Squishband (or -area) depending on the design of a cylinder head is the area were the fuel-air-mix is forcefully compressed more than in other areas with the aim of aiming the gas towards the spark plug in order to improve the speed and efficiency of combustion. Thus reducing the required amount of ignition advance.
Aircooled RD350 engine, late model electronic ignition
The first step in any modification of an engine is quite simply put the assessment of the current situation.
Take the head off and...
Wössner RD350 two-stroke piston - work of art
... be greeted by a wonderful new Wössner piston for RD350ies. (Lovely)
Modified RD350 cylinder head with a 12.5mm squishband
Unfortunately, I was also greeted by this. For the sake of fair play, I have to spill the beans and admit that this is the engine of a friend and I did the 250 to 350 conversion for him. Unfortunately, somebody had modified the cylinder heads in the past and not exactly the way this was going to be helpful for a mostly stock engine. The distance between piston crown and combustion chamber roof, was an enormous 2.2 and 2.35mm. And just if that weren't bad enough, there was a lip of the headgasket protruding into the combustion chamber just as the actual head was turned too small thus creating another hot spot.
Now the 2.2mm gap could quite well be intentional, as these heads could have been fitted to a race engine, which was running both without a basegasket and had the cylinder itself shortened by the height of the headgasket relieve. Yet this is not an excuse for machining the head too small and also for the headgasket.
Headgasket 1.35mm thick
I checked the relieve in the cylinder and it was 0.65mm on the left and 0.75mm on the right. So the given headgasket, which measured a whopping 1.35mm thickness. This was to be added to the already enormous gap, bringing the "squishband" almost 3mm away from the piston crown. Way too much. A good figure for a RD350 engine would be 0.75 to 0.8mm so that the squishband is most effective, yet still leaving a bit of room for conrod stretch and a bit of play in both the big-end- and small-end bearings.
A stock RD350 head and combustion chamber (for comparison)
Conservative 60ies and 70ies desing demanded around 50% of the combustion chamber surface to be used as squish-area. Modern designs lower this figure to around the 30% mark, which in this particular case would have been a ring around 10mm wide. As the squishband is meant to accelerate the charge towards the spark plug it should, quite logically, be angled.
Now here are two schools of thought. If you machine a very steep angle into the head (as seen in the picture above) the mixture will travel to the spark plug without being compressed overly much. It will do so, but not in a very fast way (still heaps faster than stock, but you get the idea!) Now if you used a very shallow angle, let's say around the 3 to 5 degree mark, the charge would be pre-compressed, heated up and would burn a lot faster, because it has already picked up a lot more speed.
There's many ways to actually measure the distance from piston crown to the roof depending on the size of the gap. In this particular case the distance was too big to use solder or twisted strands of solder, so I had to resort to some sort of clay in order to get the cylinder heads in the right ball-park to start with.
In order to make your actual measurement, the engine has to be reassembled, in this case meaning that the heads had to reinstalled and torqued to spec. (20Nm) Then the engine is turned over until the clay hits the cylinder head, everything is removed and the crank is turned until it reaches TDC. Now the height of the protruding clay is measured as this defines the amount of material to be removed from the head.
The cylinder head is installed on the lathe adapter in order to be machined.
As I am using an almost seventy year old lathe, DRO's are a no-go. So I had to restort to an ARO (analogue readout, aka. a dial-indicator) in order to only take off the required 1.1mm on the left and 1.2mm on the right cylinder.
As it turned out after a few revolutions, not only was the cylinder's squishband machined too wide, the cylinder head had also been tilted somewhat during the operation and as such wasn't perfectly perpendicular to the piston crown and sparkplug.
These two videos are meant to give a slight impression, why this was actually a quite time consuming process. As carbide inserts were used, the feeds and RPMs had to be a balance between very modest feeds (0.1mm feed) and a relatively wavy surface finish and the imbalance caused by the off-center sparkplug thus inducing quite a bit of vibration. The speeds shown are the slowest and second slowest backgear speeds.
Last order of the day was to cc the actual combustion chamber by filling a graduated vessel with water. (In this case to the 600ml-mark.) The next step was to form a clay mould of the combustion chamber and then drop this in the bowl and measure the extra milliletres with a syringe, which came out at a spot-on perfect 30cc.
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