How flat is flat? - Alfa Romeo Bulletin Board & Forums
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post #1 of 18 (permalink) Old 07-07-2019, 01:10 PM Thread Starter
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How flat is flat?

A fascinating, informative video describing state of the art in this field. Best viewed at 1.5 speed.

No [email protected]#$ comments about Alfas, please .

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post #2 of 18 (permalink) Old 07-08-2019, 07:11 AM
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Excellent video. I can bring the topic into cars and Alfas if you will allow me.

When we send our heads out for resurfacing, I have always wondered exactly what a machine shop was doing with it. I had always imagined a large machine like the one in the video they called a fly-cutter. This appears to be pretty precise.

However, I was also searching some things a couple months ago and came across a few videos where a guy shows how to resurface an aluminum cylinder head with sand paper!! He lays out the sandpaper on the top of a flat surface, like a large pane of glass and then proceeds to drag the head across it with his hands. He periodically turns it over to examine it and shows the high spots getting cut down where as the low spots are still dirty. He keeps going this way until it is all fresh aluminum. Then he switches to finer sand paper to make a better surface. He also periodically uses a flat edge and feeler gauges to check flatness, just like in your video. He explains that the final surface finish depends on the type of gasket being used. Older type engines using a thicker softer gasket don't need as fine a finish as newer engines with thinner gaskets. It all seems logical and in those late Japanese aluminum heads, he has no problem reassembling and running the engine.

So my question is, would anyone do that to our aluminum heads?? Is a complex machine doing a better job than this?

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post #3 of 18 (permalink) Old 07-08-2019, 07:50 AM
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Quote:
Originally Posted by iachella View Post
Excellent video. I can bring the topic into cars and Alfas if you will allow me.

When we send our heads out for resurfacing, I have always wondered exactly what a machine shop was doing with it. I had always imagined a large machine like the one in the video they called a fly-cutter. This appears to be pretty precise.

However, I was also searching some things a couple months ago and came across a few videos where a guy shows how to resurface an aluminum cylinder head with sand paper!! He lays out the sandpaper on the top of a flat surface, like a large pane of glass and then proceeds to drag the head across it with his hands. He periodically turns it over to examine it and shows the high spots getting cut down where as the low spots are still dirty. He keeps going this way until it is all fresh aluminum. Then he switches to finer sand paper to make a better surface. He also periodically uses a flat edge and feeler gauges to check flatness, just like in your video. He explains that the final surface finish depends on the type of gasket being used. Older type engines using a thicker softer gasket don't need as fine a finish as newer engines with thinner gaskets. It all seems logical and in those late Japanese aluminum heads, he has no problem reassembling and running the engine.

So my question is, would anyone do that to our aluminum heads?? Is a complex machine doing a better job than this?
I could see doing that with a motorcycle head (preferably single cylinder), but an Alfa head is 40+ pounds, and I think the sandpaper would rip. Also, finding a piece of sandpaper big enough would be challenging.

Mike
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post #4 of 18 (permalink) Old 07-08-2019, 08:25 AM
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Quote:
an Alfa head is 40+ pounds, and I think the sandpaper would rip
They were doing on a 4 cylinder head just like ours. No ripping. Here are a couple videos.


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post #5 of 18 (permalink) Old 07-08-2019, 08:30 AM
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They were doing on a 4 cylinder head just like ours. No ripping. Here are a couple videos.

https://www.youtube.com/watch?v=HyKN52HD6RU
https://www.youtube.com/watch?v=UDPsEwXXbMg
Interesting. Wonder where they got the sandpaper?

