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Old 09-22-2011, 09:55 PM   #1
allison-tech
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Question Milling a head for compression?

Can the stock head be milled for better compression? If so, how far can you go? If valve clearance is an issue, could the head be milled with a slight angle? I realize, of course, it would have to be a very slight angle along with slightly elongating the holes for the head-bolts to compensate.

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Old 09-22-2011, 10:59 PM   #2
Tom Wesenberg
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Default Re: Milling a head for compression?

You couldn't mill the stock head enough to raise the compression enough to matter. Best to just buy the high compression head. Milling at an angle would ruin the head.
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Old 09-22-2011, 11:32 PM   #3
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Default Re: Milling a head for compression?

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Originally Posted by allison-tech View Post
Can the stock head be milled for better compression? If so, how far can you go? If valve clearance is an issue, could the head be milled with a slight angle? I realize, of course, it would have to be a very slight angle along with slightly elongating the holes for the head-bolts to compensate.
Yes, you can mill enough to help quite a bit....
You may need to have flat top pistons.
I have taken .125 off many times...I never checked one to see what compression it ended up but the customer's said it really made a difference in performance.
Angle milling is common practice on racing heads.
Valve clearance is usually not a problem on a model A but should be clayed to check.
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Old 09-23-2011, 09:15 AM   #4
Tom Wesenberg
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Default Re: Milling a head for compression?

If you mill the head at an angle, how are the head studs going to fit the holes? And if you redrill the head stud holes at an angle to match, you still have the head nuts not seating squarely to the head, unless you also mill the top surface for the nuts. Looks like a lot of work for little gain. I'd just buy the new 5.5 head and be done with it. I don't think you could get near 5.5 by milling a stock head.
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Old 09-23-2011, 09:29 AM   #5
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Default Re: Milling a head for compression?

Thank-you both for the responses, I would like to get the most out of my engine w/o any bolt-on aftermarket access. I try to keep in mind that this car, as a whole assy. was built for low HP. My performance is achieved w/a correct tune & I would like to get as much as I can w/o needing to upgrade other components to handle the extra HP. I'd love to get 50 hp w/my original parts. I think I will take off .125 and see how it does.
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Old 09-23-2011, 09:34 AM   #6
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Default Re: Milling a head for compression?

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Tom, about how much hp is gained w/the 5.5 head?
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Old 09-23-2011, 10:12 AM   #7
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Default Re: Milling a head for compression?

At one time someone sold cast ,combustion chamber fillers,that you weled in,to raise compression. I saw them on E-bay one time. Also,low buck racers would just build up weld bead to fill the combustion chamber. I met a guy at a car show with a doodle bug tractor,that had a head, he filled with scrap iron welded in. He said he had been using the tractor since 1950.
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Old 09-23-2011, 10:28 AM   #8
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Default Re: Milling a head for compression?

The compression was kept low for three reasons,
1) The gas of the period was low octane.
2) Model A's were designed to be handcranked in an emergency
3) Bearing loading, Henry Ford did not want his cars showing up back at the agencies with burned out bearings. Few things would have ended Ford motor Co as that.

The gas today is not an issue. Ditto for the handcranking. Higher compresssion usually means shorter babbit life. However, It's been said many times the quickest and cheapest way to a more powerful model A is through a HC head. Next is a cam. Next, the sky's the limit. Lighten flywheel, bore it .125, bigger valves, counter balanced crank w/inserts, multible downdraft carburetors. better exhaust scavenging! Whatever you can think of and afford.
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Thank-you both for the responses, I would like to get the most out of my engine w/o any bolt-on aftermarket access. I try to keep in mind that this car, as a whole assy. was built for low HP. My performance is achieved w/a correct tune & I would like to get as much as I can w/o needing to upgrade other components to handle the extra HP. I'd love to get 50 hp w/my original parts. I think I will take off .125 and see how it does.
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Old 09-23-2011, 10:31 AM   #9
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Default Re: Milling a head for compression?

That sounds interesting, I think I would always be worried about an insert coming loose, but the weld-fill sounds safe w/some more knowledge. Again, I'll be pretty happy to get 50 hp, so I'm not wanting to go overboard in mod.'s
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Old 09-23-2011, 10:31 AM   #10
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Default Re: Milling a head for compression?

If you are trying to keep it "stock looking", the Snyder 5.5 head would look more stock than a modified original would. And a whole lot less trouble in the long run.

I'm a little confused about you wanting to keep it original, but at the same time modifying it
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Old 09-23-2011, 10:48 AM   #11
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Default Re: Milling a head for compression?

