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Old 09-10-2020, 02:02 PM   #1
Mart
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Default Head skimming: Any tips?

I just did a test and I can just about get a head on my small milling machine. no overlap at either end or side but I can just about cover the surface.

I tried with parallels under the ends and middle (Under the head nut bosses) and bolts down through the end spark plug holes.

I was hardly touching the ends but taking quite a bit off the middle.

So a couple of questions.

Are the head nut bosses a reliable surface to mount the head on?

Is it better to mount the head face down and level the nut bosses first? Too late on my test head.

Is it better to use just 3 mounting points so the head isn't pulled into a bind?

Any hints and tips gratefully received.

Mart.
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Old 09-10-2020, 02:24 PM   #2
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Default Re: Head skimming: Any tips?

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Originally Posted by Mart View Post
I just did a test and I can just about get a head on my small milling machine. no overlap at either end or side but I can just about cover the surface.

I tried with parallels under the ends and middle (Under the head nut bosses) and bolts down through the end spark plug holes.

I was hardly touching the ends but taking quite a bit off the middle.

So a couple of questions.

Are the head nut bosses a reliable surface to mount the head on?

Is it better to mount the head face down and level the nut bosses first? Too late on my test head.

Is it better to use just 3 mounting points so the head isn't pulled into a bind?

Any hints and tips gratefully received.

Mart.
Mart, When I used to do these, I'd mount the head on a number of adjustable supports, usually six. The difficult (time consuming) issue was leveling the head as best I could.
I clamped it in many places (usually six) and would mill as much as possible around the clamps. Remove one clamp at a time - mill. Replace that clamp and move to the next.
The bosses are close but in my opinion, not nearly close enough to solely utilize.
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Old 09-10-2020, 03:44 PM   #3
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Default Re: Head skimming: Any tips?

Take the heads to a shop that has a surface grinder. Using a milling machine can quickly damage the surface. If you don't know whether the heads have been surfaced before, you can check valve head clearance by placing modeling clay in the valve area, then placing the head on the block, and turning the crankshaft. Then observe the clay to see if there is sufficient clearance.
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Old 09-10-2020, 03:46 PM   #4
Lawson Cox
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Default Re: Head skimming: Any tips?

Take it to a good reliable machine shop and 'splain what you want done. Couldn't be all that much trouble or money. Some things are better paid for than experimenting with yourself. My 2.
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Old 09-10-2020, 04:26 PM   #5
Lawrie
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Default Re: Head skimming: Any tips?

Mart ,can you spin it it that lathe of yours
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Old 09-10-2020, 04:27 PM   #6
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Default Re: Head skimming: Any tips?

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I agree with Lawson...
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Old 09-10-2020, 10:36 PM   #7
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Default Re: Head skimming: Any tips?

Every time the heads are surfaced the dome diameter gets smaller-easy to measure. Newc
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Old 09-10-2020, 10:48 PM   #8
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Default Re: Head skimming: Any tips?

When a local machine shop surfaced a pair of finned heads I had, they tapped the stud holes in the four corners. [Probably 1/2 unf?] Screwed in bolts and clamped the bolt heads. Having this setup allowed them to jack up/lower each end of head[s] to set up.
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Old 09-10-2020, 11:29 PM   #9
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Default Re: Head skimming: Any tips?

https://youtu.be/YPe4ddzlr-k
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Old 09-11-2020, 04:13 AM   #10
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Default Re: Head skimming: Any tips?

Thanks for all the advice, fellas. I did a test on an otherwise scrap head last night and it came out ok. I'm just playing about with this, with the aim of being able to fine tune the over the piston clearance somewhere in the future.

I was just pleased to find I could actually get one on there and physically cover the whole surface. I always assumed that my machine wasn't big enough.

Once I get it working as I would like I'll no doubt show it in a video.

Mart.
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Old 09-11-2020, 05:48 AM   #11
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Default Re: Head skimming: Any tips?

