Checking Head Surface for Flat?

[Terry, NJ]

Mike! My head is swimming! I can't figure out if you impressed me with your intelligence or dazzled me with BS LOL. I went to school in the late fifties, early sixties and there was no NC back then. In the mid seventies, I was running a P&W Jig borer that was NC and I had to learn something of the X-Y coordinates to set up the work on the machine. I didn't like it. I went back to figuring die clearances and related stuff and then on to model making later. Hence the trade has moved on. A friend showed a turning center at work one day and I was amazed at what in could do in seconds. I know I'm a dinosaur with this stuff. But I think I'm correct in this ,at least one could check for warpage by putting it on four Jo Blocks or by rocking it on a do all surface plate. About flatness off a machine, The Blanchards I have run probably wouldn't give better than .002 flatness anyway, unless you let it spark off for a while. Wonderful machine though!
Terry

Originally Posted by MikeK

Terry, In most cases diagonal checks of an untrue plane will reveal either a convex or concave form. But not always!

I posted the mathematically generated plane shown because that plane WILL show a true flat line across opposite corners. Yes, it is a special case, not all twisted planes exhibit that characteristic. You can generate it by starting with two diagonal lines defined by X&Y co-ords bonded at their midpoint in a 3D CAD program at Z = 0. Then assign a random Z variance to only three of the four end points. The fourth end will generate it's own Z if the lines remain bonded at some point. Then connect the four corners with straight lines. Voila! A warped plane that is straight across every edge and both diagonals.

With the plane shown, if you pick one corner and a point 1/4, 1/2, or 3/4 along the opposite side then yes, you will cross a slight convex, but that point would be near impossible to detect because it would cross the cylinder or chamber hollows with nothing to 'feel' against.

You will easily get the plane shown if you take a head and support it on blocks by the four corner stud holes with a few thousanths 'wobble' across those corners and then bolt it down through #1 & #4 plug holes for surfacing.

I won't mention any type or names, but some new heads that check flat with a straight edge do not pass muster (to me) on a surface plate. A new head should not be 'off' 0.002".

Years ago, when I used to build engines for short track racing we checked head/block matings with plastigauge at perimeter points and between cylinders. 0.003 steel shim-stock was placed in opposite corners and just the four corner studs were torqued to 25 ft-lbs. By mid season both blocks and heads were warped, but if they had the same mating warp with not more than 0.002 variance they would hold a head gasket just fine to run again another weekend.