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Old 08-30-07, 01:39 PM   #1
phantomcow2
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Tolerances of a cylinder head?

I am referring to the machined portion of the top of the cylinder head. What are the tolerances?
Is the surface finish just a milled surface? Ground? Ground and sanded? Sanded and lapped with polishing compound? Or do they rely on a gasket material to fill in the micro scratches from milling.

And the most critical part of this question, what are the tolerances for flatness of the surface? Is it loose as a goose like .005"? .002"? Or are we talking in the sub .001" range? If one had a cylinder head out on a bench and was checking for flatness, where would they want to measure from?
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Old 08-30-07, 01:40 PM   #2
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The cylinder heads in my car are very tolerant of me. They've never once called me a bad name.
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Old 08-30-07, 01:52 PM   #3
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Tolerance is more dependant on head gasket material and thickness, as well as valve and piston clearance. When using a thick, compliant gasket a couple thousandths of an inch is probably OK, but if you are using gaskets with more metal you'd want it milled as flat as possible. For uniform combustion chamber volume, I've always zero decked blocks where possible, and getting a smooth, even surface on the heads prevents air/fuel from leaking into that space.
Bottom line, I've always had heads plate honed or milled to as tight a tolerance as I could get them on every build. Take your head(s) to a machine shop if you are concerned.

Micro scratches/swirls are are ok, but anything else would need to be taken out.
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Old 08-30-07, 01:55 PM   #4
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My understanding is that the tolerance is a 'decent' on a scale of 1-10
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Old 08-30-07, 01:59 PM   #5
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I would guess that there are no typical answers for any of this. Before the 70's, tolerances may have been implied by manufacturing and surface prep techniques, but this practice is frowned upon because it specifies a process. What an engineer really wants to specify is the resultant material, not what steps it takes to achieve it. Nowadays, Geometric dimensioning and tolerancing are used (per ANSI Y14M, or other industry standard).
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Old 08-30-07, 02:11 PM   #6
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Why is something referred to as "loose as a goose"? Have not been in a single machine shop that had a goose in it. Not one I tell you, and I have been in more than a few machine shops.
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Old 08-30-07, 02:25 PM   #7
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Why is something referred to as "loose as a goose"? Have not been in a single machine shop that had a goose in it. Not one I tell you, and I have been in more than a few machine shops.
Gots nuthin' to do with a machine shop and everything to do with (1) the rhyme and (2) the speed with which a goose turns input (food) into output (yuch), thereby leading W.C. Fields to once bellow at the feathered friends befouling his lawn: "If you've got to sh*t, sh*t green."
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Old 08-30-07, 02:29 PM   #8
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Gots nuthin' to do with a machine shop and everything to do with (1) the rhyme and (2) the speed with which a goose turns input (food) into output (yuch), thereby leading W.C. Fields to once bellow at the feathered friends befouling his lawn: "If you've got to sh*t, sh*t green."
ah, okay, makes sense now. Thanks. And as a your welcome here is another goose strory.

Your story brought to mind a story a friend from west Texas told me. They had geese on their farm. As a boy, he would get a long piece of string and a piece of raw bacon. Tie bacon to string, feed to goose. Bacon on strings passes through goose, next goose eats it, and so on and so on.

Soon he had all the geese on one string!
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Old 08-30-07, 02:29 PM   #9
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Well, I am the machine shop in this case. I ask about tolerances because the tighter the tolerances, the longer it takes for me to mentally prepare for it. I'm considering a little modification, but first I'll start with a lawn mower.
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Old 08-30-07, 02:33 PM   #10
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I am referring to the machined portion of the top of the cylinder head. What are the tolerances?
Is the surface finish just a milled surface? Ground? Ground and sanded? Sanded and lapped with polishing compound? Or do they rely on a gasket material to fill in the micro scratches from milling.

And the most critical part of this question, what are the tolerances for flatness of the surface? Is it loose as a goose like .005"? .002"? Or are we talking in the sub .001" range? If one had a cylinder head out on a bench and was checking for flatness, where would they want to measure from?
Depends on the manufacturer. If I remember correctly Rolls Royce uses a CNC grinder to grind their heads to be very flat. So flat I don't they use head gaskets. My motorcycle on the other hand, just machined. I have no idea what the tolerance for flatness on my motorcycle heads are. Again, each manufacturer's specs are probably a little different.
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Old 08-30-07, 02:39 PM   #11
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Wouldn't the head gasket surface be the BOTTOM of the head(when bolted to the block)?

