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Old 04-14-2019, 09:14 PM   #1
pearson222
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Default Heads below spec height after resurfacing?

I recently picked up a set of rebuilt heads. I dropped them off at a local machine shop to be looked over. I was called back and told that they were resurfaced on a "head resurfacer" which is a large belt sander and the surface is not flat. The machinist said in order to correct the surfaces they would have to mill a decent amount off and would result in the heads being below spec height.

Has anyone heard of this type of a problem or know anything about spec height of heads and whether they can be run below spec height or not? I really do not want to have to scrap the castings and spend a ton of time and money of redoing all the work.
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Old 04-15-2019, 08:21 AM   #2
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The "head resurfacer" they refer to is 2nd worst tool in the world for surfacing heads. A scotchbrite pad on an air tool being #1. The shops that attempt to do heads on these machines are hacks and should be out of business. Anytime anyone needs a head or block surfaced, ask the shop if their machine uses a PCD insert. If they don't know what that means, try the next shop.
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Old 04-15-2019, 10:06 AM   #3
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^totally agree the 'belt sander' is a hack job tool for doing that.
But I don't really see how anybody needs a pcd insert to mill aluminum, and the insert has nothing to do with the quality of the machine they use.

As for this guys question, if they are 'below' spec it just means the combustion chambers are now going to be significantly smaller and the valves are brought down closer now.
It certainly doesn't mean you can't run them, you would just have to check out those details to make sure you don't have an issue with compression ratio or valve interference.

Worst case use a thicker gasket to step the heads back up.

Plus, ask him what a 'decent' amount is?
Who knows what he means and also ask how much below spec?
What if he says .001" below spec?
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Old 04-15-2019, 11:02 AM   #4
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Quote:
Originally Posted by Motion Machine View Post
The "head resurfacer" they refer to is 2nd worst tool in the world for surfacing heads. A scotchbrite pad on an air tool being #1. The shops that attempt to do heads on these machines are hacks and should be out of business. Anytime anyone needs a head or block surfaced, ask the shop if their machine uses a PCD insert. If they don't know what that means, try the next shop.
THIS!!! A whiz wheel on a grinder has cost my customers .005" in head material due to carelessness.

If below spec you'll need to run a thicker head gasket. Tomei makes some, JE makes some, IAG has some now. You have some options here so not all hope is lost.
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Old 04-15-2019, 12:13 PM   #5
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One doesn't "need" a PCD to resurface aluminum, but it gives a superior finish by far over anything else, including CBN. If a shop has a PCD equipped surfacer (it will also be CBN), then that tells you that he has a more modern machine and cares about the surface finish. So many shops have old carbide surfacing equipment and are too cheap to pay $30K for a new machine so they keep putting out rough surfaces on aluminum. Simply asking if they have PCD will tell you a lot.
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Old 04-15-2019, 02:14 PM   #6
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Quote:
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If a shop has a PCD equipped surfacer (it will also be CBN), then that tells you that he has a more modern machine and cares about the surface finish.
still not following you.
We have some cnc's here that are over 30 years old and we regularly run CBN inserts as well as PCD tipped inserts or ceramics as needed for super hard nickel castings or hard turning tool steels post heat treating.

So for one, I don't get at all the reference to a more modern machine having a 'capability' to use PCD. It's just an insert, put it in anything you want.
If the spindle is junk that's one thing, but a super duper insert won't help in that case anyway.

And since when does aluminum need an ultra hard insert like PCD or CBN? Frankly it seems like a waste of expensive inserts since aluminum machines like butter.
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Old 04-15-2019, 03:31 PM   #7
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"automotive" machine shop and a general machine shop like you are referencing are 2 completely different things who have completely different machines, other than maybe a manual lathe and a knee mill. In the automotive world, shops that started in the 50's and 60's all had either a grinder or a slow multi-tooth carbide machine. Aluminum was rare then. A lot of those shops are still in business and have not upgraded to CBN/PCD, which hit the market in the 90's. So they still try to cut aluminum with those old machines. I see it all the time. Here's a pic of a head that came in a couple of weeks ago. The shop told the owner that his head is junk, that's why it's so rough. No, it's your POS machine. When I surface this, it'll get a 12-15 Ra easily with my PCD. With a CBN it would be a 20. This is why I tell guys to look for a shop with a PCD machine, it eliminates the possibility of them having an old whipped out carbide or grinding machine, neither of which have been made for this market for many years.
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File Type: jpg Daimler Ra finish 001 (Small).jpg (44.3 KB, 34 views)
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Old 04-15-2019, 04:02 PM   #8
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The shop told the owner that his head is junk, that's why it's so rough. No, it's your POS machine. When I surface this, it'll get a 12-15 Ra easily with my PCD. With a CBN it would be a 20.
well a 400 something finish is ridiculous
I could pick inserts out of the scrap carbide can and do it on a knee mill with a worn out spindle and get a better finish than that.

