That's a giant fly cutter.

Thats so fucking coool!!! Does anyone know if that has an effect on engine performance? If it does, what sort of effects?

As mentioned in the imgur comments: "Flattening the underside of an engine head after it has been deformed by the heat and possibly has blown the gasket." A quite positive reliability effect in that a flat surface is a better gasket mating surface - means much less likely to have a gasket and overheating problem

Skimming the cylinder head (and block) makes the combustion chamber smaller which increases compression. Higher compression (until a certain point) makes more power and the engine more efficient. There is a downside though, higher compression means higher combustion temperature which means higher NOx emissions. That is also the reason engine manufacturers keep lowering compression on their diesel engines, as they’re notoriously high NOx emitters.

With a head as warped as this, skimming is a necessity. It simply won’t seal otherwise. Today’s headgaskets are often steelshims or multilayer steel (mls) that requires completely flat matingsurfaces. Back in the day with composite materials you could get away with warped heads and blocks, but they wouldn’t hold as much pressure (power) as modern gaskets.

The real question is: what the heck bent the block out of plane that they needed eight passes to flatten it?

And, if the head is that much out of shape, how are the cylinder borings, with regards to shape and alignment?

The question is: when they need eight passes to get the head back to plane, how bad is the rest of the block?

Not necessarily that bad if it’s a closed deck iron block. If it’s a modern open deck aluminium block, yea that’s a different story.

But the reason they take that many passes is to ease up the final pass, you don’t want to remove more than absolutely necessary.

Thank you for explaining!!! ❤️

Higher compression/temperature can also lead to pre-detonation and thoroughly destroy an engine. So you may need to switch to a higher octane fuel because they have a higher combustion temp.

Absolutely, which is why I wrote “until a certain point”. Pre-detonation and lean afr under power are the two quickest ways to kill an engine.

No problem at all 😊

I can't tell from the video, but maybe the depth of cut isn't too deep. The first few skim a large part so maybe the warp in the center isn't as bad as the video makes it look. Could be over torqued bolts on ends and loose in center plus heat.