Well, unless it's a Porsche, VW bug, or Subaru.
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Old 08-30-07, 02:45 PM   #12
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A CNC surface grinder with an aluminum wheel would do that, yep. My only concern about relying on that surface to be so flat it you don't need a gasket, is that because it's aluminum it could easily be damaged by even the slightest thing. Then your surface isn't perfectly flat.
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Old 08-30-07, 03:25 PM   #13
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Old 08-31-07, 11:25 AM   #14
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The head tolerance also has a lot to do with the deck surface as well. The manuals I've seen typically goes to 0.004" on each, but if the mating surfaces are warped in opposite directions, it won't seal correctly. And the gasket does make a difference. Thicker fibre gaskets tolerate more warpage. But I prefer the all-metal Cometic gaskets as they handle higher boost-pressures and are more tolerant of tuning mistakes.
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Old 08-31-07, 11:43 AM   #15
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It is a good idea, if you remove the head, to get it machined and cleaned with new valves, and guides. If you are just replacing the gasket, you may not have to mess with the head if you didn't have problems before. The only thing to remember is to make sure the gasket is the same thickness as the old one. Now if you want to go thinner you may have a problem.
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Old 08-31-07, 11:45 AM   #16
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The head tolerance also has a lot to do with the deck surface as well. The manuals I've seen typically goes to 0.004" on each, but if the mating surfaces are warped in opposite directions, it won't seal correctly. And the gasket does make a difference. Thicker fibre gaskets tolerate more warpage. But I prefer the all-metal Cometic gaskets as they handle higher boost-pressures and are more tolerant of tuning mistakes.
^^^^
what he said. of course, a lot has to do on what kind of cylinder head it is and which end you're talking about - i may have missed it, but you didn't really fill us in on much. block side or valve cover side or what? if it's an older valve cover side, i wouldn't go crazy over t, but on the flip side, with modern engines, you can't get em too flat.

as far as practicing on something cheap... thats a great idea. maybe jsharr will let you borrow his car to test it on? :-)
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Old 08-31-07, 01:11 PM   #17
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Well, my plan was to mill little grooves into the combustion chamber of my cylinder head, and then mill off a little bit from the top to increase compression. Hopefully I can get to 10:1 this way.
I was going to go to a scrap yard or something and find a replacement cylinder head with 1/2 the miles and stick that in.
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Old 08-31-07, 01:58 PM   #18
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Why mill little grooves? Any edge or ridge will retain heat and be a source for detonation & pre-ignition. Honda 4v heads are some of the best out there already. Extremely hard to improve on them.

Going to higher-compression typically requires going to a head-design with narrower included-angle between the valves to keep the combustion-chamber shape as spherical as possible. Otherwise you end up with a too thin and crescent-shaped one that has slow flame-front propagation speed.

If you look at the top of the piston, you'll see that it has a tent-shaped bump sticking up? This bump to reduce combustion-chamber volume and increase compression lengthens the distance from the spark-plug to the edge of the cylinder and slows down the time between ignition to full-combustion. In order to combat that and retain high-compression requires having the angle between the valves to be more parallel so that they are flatter with each other. This then lets you move them closer to the piston for high compression AND reduce the height of the bump on the piston. This then shortens the distance the flame has to travel and you get full-combustion quicker. This is the idea behind a "hemi" head.

Also AR of head-surface is typically 80 or smoother. The compression-ring in the gasket provides the tight metal-on-metal seal against combustion pressures. Using some Coppercoat spray on the gasket helps seal the outer edges from seeping coolant if the AR is rough.

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Old 08-31-07, 04:07 PM   #19
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Listen to DannoXYZ. Don't mill any grooves into the head. You will create hot spots and may cause pre ignition or detonation.

If you want a cheap way to raise compression go with a smaller head gasket. Dont worry about machining the head or block.
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Old 08-31-07, 04:24 PM   #20
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Ideally, tolerances will be 0 from spec.....practically speaking, though, not possible. The more precise the tolerances though, the better the outcome.

Are you wanting to shave the heads to get more compression?
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Old 08-31-07, 04:25 PM   #21
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Ideally, tolerances will be 0 from spec.....practically speaking, though, not possible. The more precise the tolerances though, the better the outcome.

Are you wanting to shave the heads to get more compression?
I just saw your post above: Consider domed pistons for a compression increase.
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Old 09-01-07, 02:53 AM   #22
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the tighter the tolerances, the longer it takes for me to mentally prepare for it.
It's all about music and lighting. What?
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Old 09-01-07, 07:18 AM   #23
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It's all about music and lighting. What?
I don't know. I have to be focused and in the mood to stay focused for a while when I have something that calls for tolerances of under .0005", or when it's something that I am modifying and can't easily replace.

As far as the grooves, happy reading:
http://somender-singh.com/content/view/7/31/

I've done tons of research, and this has been the topic of discussion on many forums. Everybody who has tried this has reported positive improvement.
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Old 09-01-07, 07:47 AM   #24
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Good gracious, Phantom! Are you building a car from scratch??
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Old 09-01-07, 10:12 AM   #25
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10:1 compression, where are you going to buy gas for that, at a Cool Blue 106 by the drum?
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