I don't know man, we don't have specialized automotive equipment but plenty of cnc's and countless jobs with surface finish requirements down to low double digits or even single digits.
I fully believe we could surface a head with nothing more than a shell mill with plain jane carbide inserts and get an Ra in the 30's no problem as long as it was on a machine with a good spindle and a decent setup.
And at some point a better finish simply does nothing.
Insert design/shape is one thing but I see zero need for something super hard like PCD.
I don't doubt the tip will last forever on a good setup but still.
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Old 04-15-2019, 05:11 PM   #9
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"I could pick inserts out of the scrap carbide can and do it on a knee mill with a worn out spindle and get a better finish than that."

You're proving my point. You can adjust the speeds and feeds as needed on a Bridgeport to get a good (maybe) finish. But on a head that's seen too much heat, you'll never get a good finish with carbide. On purpose built automotive resurfacers of that era, most are 1 feed rate and a fixed spindle rpm of about 80. No chip breaker, no radius, no rake on the inserts. Compare that to the 10" PCD/CBN milling head on my VM20 that spins at 2500 with a 35ipm feed rate. Back then, they did not foresee the influx of aluminum. Some shops will change pulleys to increase the rpm but still it's not enough for MLS gaskets and most engines today use MLS. Some shops buy those evil belt surfacers to get by but it's a crime to use them on any engine part except an exhaust manifold.
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Old 04-16-2019, 06:17 AM   #10
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Quote:
Originally Posted by Motion Machine View Post
You're proving my point. You can adjust the speeds and feeds as needed on a Bridgeport to get a good (maybe) finish. But on a head that's seen too much heat, you'll never get a good finish with carbide. On purpose built automotive resurfacers of that era, most are 1 feed rate and a fixed spindle rpm of about 80. No chip breaker, no radius, no rake on the inserts.
No, your point is that you need basically the hardest synthetic material on earth to machine aluminum, which is downright false.

The underlined part is why.
80 rpm's (almost shy by a factor of 100 for aluminum, at least 50)
1 feed rate (I don't even know where to begin there)
No chip breaker (hmmmm, ok, so you are talking about archaic inserts now)
No radius (again, what????? So inserts from 1964?)
No rake (and again, inserts from 1965)

So all of those reasons are why it doesn't work when every aspect of a setup is wrong, none have to do with PCD and nothing I said makes your point that you need PCD to machine aluminum.
I see it daily achieving glass smooth finishes with regular carbide or even HSS is more than capable of Ra's in low double digits. Below 40 or so does it really make a difference (NO)
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Old 04-16-2019, 08:28 AM   #11
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I'm sure my customers are happy that I am using the best available, yet completely unnecessary, tooling to give them a superior finish to seal their motor. Given a choice, I'd bet they'd choose the experience of someone who is actually in the engine machining business over someone who is not. I'm sure you'll want the last word so take it away, I recognize futility when I see it.
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Old 04-16-2019, 09:02 AM   #12
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I spoke to the machine shop and he gave me an overview of what's going on-

He measured the overall cylinder head height and it is uneven on both heads. Some spots are low and some spots are high. Since the surface is so uneven he would have to register the heads using the head stud holes and deck the surface at 90 degrees of the holes. He doesnt know exactly how much below min spec (127.2mm) they would come out as they are both different.

I have two options:

1.) Do the above and then spec a cometic head gasket to make up the difference in lost material.

2.) Transplant the internals into my stock heads which have plenty of height left to skim correctly.

Are their downsides to using cometic head gaskets? Longevity? I am mostly concerned about piston to valve clearance. I feel like most people never take the time to check this and just bolt the heads on. Any suggestions on which route to go?
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Old 04-16-2019, 09:25 AM   #13
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The top of a Subaru head is exactly square to the bottom. I've checked this numerous times on a granite surface plate and a height gage. The shop only needs to mount the head inverted between parallels. All the shops I know surface Subaru heads this way or any other head that has a machined valve cover surface that is square to the deck. Your shop is turning the simplest surface job into something it isn't. Is this an automotive you're dealing with?
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