Here is some info that I copied off the old ford barn a few years back. The minimum thickness of the head after milling to the MAX shouldn't exceed 240 . Original thickness of the unmilled head was 394 . Measure the thickness of the casting from the underside in the edges of the water jacket holes near the distributor, near stud holes 4 & 5 . This measurement will let you know how much has been removed in the past and how much to remove for the desired compression ratio. Stock ratio was 4.22 If you remove .025 the ratio changes to 4.40 , remove .050 and it jumps to 4.59 , remove .075 and the ratio will be 4.81 , remove .0100 and the max ratio will end up at 5.04. The 5.04 is not that far from the 5.2 ratio of the original police head with the large letter B . The design of the original combustion chamber won't be as efficient as the heart shaped combustion chamber on the police head or the other high CR heads that are now available. You could try smoothing the rough cast in the combustion chambers and measureing the CC in each chamber and trying to equalize. This on the other hand will slightly increase chamber size and lower CR slightly to the point that you may have to mill a little more to get back to the 5.04 reading. This is not my recommendation but was considered to be on the safe side of how much to mill. I have seen heads milled to the point that the counter bore was completely removed. What you have to be careful of is the height that the piston rises above deck height. The stock engine had the pistons rising about .032 above deck height. You may need to calculate the thickness of the head gasket when torqued minus the amount that the pistons rise above deck height. You need to maintain a distance of at least .030-.035 between the top of the piston and the top of the combustion chamber over the piston when the head is torqued down.
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Old 09-23-2011, 10:48 AM   #12
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Default Re: Milling a head for compression?

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Originally Posted by allison-tech View Post
Thank-you both for the responses, I would like to get the most out of my engine w/o any bolt-on aftermarket access. I try to keep in mind that this car, as a whole assy. was built for low HP. My performance is achieved w/a correct tune & I would like to get as much as I can w/o needing to upgrade other components to handle the extra HP. I'd love to get 50 hp w/my original parts. I think I will take off .125 and see how it does.
It should be easy to find a head that has been milled .125 cheap. I have thrown them away in the past since in my opinion taking .125 off makes them too weak. Take a close look at every area that you are cutting the .125 from.

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Old 09-23-2011, 11:09 AM   #13
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Default Re: Milling a head for compression?

I guess I'm wanting to stay pretty much orig. & figured that taking a little (.125 it sounds) off the head wouldn't be noticed. I.E. I've read that if I put a B model carb on it's worth enough hp to bring the total to around 50 hp, but I'd like to get there w/what it came with. Kinda' the same principle as disassembling a new motor back in the 70's/80's and re-assembling w/no changes other than matching gaskets/ports, smoothing rough casting, ect. & ending up w/10-15 more hp. Hope that makes sense, I really appreciate you guys alot.
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Old 09-23-2011, 11:55 AM   #14
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Default Re: Milling a head for compression?

The model A was rated at 40 hp at 2200 rpm the model B engine was rated at around 50 hp at 2800 rpm The things that increased the hp in the B engine was the heart shaped combustion chamber. This is what gave the noticable seat of the pants feel. The cam allowed the engine to turn an additional 600 rpm but would not give this rpm increase without the higher cr head. The B carb and manifold was 1/8 larger and the carb had a high speed circuit. There was a small difference in the ports in the block of the model B engine. All of the above mentioned differences only added 10 horsepower. You could only expect maybe three horse power from the B carb if it was used with the B intake manifold on a well tuned engine. You probably wouldn't feel much difference in power with the B carb. Compression is what gives the seat of the pants feel. In other words it is torque that you will feel more than the horsepower that more carb or a different cam will add, They will make a big difference if all are used together.
I think that the dyno test that I read showed that the Snyder 5.5 head alone would add fourteen horsepower to the stock model A engine. This in itself is higher than the model B horsepower
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Old 09-23-2011, 12:35 PM   #15
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Default Re: Milling a head for compression?

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Originally Posted by Purdy Swoft View Post
The model A was rated at 40 hp at 2200 rpm the model B engine was rated at around 50 hp at 2800 rpm The things that increased the hp in the B engine was the heart shaped combustion chamber. This is what gave the noticable seat of the pants feel. The cam allowed the engine to turn an additional 600 rpm but would not give this rpm increase without the higher cr head. The B carb and manifold was 1/8 larger and the carb had a high speed circuit. There was a small difference in the ports in the block of the model B engine. All of the above mentioned differences only added 10 horsepower. You could only expect maybe three horse power from the B carb if it was used with the B intake manifold on a well tuned engine. You probably wouldn't feel much difference in power with the B carb. Compression is what gives the seat of the pants feel. In other words it is torque that you will feel more than the horsepower that more carb or a different cam will add, They will make a big difference if all are used together.
I think that the dyno test that I read showed that the Snyder 5.5 head alone would add fourteen horsepower to the stock model A engine. This in itself is higher than the model B horsepower
Purdy, are you sure the 'B' intake manifold wasn't 1.250" machined whereas the Model A maifold was 1.000" cast? And, doesn't the air flow go up on the square when the diameter increases? In other words, just because the size is 25% larger doesn't mean the airflow increases by 25% as it would be more.

