gmedson

Hi Zvonko - If your Zeiss Jena has 2.5cc displcement, cylinder bore will be about 15.49mm. If 2.0cc, 14.01mm. Just unscrew the aluminum head muff and measure inside diameter of top of cylinder.

Your home-made aluminum cylinder muff should work just fine as long as it screws onto the cylinder nicely and the compression screw thread is reasonably good and the screw is long enough.... It is not a critical part with regard to fit.

The old standard diesel fuel recipe is 1/3 kerosene, 1/3 ethyl ether, and 1/3 oil. Most diesel enthusiastis regard castor oil as best. Component proportions are not critical. Most present-day modelers would cut the oil to about 20 percent and add the difference to the kerosene. Tiny, tiny engines generally require more ether to get them started. But if the ether is fresh, 20 percent ether is enough for engines such as yours. AND you can use an ignition improver, such as amyl nitrate. There are several chemicals that can be used. Easiest and safest is to buy an ignition improver meant for diesel engines such as in trucks. Label will tell the percentage of the actual ignition improver (at least in the USA). Do the math so you get about 1 1/2 final percent in your model engine fuel. And add the rest of the content of the can of ignition improver to your kerosene in calculating final percentages. Most of what is in the ignition improver can is low molecular weight hydrocarbons. Ignition improver allows ignition at slightly lower compression among other benefits.

You are fortunate to be able to buy your fuel components, including ether. In the USA we cannot buy ethyl ether because it is used by people who make illegal drugs!

Regards. -Gary