Mike
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post #6 of 18 (permalink) Old 07-08-2019, 10:17 AM
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Unless you have the arm stamina and some mechanical ability, just take it to a machine shop. I have done this with carbs on a granite slab that was certified less than a .001 out and it is a long long process. Just my opinion
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post #7 of 18 (permalink) Old 07-08-2019, 11:33 AM
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Really interesting. I've wondered about this - what is "flat" and how do you really measure it? At some point both the surfacing elements or grinders, and the high tech measuring probes depend on the accuracy of the machining and mechanism of the rails and carriers that move them back and forth. Is the bed that holds the object down completely parallel to the overhead mechanism that carries the grinding wheel? You get into that loop where the tools that produce the tools that do the measuring have to be more accurate....but what measures them?

I got 3/4 through the video wondering if mechanical means of measuring aren't enough - can't they use light?...and BOOM, there's the guy talking about light wavelengths! Pretty cool stuff. At the end they imply that maybe at the molecular level, you could go along with a very tiny hammer and pound those pesky atoms down!


Funny I was just looking at this at a German tool supplier last night.....

https://www.fine-tools.com/straightedge.html

And a granite reference plate....

"This granite reference plate is used to measure objects, mostly in metal work, for flatness and squareness. It is flat to within 3 m (1 m = 0.001 mm) measured from a corner across the surface."
https://www.fine-tools.com/granit-messplatte.html


<EDIT> ...
Just thought of another frame of reference - concrete slabs are nowhere near "flat." They shrink, they cup, they curl, they settle and crack. But I recall having to deal with a new high school wood basketball court slab, and I think the accuracy required was an 1/8" in 10', so that the wood sleepers didn't need shims. They had a hard time getting there even with grinding.

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post #8 of 18 (permalink) Old 09-19-2019, 06:13 AM
Richard Jemison
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Surface finish

It aint just flat that`s important. The surface should be glass smooth as the head and block are not the same temps, and the head is hotter and expands .

The head surfacer I use leaves a mirror finish.

Richard Jemison
RJR Racing

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post #9 of 18 (permalink) Old 09-20-2019, 08:22 AM
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There are two sides to a cylinder head . If it is warped on the bottom ..it is warped on top where the cams are. You draw your own conclusions.
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post #10 of 18 (permalink) Old 09-20-2019, 10:08 AM
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What would you do to the top of the head when you resurface the bottom? I have to admit I have never heard of a procedure that is done to the top when a head is resurfaced.

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post #11 of 18 (permalink) Old 09-20-2019, 12:10 PM
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There are many crazy videos on YouTube, and I put the ones where they attempt to save $50's by sanding their head surface to be very much in the crazy stupid category. Heck the head surface probably did not even need any work.

YouTube videos should all come with a warning: "It is possible an idiot made this video, but you are a bigger idiot following it"
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post #12 of 18 (permalink) Old 09-20-2019, 01:38 PM
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Quote:
Originally Posted by PSk View Post
There are many crazy videos on YouTube, and I put the ones where they attempt to save $50's by sanding their head surface to be very much in the crazy stupid category. Heck the head surface probably did not even need any work.

YouTube videos should all come with a warning: "It is possible an idiot made this video, but you are a bigger idiot following it"
Pete
Only caveat not considered. .. If you are selling the car this might be a good idea ( youtube) .. It won't fail for the first 50 miles.
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post #13 of 18 (permalink) Old 09-20-2019, 02:40 PM
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OK, fine. I did not know the cost of bringing it to a machinist, nor would I attempt this without an expert saying that it would probably be OK.

But please elaborate on the cam issue. Is this always an issue? Does the machinist take this into account? What would that procedure be?

Thanks,

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post #14 of 18 (permalink) Old 09-20-2019, 03:08 PM
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Datsun L series engines had cam towers, the design was similar to MBZ. When the head was milled it often brought the cam level too low for valve adjustment..Parts stores stocked 5/10/and 15 thou shims to put below the cam towers to get things bank in order.
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post #15 of 18 (permalink) Old 09-20-2019, 03:17 PM
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OK, great. How about Alfa heads? There are a lot of rebuilds being done and heads being resurfaced. I haven't heard of cam issues. Is it rare? What do people do?

Stefano
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