A question for the machinists

[Canuck750]

I have a cylinder head that when laid on a sheet of glass appears to be slightly warped, the sealing surface to the cylinder is the inner recessed surface but a series of fiber and steel gaskets seal the flat surfaces of head to cylinder casting.





To get a flat surface on the body of the head would it make sense to clamp the head on a mill and run a fly cutter across the surface or would this comprimise the head to cylinder seal across the gasket surfaces? Wondering if I should just leave it or not.
I think the right thing is to deck the head surface and then cut the recessed circle to the same depth of cut as the decking, am I overthinking this?

[dxhall]

Don’t think much of fly cutters for head work.  If there’s any wear in your milling machine table, your head surface as cut with the fly cutter will reflect the wear in the mill table.  On a small head you’d probably be ok, but it’s not best practice.

[moto-uno]

  Unless we're talking major surface warpage, dragging the head across various grits of sandpaper
on a flat surface (drill press table) has worked well for me for years . It's not + or- a thou or two to make
a satisfactory surface for a gasket to seal .   Peter

[RinkRat II]

    Norton makes an open mesh lapping paper that works great on a piece of glass or granite that would do the job your looking to do. Stoddard solvent works well as a lubricant . I bought a piece of granite from a monument company (think gravestone) and it was within .0005 flat.

  My $.02
  Paul B

[normzone]

Randy Walls told me a tale of in his first youth having no money to plane his motorcycle head with, and running it back and forth across the garage floor to take some meat off, and then finishing it with sand paper as others have described above. Google Randy Walls Supernova for further validation of his accomplishments. It's all just metal and surface finish in the end.

[Murray]

Chisel sharpening stone to level off the surfaces.

[Chuck in Indiana]

I think the right thing is to deck the head surface and then cut the recessed circle to the same depth of cut as the decking, am I overthinking this?

I guess I'd have to know if how much it is warped. .003" or .030." 

[Mike Tashjian]

Fly cutting and re cutting the inner sealing surface will change the compression and may cause other issues. Many  parts need a fixture to properly hold them while they are machined.  I like to use a good straight edge at various directions and feeler gauges to do the first assessment.  I use a large flat file gently next to knock down any raised burrs, then the glass plate and sand paper if needed.  The gaskets are meant to seal small imperfections and should work fine in most cases. 

[SIR REAL ED]

    Norton makes an open mesh lapping paper that works great on a piece of glass or granite that would do the job your looking to do. Stoddard solvent works well as a lubricant . I bought a piece of granite from a monument company (think gravestone) and it was within .0005 flat.

  My $.02
  Paul B

PaulB, I like it!

Chuck, you are over thinking it!  Listen to Paul B.

Measure twice before cutting, sanding, etc!

I'd get out the bluing dye or a magic marker, color between the lines then check with some 320-400 grit paper on a piece or glass or a drill press table (or you could visit a local cemetery).  After a few figure 8's you will know if you need to measure anything.

[pressureangle]

You *must* maintain the relationship between the internal cylinder sealing surface and the outer gasket surface.
There is no way to return everything to perfect without at least a slight increase in compression, and you have to verify valve to piston clearance at least.
I have seen some done where the inner ring is resurfaced, and the cylinder relieved to bring the compression seal up into the head to match. But however it's done you have very little room for error. What engine is this?

[PeteS]

I guess I'd have to know if how much it is warped. .003" or .030." 

Thats the first question. A flycutter is a better choice than sandpaper. Sandpaper will take more off the edges than the center.
You can use a dial indicator to see how much if any wear on the mill ways. I would do that before taking any cuts.
The recessed area around the circumference is the squish band. .040” is usually considered the minimum space between it and the piston when hot rodding an engine for increasing CR. I know thats not the purpose here and it doesn’t appear you would get anywhere close to that.
If this is a twin an equal amount should be taken off the other head.

Pete

[Canuck750]

Good advice and information guys, thank you, I will try flat sanding on a sheet of glass first and see how that goes.

[John Croucher]

Glass is not flat.  Find a accurate surface plate.

[Moparnut72]

Flat enough for this application. Make sure you go in a figure 8 pattern. Just get a heavier piece of glass than normal window glass.
kk

[normzone]

I've never measured any, but I've heard plate glass described as flat within .010 inches. If that's so, at some point the measurement error will exceed the desired tolerance of your modification. [Chuck] is correct about .003 versus .030, and [John Croucher] is spot on with finding a surface plate.

Additionally, [pressureangle]'s point re compression increase and decreased valve/piston clearance is real.

[PeteS]

 A lot of speculation here. It would be good to know first how far off the head is.
As for plate glass, put a straight edge on it. The piece I have, laminated safety glass, you can’t get a piece of .001” shim stock between the straight edge and the glass over 18”.
Flat enough for sandpaper work.

Pete

[Chuck in Indiana]

PaulB, I like it!

Chuck, you are over thinking it!  Listen to Paul B.

Measure twice before cutting, sanding, etc!

I'd get out the bluing dye or a magic marker, color between the lines then check with some 320-400 grit paper on a piece or glass or a drill press table (or you could visit a local cemetery).  After a few figure 8's you will know if you need to measure anything.

And just *how* is finding out how much it is warped over thinking?

