Piston Thickness & Squish
It's unfortunate that cutting the crown radius down, or making a flat top piston out of them means you have to move into an after market head. With most other companies I'd say they're just selling you more parts, but in the case of Group K, my gut tells me that they've probably tested the heck out of modified stock parts and just could not get reliability to where they needed it to be, and to make it profitable at the same time. However I wonder what would happen if they relaxed the relationship between the piston crown radius and the combustion chamber a bit - say using a +2 to +3 degree positive chamber shape to lessen the detonation problems they experienced, or if detuning the kit that much would have not gained much, if any power. The Group K approach to building this engine with just surface decking, gasket and head changes either means Kawasaki hit a home run out of the box and performance can't be improved very much using the same octane fuel or Group K found it more profitable to sell their machine shop services and the always necessary very profitable replacement consumables customers have to buy when performing engine tear downs. The fact that the kit uses replacement carburetor jets only, and requires that the stock airbox be retained tells a somewhat dismal story of what you can really plan to expect from the modification. I don't like that after all their experience with coaxing power from this engine they recommend installing an exhaust outlet with a larger opening, letting useful heat escape without ever becoming part of the power making process. That alone negates much of all the modifications performed and would be enough of a reason for me to not have the work done at all.
When cutting a piston crown like we're talking about here performance can improve because of several things. The act of removing material lightens the piston letting it accelerate more quickly, and the heat path is shortened from the center of the piston (usually directly below the spark plug) to the edge, improving heat transfer from the piston to the rings, through the rings to the cylinder wall and from there to the cooling system. Mechanically, a flat piston crown is a more efficient shape, and the design allows more compact combustion chamber shapes and tighter squish clearances. Some pistons will not be able to have their crowns cut flat or as flat due to the amount of material the original part was supplied with. Some after market pistons, like Wiseco pistons are often designed as a direct replacement for the OEM version but because of the superior material and manufacturing technique they are considerably lighter, offering improved performance due to that alone. When those parts can then be modified to further lighten them and improve their ability to produce more work, the increase in performance goes up geometrically.
I have had very good success and reliability reducing the center thickness of Wiseco forged pistons to .155" for bores to around 69 mm's and .175" for most other larger bore engines. In some cases I have run with pistons as thin as .100", but only for short duration events. For normal bore sizes, I consider .100" center thickness to be past acceptable reliability margins and well into unacceptable reliability and generally, unnecessary. Cast pistons can be cut as well though they take a performance hit because they run hotter, retain heat much longer and they're generally over built. Obviously, higher compression ratio engines using forged, or cast pistons can't have their center thickness reduced as much as a lower compression ratio engine due to higher combustion chamber temperatures.
The amount of "drumming" produced from these cut-flat pistons, though present, can be all but forgotten. Let the OEM manufacturers build their cast flat tops pistons with so much additional crown thickness to combat its effects that most of the mechanical advantage is lost, and weight is often the on par with the highly crowned part it's designed to replace.
Regarding its thickness - Check it to be sure, but most of the larger pistons start at about .215" thick. With that amount of compression, it must have a very high compression ratio so you'd be better off not going to the thin side of the range. I feel comfortable with telling you to remove no more than .040", keeping it at about .175".
The method for removing material varies due to the design of the piston, but generally speaking any way you can hold the piston firmly, without distorting it, will do. I've done it on both types of machines with success. I even went through the trouble of making elaborate jigs and fixtures to be able to hold them while machining, without inducing any distortion and realized that they came out no better than they did when I just trued them up in a lathe.
It's a good idea to cut an ordinary piece of writing paper the same size as the outer circumferance of the piston and squeeze it between the surface of the piston and the jaws of the lathe chuck - I've successfully used high quality masking tape for this too. Even if you have soft jaws, use it. True it when still fairly loose in the chuck, then tighten it - just enough so it won't come out during machining. I know that's not telling you how tight it needs to be, but consider this, you could tighten it much, much tighter if you wanted to. So, use your best judgment. It's easier to judge if you pick a cut depth - say .010" that you'll use. That might give you some sort of idea of how tightly it needs to be held. In this case plan on four .010" cuts, or you could do like I do and make three .010" cuts, one .008" cut and one .002" cut to be sure the final finish is unblemished (when I first started cutting pistons like this I used .005" per cut just to be extra cautious). I like to soften the hard edge left by the cut at the edge so I spend some time with a piece of 600 wet paper and some light oil, nicely rounding it, to remove any possibility that it would cause that area to pre ignite the mixture, but I don't want to spend so much time rounding it there to change the piston timing height (unless that's what I'm shooting for).
It's a good idea to have the tool height perfectly centered so it doesn't leave a nub in the center of the piston for a couple of reasons. It will later have to be removed, usually by sanding it off with fine sand paper because if it wasn't, combustion temperatures would heat it enough to make it glow creating preignition. And cutting a delicate piece of material is difficult enough without causing undue friction and heat due to the vertical alignment of the tool causing more trouble. A tool that is too high or too low stresses the part more than usual, and the condition gets worse the closer to center the tool comes. Right at the center the condition is awful, at its worse, and will wreak the most havoc then if it's going to wreak any at all. If the piston comes out of the lathe while being machined due to tool height misalignment, even when spinning only a couple of hundred rpm's it will most likely be ruined because it's all but impossible to be able to turn off the machine and have its inertia stopped before the piston twists and gets caught oddly between the jaws of the chuck and the tool, creating some awful gouges.
I like to check squish thickness using the NSEW method. Run a strip on material from one side of the bore to the other in two places. Front to back, and left to right. When referring to the location of the end result thickness' within the bore, use the points north, south, east, or west, with north being straight ahead pointing out the exhaust port. These references let everyone speak the same language.
In a perfect world squish thickness would be the same all the way around the bore when done, but in practicality that isn't going to happen every time. Stacking tolerances come into play here too from the head and head gasket, the squish area of the dome, torque on the studs, the deck surfaces, piston crown, the condition of the TE and BE and main bearings, the condition of the bore, the fit of the piston within the bore, whether or not the rings are on the piston, things I can't think of right now, luck and Murphy's laws. I consider it good enough if I can reproduce the same result three times.
A few thousandths difference is not bad, in fact I'd consider anything under .004" pretty good. That's often times improving the stock condition by more than twice that amount. At .005" I start thinking of the things I can change to improve the situation. I like .0015 to .0025" for most builds. Obviously, for maximum effect .001" or less is preferred and is often attainable with considerable effort. In some cases it is impossible to get less than .005" or .006" of a difference due to the nature of the build. That's just the way it is and we simply have to live with it, though it bugs the living heck out of me.
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