Also, while I agree there was "only 10 horsepower" difference, I would think that a more realistic approach to this mindset would be the thought of the 'B' engine had 25% more power over the A, and the torque level was increased too.
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Old 09-23-2011, 12:37 PM   #16
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Default Re: Milling a head for compression?

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If you mill the head at an angle, how are the head studs going to fit the holes? And if you redrill the head stud holes at an angle to match, you still have the head nuts not seating squarely to the head, unless you also mill the top surface for the nuts. Looks like a lot of work for little gain. I'd just buy the new 5.5 head and be done with it. I don't think you could get near 5.5 by milling a stock head.
Yes you need to mill the holes and spot face them.
When you are straining for the last hp on a race engine, the extra effort is well worth it.
Milling and machining is not for the average back yard mechanic.
Like you say, much easier to buy a head.
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Old 09-23-2011, 12:52 PM   #17
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Smile Re: Milling a head for compression?

Quote:
Originally Posted by Purdy Swoft View Post
The model A was rated at 40 hp at 2200 rpm the model B engine was rated at around 50 hp at 2800 rpm The things that increased the hp in the B engine was the heart shaped combustion chamber. This is what gave the noticable seat of the pants feel. The cam allowed the engine to turn an additional 600 rpm but would not give this rpm increase without the higher cr head. The B carb and manifold was 1/8 larger and the carb had a high speed circuit. There was a small difference in the ports in the block of the model B engine. All of the above mentioned differences only added 10 horsepower. You could only expect maybe three horse power from the B carb if it was used with the B intake manifold on a well tuned engine. You probably wouldn't feel much difference in power with the B carb. Compression is what gives the seat of the pants feel. In other words it is torque that you will feel more than the horsepower that more carb or a different cam will add, They will make a big difference if all are used together.
I think that the dyno test that I read showed that the Snyder 5.5 head alone would add fourteen horsepower to the stock model A engine. This in itself is higher than the model B horsepower
From what I've learned today, it sounds like the HC head is the way to go. I wonder if it would be too time consuming to modify my OE combustion chambers along w/milling off .100 & get that "seat of the pants" feel? I guess I'm too curious sometimes, but usually just in instances where I can't find someone else who has done it before. I'd prefer to learn from others experience (which is why I'm so excited about this site). I really appreciate your correspondence, sincerely, Lewis K. Large
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Old 09-23-2011, 12:59 PM   #18
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Default Re: Milling a head for compression?

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Originally Posted by BRENT in 10-uh-C View Post
Purdy, are you sure the 'B' intake manifold wasn't 1.250" machined whereas the Model A maifold was 1.000" cast? And, doesn't the air flow go up on the square when the diameter increases? In other words, just because the size is 25% larger doesn't mean the airflow increases by 25% as it would be more.

Also, while I agree there was "only 10 horsepower" difference, I would think that a more realistic approach to this mindset would be the thought of the 'B' engine had 25% more power over the A, and the torque level was increased too.
There must be some confusion here . No offence but reread my post. I didn't go into specifics on the the exact differences in dimension of the two manifolds or their respective flow rate. I simply pointed out the fact that the Model B carburetor had a 1/8 inch larger throat and the manifold was larger and this was one of the things that increased the horsepower.

Whether you call the increase, ten horsepower or a 25% horsepower increase still means the same. The point of my post was that the addition of a model B carburetor will NOT give a 10 horsepower increase to the model A engine by any strech of the imagination.
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Old 09-23-2011, 01:02 PM   #19
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Default Re: Milling a head for compression?

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From what I've learned today, it sounds like the HC head is the way to go...
You won't be sorry, and if you opt for one of the Snyder heads, it takes a real close eye to tell it from the OEM Ford head (until you start driving it).
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Old 09-23-2011, 01:08 PM   #20
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Default Re: Milling a head for compression?

Knowing my nature( I really should have 9 lives ) I'll be doing some combustion work soon, but 1st, I really like the fact that I can start my eng. w/a crank. Can ya'll still start your HC eng's w/the crank? I also don't want to shorten the life of the babbit, Terry made some good points to ponder, along w/all the other posts.
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