In nowadays lapped piston 2,5 cc engines, tapered liners are widely used as well as a "barrel" shaped pistons (see fig. 12).

A taper liner gives better sealing at the point where pressure in the cylinder is high, and reduces friction where sealing is less important.
From experiments on different engines we found a taper of the liner between 0.015 and 0.025 mm in diameter per cm. to give satisfying results.
This is a bit less than Rossi, and about equal to Nelson and Bugl. A roundness of the liner within 0.0015 mm in diameter in the FMV cylinders turned out to be (too?) good.
The only effect of ovality (up to 0.005 mm) seems to be more difficult starting, unless the piston is oval in the same way, but how can you make that!
The "barrel" shaped piston has two advantages:
Firstly a wedge is formed to press the oil (if any) between the piston skirt and the cylinder to improve lubrication.
Secondly it allows the piston to "find its way" if the cylinder is deformed from thermal reasons.
The way we barreled the FMV's piston is drawn in fig. 14a.
The angles were turned avert pre-honing the pistons to about 0.005 mm from its size.

4.2.3 Transfer and exhaust ports in the liner.

As we have stated before, we don't believe in the effects of different portings.
We are convinced that the huge third port in the Nelson is the reason that it goes so well. Maybe it helps, but with a smaller third port is wouldn't be much worse. For a teamrace engine the exhaust width shouldn't be too wide, 10-11 mm is enough. A bigger opening there may waste fresh mixture.
For mechanical reasons it is favorable to keep the ports as small as possible, giving more cylinder wall for the piston for guidance.
The inclination of the third port is in most engines 50°-60°, no differences were found between main transfer ports inclined between 0° and 15°.
In figure 13 a section of the cylinder parts is drawn.

4.2.4 Piston design and cooling.

About the outside shapes of the piston a few things were said before.
In the following a few other aspects of the FMV piston design are treated.

The design had to meet two demands: lightness and limited con-rod end play.
With a piston pin directly supported by the top of the piston a very thin (0.4 - 0.5 mm) piston skirt is possible. The small end construction is shown in fig. 14a.

The FMV cast iron piston weight is around 3.6 gram, to compare: Rossi cast iron 5.2 gram; K&B-Nelson 6.1 gram.
The piston pin is short, and thus light at 0.6 grams, 0.4 gram lighter than Rossi and Nelson.

In principle Bugl's set up (fig. 14b) has the same advantages, but piston + piston-pin construction is still about 5.2 gram.

The piston pin in the FMV is kept in place by a wire clip of 0.6 mm diameter going through the pin into the piston, see fig. 14a.
The thing looks nice and simple, but needs further improvement in order to enable the separation of the con-rod and the piston in the motor. With our small end construction it is quite a hard job to get the con-rod of the shaft with the piston on.

By chance we found out that a very thin piston skirt has one more advantage. Because the piston can be easily radially compressed the diameter of the piston "follows" the taper of the liner, making the motor almost completely insensitive to piston fit.
It acts more or less like a piston ring.
Unlike a Rossi-type piston, that will stick suddenly when pushed by hand towards T.D.C in a tapered liner, the FMV piston only gives a gradual increase in resistance.
No fatigue failure in the piston was found until now, so maybe thinner walls are possible for improved performance.

Since it was shown before that a too high piston wall temperature causes excessive expansion and lack of lubrication, the cooling of the piston is important.
There are two main ways the piston can be cooled.
In first place there's cool air and fuel at the underside of the piston. If, like in the Bugl all fresh gases flow through the piston, this can be a main way of cooling.
In Nelsons, Rossi's and FMV's we can only hope for "refreshment" from that side.
In the second place, there is a given amount of metallic contact between piston and liner, especially with tight fits.
With a loosely fitted piston, hot gas will leak between piston and liner heating up the walls and preventing the already limited metallic contact. This must be the main reason that worn out pistons and liners overheat quite easily.

One of the main sources of the heating of the piston will be radiation from the combustion process.
A black piston (carbon deposition) receives far more heat than a shining type.
So the cleaner it is, the cooler it stays. Striving for higher thermal efficiency with inevitable increased gas temperature, the piston temperature could turn out to be a bottle neck and may be it already is, due to possible oil flashing.
At this moment we're thinking about a super shining piston top as a useful way to limit piston temperatures.
Preventing carbon built up might be our future goal.