Did not occur to me that one would consider skimming only the gasket surface... simply because it doesn't work!
In this whole skimming process lies the greatest danger to these heads, reducing overall thickness. While skimming may be par for the course with all sorts of auto engines, it can be way over the top for the soft and fragile triple heads. Tolerances on both sides of the head are quite tight, there's not much room for error.
Skimming the combustion side raises compression, not always a bad thing. But it also reduces the gap between the liner spigot (1000 & 750 only) and head. This needs to be observed in order to have the head gasket seal properly. No big deal, the spigot can be modified to suit (1200 do not have this spigot). But, this allows for more slop in the timing chain that needs to be taken up by the tensioner. The ensuing shorter chain run can also throw cam timing off by a couple degrees, if you're really anal about cam timing, you're in for a surprise! Skimming the cam side brings down the cams and reduces required shim thickness, this has definite limits as long as you're dealing with stock valves. These will tolerate a certain amount of tipping, but no more than perhaps 0.8mm, the shims will at one point rest on the valve spring collets, making adjustment impossible (of course the shims can be modified and often are, a sheer endless chain of bodges imho). Shorter valves are available to counter this, but then you may have to start thinking about excessive spring pressure, especially if valve seat inserts are installed. The surface supporting the cam blocks needs to be on the same level as the cam cover gasket surface, otherwise the gasket will not work unless the cam blocks are shortened at the top. This also has its limits, with stock items at least, these carry a cast-in M6 nut that will pull out if too much of the retaining alloy is removed... fuckin' can of worms.
Most heads are only bent and can often be re-bent back to shape on a hydraulic press. This takes time, a little heat and some experience. I have been able to reclaim bent heads to the point that skimming would be totally unnecessary. In fact, many bent heads can be ignored, when buttoned down, they may end up perfectly flat again, allowing the cams to spin freely (bolt down to a milling machine bed to check!). Due to excessive heat, the alloy can show areas of recession, especially between cylinders #1 and #2 and around the chain tunnel. Cam cover gasket surface is also prone to this, depending on how straight the remainder of the head is, reclaiming by welding (laser preferably to avoid distortion) might be considered. Recession and warping in 2 or more planes needs a lot of thought before randomly strapping the head to the machine bed for skimming.
I have had to reclaim many skimmed heads that had over-enthusiastically 1mm and more removed from their overall thickness/height. This calls for endless adjustments/mods in nearly all components in order to get the head working satisfactorily long-term.
piet
any new heads going?
