that were cut flat.

[MikeK]

Just for the record, an off-the assembly line A engine with every dimension at zero tolerance deviation would have a block height of 11.500" and the pistons would be 0.03125" above deck. Most blocks came off a little proud, about 11.505". The odd stock piston pin height, 1.90625" (1 29/32") , doesn't hold true with many replacement pistons. They are often shorter if you have a collection of old replacements and measure them. Stock rods were 7.500" I've seen rebuilt rods that were off as much as 0.025"

Add to all this the horrors that come with crank regrinds like off tolerance throw variances and you have a mess that requires a competent, experienced engine builder. It is not possible to give any one set answer about how much piston pop-up is correct or acceptable for a theoretical pencil and paper question about a particular head on a rebuild. Accurate measurement with parts in hand is required.