The head should be completely relaxed when being leveled/positioned on the milling machine. If the head is warped excessively it should be straightened or completely machined at the gasket face and in the chambers. Too, it must be supported so as not to deflect when cutting pressure is applied. I have a special fixture which locates the head at the 4 corner holes on adjustable, hollow, dowels and 4 more , adjustable, supports at the sparkplug faces. After the tedious job of finding the best position, with all four corners touching, bolts are used at the 4 corners to clamp. Then the supports at the sparkplugs are applied while observing a Test Indicator, resting on the gasket face, to avoid changing the position or flexing the head. Finally, clamping can be accomplished through the sparkplug holes with the rest pads supporting. You will find it impressive to see how easily the head can be flexed. When work is done in the chamber the supports at the sparkplugs are not clamped/bolted. When cutting at the gasket face the sparkplug clamps are used. Finding the best condition at which to set the head can take some patience and time. I use a Platten surfacing machine after milling cuts to achieve the finish I desire.


I found a picture which partially shows the fixture I made for head work on pages 58 and 62 of my book.
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Old 09-11-2020, 09:59 AM   #12
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Default Re: Head skimming: Any tips?

Mart, I found that on my Bridgeport 9x42, I can get a head on there but have to use all of the table with the Bridgeport head all the way to the right. So having done that I use all of the table. But my machine has wear in it and will cut things into a banana. Its because of wear. With the table all the way to one side the weight of the table is pulling that end down, then when in the middle it levels off and goes to the other end and it hangs from the weight again, therefor cutting the head into a banana high in the middle. Never found a way to get it out of it.
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Old 09-11-2020, 10:21 AM   #13
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Default Re: Head skimming: Any tips?

Quote:
Originally Posted by Lawson Cox View Post
Take it to a good reliable machine shop and 'splain what you want done. Couldn't be all that much trouble or money. Some things are better paid for than experimenting with yourself. My 2.

This is why we do stuff ourselves...
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Old 09-11-2020, 05:56 PM   #14
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Default Re: Head skimming: Any tips?

Anything can be bought.
Some folks enjoy being creative and learning.
I'm with Mart.
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Old 09-14-2020, 12:08 PM   #15
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Default Re: Head skimming: Any tips?

Flatrod has a good point. Knee mills are good for a lot of different milling jobs but the table can be a problem if things aren't in a good tight condition with it. Most machines that are designed to deck things with accuracy have full support underneath to make certain there can be no deflection under cutting load. A regular knee mill would be fine for small stuff like motorcycle heads but may not have the support it needs for the longer stuff.

If it works for you then that's great. A good surface cut should be very smooth. If a person has what I refer to as a grave stone or surface plate, it's easy to check the job. Lapping machines are another way but those things can get expensive. They can get the surface better than a gnat's ahrs by far.
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Old 09-14-2020, 02:37 PM   #16
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Default Re: Head skimming: Any tips?

Thanks for all the input guys. I did a test and it is ok-ish. I think as my mill is so lightly used it is ok re sag, but just holding the head is the difficult part. I'm working on something else now, but will revisit this subject in the future.

Mart.
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Old 09-14-2020, 04:52 PM   #17
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Default Re: Head skimming: Any tips?

Put your head with the chambers down on the mills table and underlay the banana with shims as best as you can before tightening down thus holding the warped head in its true warped stage on the table. Do a small fly cut on every bolthole surface, of course all to the same depth. When finished you have 24 points to lay the head on, all of them in the same plane and level. When head is mounted on these points and tightened down on the table you can skim the gasket surface and get a straight and flat head again.
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Old 09-14-2020, 05:43 PM   #18
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Smile Re: Head skimming: Any tips?

When I milled the heads using my bridgeport, I balanced the head with five 1/2 threaded rod on flat bar plates bolted to table. Using nuts and adjusting the head. The nuts were resting on bolt hole bosses. The head was secured by clamping down with 3/8 bolts through spark plug holes. The important thing is to indicate at the bottom of combustion chambers on each end of head lengthwise and your set to go.

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Old 09-15-2020, 08:14 AM   #19
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Default Re: Head skimming: Any tips?

Same here, I use 5 small jack stands. 4 at the outer corners and one in the middle. pull the head down with bolts through the spark plug holes. Adjustable or shim able as you need. Also made t slots in 3/8 to fit the slots in my table.
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Old 09-15-2020, 01:07 PM   #20
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Default Re: Head skimming: Any tips?

The machine shop that I use has a dedicated head mill. They thread the holes and attach it to a fixture that tilts to perfectly index the head. they told me that clamping the head down distorts the head and when removed it is not flat.

They do a lot of race engines so they are kind of out of my league and my home shop Bridgeport. I think Mart could do one and have acceptable accuracy.

John
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