[Canuck750]

The cylinder head I am working with is off a Benelli Sprite 200 egg motor. The head has a very slight gap about one axis across the center, I have had the head on a piece of laminated safety glass, I don’t have a surface plate but I do have a milling machine deck I could use.
My mill also has a 4 tooth cutting head that is about 2” across, probably much better than a fly cutter, I will try and measure with feeler gauges, it’s not much but there is a very slight gap, perhaps the spigot on the cylinder is enough to create the gas seal and the flat mating surfaces are sealed for oil with the gasket stack.

Figuring out the cuts to the head recess, squish band etc is way above my skill level so I think it best I don’t go there.

Thanks for explaining the factors I would need to consider.

[lucky phil]

Glass is not flat.  Find a accurate surface plate.

Correct. I have 1 large Granite surface plate and 2 steel surface plates in my workshop and I've tested plate glass on the certified granite plate and it's not flat. Fly cut the head surface and also the cylinder recess the same amount. Looks like one area and the compression sealing face may need some welding as well.

Phil

[Canuck750]

Correct. I have 1 large Granite surface plate and 2 steel surface plates in my workshop and I've tested plate glass on the certified granite plate and it's not flat. Fly cut the head surface and also the cylinder recess the same amount. Looks like one area and the compression sealing face may need some welding as well.

Phil

I can get a 9" x 18" x 2" granite surface plate locally for under $100.00, looks like its next on the want list.

[lucky phil]

I can get a 9" x 18" x 2" granite surface plate locally for under $100.00, looks like its next on the want list.

Good move you'll be using it all the time considering how much mechanical work you do. One of the best investments I ever made. One of my steel plates is a specialist unit that has dual rollers and clamps for a roll of wet and dry thats held across the plate. Specifically designed for tasks like the one you have. When the paper is used up you simply release the clamps pull a new section of paper from the roll and clamp it down again. With Ducati WSB engine heads back in the 90's the heads were put on a lapping plate with diamond paste and lapped to get a flat and appropriate surface to give the gasket some bite. Stopped them weeping coolant around the exhaust port side of the head.

Phil

[SIR REAL ED]

And just *how* is finding out how much it is warped over thinking?

"over thinking" was bad use of language on my part.  My bad. But.... to be honest, you started it!

Better use of language may be "thinking 10 steps ahead to solve a problem that might not exist."  Let face it, we all love to go into full "rocket scientist" mode or "GP mechanic" mode. Especially when we can use that thinking to rationalize buying more cool stuff/tools!  The depth and breadth of technical knowledge and expertise at WG is impressive.  I thank you all for your help in both problem solving and your support in rationalizing buying more cool sh*t!

If we go into "redneck farmer with few resources" mode (my default due to upbringing), a quick check would be to set the head on the cylinder with no head gasket and shine a light inside the cylinder to see if a gap exists.  If the gap exists, check the gap with a feeler gage, or even compare the gap to the thickness of the stock head gasket.  Then check the depth of the "o-ring" groove in multiple places.  Make a plan of attack from there.

Decades of working with both redneck farmers and Phd Engineers has taught me both have vastly different view points when it comes to solving problems.  Same with doctors when it comes to medical issues.

Always fascinating to see how many ways a problem can be either solved or created.

If you really, really want to have fun, go to a dental supply website and buy the material dentists use to make dental impressions.  I've used Extrude Extra in the past.  That stuff is so accurate it will reproduce fingerprints.  Great for measuring gaps, etc. that can't be accessed after assembly.

[SIR REAL ED]

And just *how* is finding out how much it is warped over thinking?

One of my neighbors has a great response when someone accuses him of "over thinking" a problem.

"OK, what happens when you under think it?"

[pressureangle]

The cylinder head I am working with is off a Benelli Sprite 200 egg motor. The head has a very slight gap about one axis across the center, I have had the head on a piece of laminated safety glass, I don’t have a surface plate but I do have a milling machine deck I could use.
My mill also has a 4 tooth cutting head that is about 2” across, probably much better than a fly cutter, I will try and measure with feeler gauges, it’s not much but there is a very slight gap, perhaps the spigot on the cylinder is enough to create the gas seal and the flat mating surfaces are sealed for oil with the gasket stack.

Figuring out the cuts to the head recess, squish band etc is way above my skill level so I think it best I don’t go there.

Thanks for explaining the factors I would need to consider.

A lot of old European motors use gasketless spigots for compression seal, as you have here. Yours has some damage to the sealing surface, as well as the warpage on the gasket surface. As you remove material from the gasket surface, you'll reduce compression of that gasket because the compression seal can't move at all. I'd take this opportunity to lap the compression ring, using medium valve grinding compound. It's not as good as machining out the flaws, but it will reduce the strain on those damaged areas and also give back a little of the lost gasket surface compression. Take just enough material to see that your seal doesn't have any light points that may lead to leakage and further damage.

[Canuck750]

A lot of old European motors use gasketless spigots for compression seal, as you have here. Yours has some damage to the sealing surface, as well as the warpage on the gasket surface. As you remove material from the gasket surface, you'll reduce compression of that gasket because the compression seal can't move at all. I'd take this opportunity to lap the compression ring, using medium valve grinding compound. It's not as good as machining out the flaws, but it will reduce the strain on those damaged areas and also give back a little of the lost gasket surface compression. Take just enough material to see that your seal doesn't have any light points that may lead to leakage and further damage.

I will do that, I may take it to a friend who is a retired professional welder with tons of experience welding aluminum and cast iron, then get a pro to machine it.