head milling question

[austinsailor]

I'm working my way through this 251 I'm rebuilding. With my Fargo head, removing .010 from the block, and .100 from the head, I figure the ratio at apx 8 to 1.

Is it safe to remove this much?

I'd like to get the compression up even more, do I dare take even more off? I seem to recall reading even more has been removed with success, like .125. This would give about 8.4 to 1.

I have a pretty hot cam grind coming from Earl Edgerton, I'd like to make this thing at least act like it's something special.

Thoughts or experience?

[Plymouthy Adams]

your call..you can go a bit more..but just keep in mind..shoud somethng go amiss..you will need a new head as there is no room left for dressing..if you go just half of that..you get a nice bump but still have a margin for future maintenance.....

[austinsailor]

When you say a bit more, are you saying even more that .125? How safe is .1? .125?

[Don Coatney]

Have you looked at your valve to head clearance?

[greg g]

Were it me I would build it at .010 and .070 then maybe go for more if you think you need it. Make sure you check the valve to head clearence. Especially if your cam ground is going to give you increased lift. And look around for a solid copper head gasket. these were pretty common in marine use and resist the blow out common to the type of failure we are used to seeing. I thi the above combination will be pretty close to 9 to 1. the copper gasket is also thiner than the composite and will also tighten things up.

[austinsailor]

I'm starting with .5 depth in the head, the gasket adds about .075. Lift will be .410.

.575 - .410 = 0.165 to play with. Depends a bit on how high the valves are above the head to start. Might need a bit of relief. Guess I'll wait until the motor is assembled, the do some closer measurements. My machinist tells me that there is nothing needed to be added if it clears. Unlike a rod at high speeds, no stretch or anything to be concerned with.

I'm still wondering just how safe it is to remove .1 or more.

I'm also wondering how the 9:1 or so is figured. My numbers, calculating the head volume, cylinder displacement, allowing for the volume in the head gasket, don't come close to that.

I figure the cylinder at 42.74 ci (3 7/16 bored .040 over), the head gasket at .92 (apx 3.2 by .38 by .075) and the head at 5.7972 ci.

Taking off .010 of the block and .1 of the head makes 5.384, into 42.74 = 7.938.

Am I missing something?

[Plymouthy Adams]

you just have to make the call to do the deed after all the investigation you have done...1 is very doable..

[BeBop138]

On my 265 I took 65 thou and then turned chicken and had the valve pockets relieved---just to be safe:o...................Lee

[dezeldoc]

Don't forget if someone in the past has cut the head and you don't know about it, that is material already gone, so you might want to take .90 and see from there.

[Plymouthy Adams]

to determine the head for original or cut you must CC the head and do the CR calcs..

[austinsailor]

done that. Shows to be 6.5 to 1.

[james curl]

On my 218 I had the block surfaced .010 and the head .100. When I checked the valve clearance with solder I had almost no clearance in the radius of the combustion chamber above the valves.. I used Chevrolet stainless steel exhaust valves for the intake and exhaust. The top surface stands proud of the top of the block when closed. I found that I had to re-cut the radius on the outside of the head to allow the valve enough clearance not to hit the head. I used a 3/8" round burr to tighten the radius only on the outside of the combustion chamber above the valves.

Edited February 10, 2011 by james curl
addition

[austinsailor]

Thanks. That is good info. Sounds like I'm not out of line at .100, but I need to check when done.

If you have it on the road now, do you see any difference?

[james curl]

I have had it on the road since 2007, it has about 20,000 miles on it now. I did dual exhaust and carburetors, +.040 over bore and a mild cam regrind. I think it runs very good for a 218, much stronger than the old 75,000 mile original it replaced. I posted a long post about my road trip to Cal and on to Bonneville salt flats for speed week in 2007. Just search for threads on Bonneville and you should turn it up.

[Don Coatney]

I have had it on the road since 2007, it has about 20,000 miles on it now. I did dual exhaust and carburetors, +.040 over bore and a mild cam regrind. I think it runs very good for a 218, much stronger than the old 75,000 mile original it replaced. I posted a long post about my road trip to Cal and on to Bonneville salt flats for speed week in 2007. Just search for threads on Bonneville and you should turn it up.

