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Surfacing a cylinder head

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I have a 40 Chrysler with a 241.   Has a bad head gasket,   So when I replace that.  I want to plane the head. How can I measure mine to see if it’s already been cut or not?   And. What are some successful cuts that some of you have done.   I’m thinking 50-60 thousands will do the trick...

 

 

thanks fellas!

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Unless you know for certain if its been ground before then I'd ask the machine shop to check it for level and take the bare minimium to achieve it as level..........or yeh I suppose maybe 30-40 thou which should up the compression a little.....stock it was either 6.5:1 with the "nascar" optional 7.1:1 so you might bring it up to the mid to high 7's........maybe.......lol......doubt if anyone knows the stock thickness after nearly years.........andyd.

Edited by Andydodge

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Does it matter with these low ratios?  Lowering compression don't do much with stock engine. Theoretically you can plane it down just until it touches open valve.

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7 hours ago, chrysler1941 said:

Does it matter with these low ratios?  Lowering compression don't do much with stock engine. Theoretically you can plane it down just until it touches open valve.

Actually you want to leave some room. There needs to be flow over the top of the valve to aid in cooling.

 

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Once I had to worked on an old four cylinder flathead  Willys Jeep.

It had popped a head gasket because of corrosion.

I got about 5 phone calls from the machine shop, "It didn't clean up yet, what should we do?"

Answer, "Take more off till it does."

At .125 or an 1/8 of an inch it cleaned up enough we knew it would seal. The studs now too long and required a box of washers under the nuts to have enough threads. 

Needless to say it had a lot more power!

Enough to make it scary to drive. In my humble opinion, in the general condition you'll find old Jeeps in, they should never exceed 35 mph.

If the water jackets don't tapper outward (opening up the passages), you can mill a flathead a surprising amount. That's only because they start off with such low compression. Overhead Valve heads start higher and have strict limits as to how much they will tolerate. The machine shop trade has a best practice book from their trade association which has that data collected from the experience of all their members and the manufacturers. I remember seeing a notice from Oldsmobile regarding Diesel heads in a place I worked. It said ".010 warpage and under reuse. Over .010 replace. Re-surfacing of Oldsmobile Diesel Cylinder Heads is not recommended."

So there's the thick and the thin of it.

 

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A stock head measures nominally 2 inches from a flat where a head bolt sits to the underside of the head where the gasket sits.  My 230 rebuild included 040 head and.010 block surfacing.  With the .030 overbore and the single piece marine copper head gasket, the machinist calculated a 8.4 to 1 CR, up from factory 71 to1. Woke it up nicely.

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1 hour ago, rallyace said:

Actually you want to leave some room. There needs to be flow over the top of the valve to aid in cooling.

 

 Yes naturally, didn't mean touching it 😉

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anyone know stock head height on these??   getting ready to pull it and want to know if its been cut before.   

Thank you!

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On 9/11/2019 at 8:28 AM, greg g said:

A stock head measures nominally 2 inches from a flat where a head bolt sits to the underside of the head where the gasket sits.  My 230 rebuild included 040 head and.010 block surfacing.  With the .030 overbore and the single piece marine copper head gasket, the machinist calculated a 8.4 to 1 CR, up from factory 71 to1. Woke it up nicely.

Thanks i missed this the 1st time thru. 

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My experience is that the heads are all over the place. I would stick the head on with the old gasket, but first stick a 1/8 inch disc of clay over each valve.  Turn the engine over hand by hand and then take the head off.  Use the back side of a caliper to measure the depth of the clay were the valve hit it.  You then know how much you can cut the head. As one person said, make sure you have room over the valve when open for air to move over the valve and also for heat expansion and stretch at high rpm.

 

I would also CC the heads and get them to within 1cc each BEFORE you mill it.  It will run smoother if you do. Also, polishing the chambers delays the inevitable carbon collecting on it. It does not stop it, it just slows it down and also makes cleaning it the next time a lot easier.

 

James.