When I paid James a visit last summer he allowed me the pleasure of some wheel time in his car. I must say it runs and drives great.

[moose]

Taking off .010 of the block and .1 of the head makes 5.384, into 42.74 = 7.938.

Am I missing something?

42.74/5.384 = 7.938 This is the ratio of cylinder volume to head volume.

To figure the compression ratio, measure the total volume when the piston is at the bottom of the cylinder(cylinder plus head), then divide by the volume in the head at the top of the compression stroke.

(42.74+5.384)/5.384 = 8.94

[greg g]

so in other words close to 9 to 1...

[austinsailor]

So I was figuring it wrong. I thought it was the ratio of the displacement part to the compression chamber. But it's really total volume before and after compression?

This means I'm getting a pretty good compression ratio, I think this is a good thing.

I'm glad I asked.

[moose]

Maybe you should do some more measuring. The 251 you're using probably started with 7 to 1 CR.

From this we can calculate the cylinder head volume(with gasket) X

251/6=41.83 ci. per cyl

7/1 = (41.83 + X)/X

X = 6.97

The head's open surface area might be 1.5 times the cylinder area

Cylinder area = 9.275 sqin. C

head area = 13.9 sqin. H

Then if you take 0.1 in from the head, and .01 from the block you would be taking 1.39 ci. and .093 from X or,

X - 0.1*H -0.01*C = 5.48

So CR = (42.74 + 5.48)/5.48 = 8.79

These numbers are coming from the stock numbers (and just an exercise for my math history) but it seems like this head of yours may have been milled before, and maybe you should be careful taking another tenth off. I say go .060 or .080 and save some metal for the next time! It's still going to be surprising how well it will run:eek:

Edited February 10, 2011 by moose___
decimal placement

[James_Douglas]

My milling chart shows that depending on the original CR of the engine and head that if you take about 0.070 off of a STOCK head you will end up in the 7.5 to 7.75 CR range.

I CCed a NOS head and it was 110 CC with a thickness of 1.930.

I calculate that you get a 7CC reduction in head volume for every 0.024 you take off or One CC for every 0.0034 you mill off.

Felpro gaskets run about 17-19 CC's in volume.

My deck was at zero on this engine and I took about 0.070 to 0.090 off of the head as I remember.

My final hot running compression ratio's are in the 145 range.

This suggests a final CR in the 8.0 to 8.5 range. Pulling the 4860 pound 9 passenger suburban up Waldo Grade heading to the Golden Gate Bridge I do not get any detonation with stock timing specifications.

Here is an old post on this topic.

http://www430.pair.com/p15d24/mopar_forum/showthread.php?p=4452&highlight=mill+head#post4452

Hope this helps, James

Edited February 11, 2011 by James_Douglas

[Don Coatney]

James;

Good to see you posting here again. You should fill everyone here in on your mothers Desoto.

[randygall]

with this set up, do you still run the stock spark plugs?

[Tony WestOZ]

You need to measure your head volume before anything is taken off it.

I did three heads awhile ago and got three different results.

All heads I assumed were stock but even the factory changed compression over the years so it’s hard to know what is standard.

Also, because an engine might be a latter model don`t assume it will have higher compression. The nos engine I have has a 6.2 comp and I believe that is a mid 60`s flat head.

results I have,

Year, Head thickness, head volume.

53, 1.925", 6.065 cu"

58, 1.885", 5.639 cu"

nos, 1.966", 6.896 cu"

End result if I use the nos block which has 0.010" of the block (standard) with the head from the 58 (looks to be about 0.080" off from the nos head) I will end up with about 8.124:1 compression.

Another area that is sometimes forgotten in doing these calculations is how far down the bore the pistons sit. On the 251`s I have done its 0.017" is normal on a 50`s standard block.

[MarkAubuchon]

Talk with George Asche, I believe he has gone too far before on milling and now has a nice "tapping noise" Remeber the valve springs, big cam, higher RPMS, might result in a little float and hit.

[James_Douglas]

Some may find the attched milling charts helpfull...James