Edited by James_Douglas

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On compression ratios:

Boring a block does nothing to increase a compression ratio and may even lower it. A bigger bore increases the area over the piston.

Stroking a crank increases the CR dramatically.

So if you have a short stroke engine with the same CR as a long stroke engine, then the head has a smaller chamber than than the long stroke. In that case you could get more CR using short stroke head.

 

Just when you thought it was safe to go back in the water:

Because of the way machinists re-grind cranks, you might find a 6 cylinder with 6 different strokes!

Wear or dings say .010 deep would take .020 to clean up with the original stroke. The grinder machines on both sides of the journal. So the savy machinists adjust the centerline of that cylinder to clean the journal at .010. Personally I hate it but that’s standard practice.

You can complain all you want but they are not going to listen to you.

 

SO you can carefully adjust the volume of the chambers to make them all equal and still have the CRs all over the place because of the crankshafts strokes.

Next is the camshaft:

A stock docile cam will have much more pressure on a compression gauge than a hot cam.

Because of that it will start easier too. Something to think about when you’re wedded to 6 volts.

Automotive engineering is all about the compromises you are willing to make. Most of them have been made before you even see the car you plan to soup up. Flatheads, siamese intake ports are all the stuff we have to live with, don’t over think it.

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The only way to know what CR you have is to measure it.

I tested 5 heads and could find no relationship between head thickness and CR. I you don`t know the history of the head start from scratch.

 

864681776_DSC05816e.jpg.aecb1cdf94eec1f3d16fe73fc33a6d54.jpg

  

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3 hours ago, Loren said:

On compression ratios:

Boring a block does nothing to increase a compression ratio and may even lower it. A bigger bore increases the area over the piston.

Stroking a crank increases the CR dramatically.

So if you have a short stroke engine with the same CR as a long stroke engine, then the head has a smaller chamber than than the long stroke. In that case you could get more CR using short stroke head.

 

Just when you thought it was safe to go back in the water:

Because of the way machinists re-grind cranks, you might find a 6 cylinder with 6 different strokes!

Wear or dings say .010 deep would take .020 to clean up with the original stroke. The grinder machines on both sides of the journal. So the savy machinists adjust the centerline of that cylinder to clean the journal at .010. Personally I hate it but that’s standard practice.

You can complain all you want but they are not going to listen to you.

 

SO you can carefully adjust the volume of the chambers to make them all equal and still have the CRs all over the place because of the crankshafts strokes.

Next is the camshaft:

A stock docile cam will have much more pressure on a compression gauge than a hot cam.

Because of that it will start easier too. Something to think about when you’re wedded to 6 volts.

Automotive engineering is all about the compromises you are willing to make. Most of them have been made before you even see the car you plan to soup up. Flatheads, siamese intake ports are all the stuff we have to live with, don’t over think it.

But, remember boring also increases the volume of the cylinder.  And, assuming the block has been zero decked, there is no increase in the volume above the piston.  Even if not zero decked, the bore increase at TDC changes the volume over the piston very little, compared to the cylinder volume change since most of the tdc volume is in the head.

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Also remember the area from the top of the piston to the top of the top ring is part of the area of the combustion chamber.

When you bore a block you are in fact increasing the size of the combustion chamber. No matter how close the piston comes to the cylinder head and because of piston rock there has to be considerable clearance, which increases as the piston's diameter increases.

Increases in cylinder bore can actually lower compression ratios in short stroke engines because of this.

In long stroke engines there isn't enough area increase to make a measurable increase in CR.

As a general rule hot rodders don't consider increases in cylinder volume for purposes of compression ratio from boring to be worth calculating.

I know it is counter intuitive.

 

 

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X2 on the use of clay to check you clearances. This is what happens when you just assume that the head hasn't been planed and you ask the machinist to bump the compression (regrinding the cam for performance didn't help either). Machine shop said they only took 40 thou but measuring it to another head it looks like about 1/8" been removed over it's life. Thankful I decided to check the head clearance by hand before tightening everything down and hitting it with the starter!

 

Edited by 40plyrod

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