gmedson
Posts: 29
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Joined: 3/23/2005
Last Login: 11/26/2009
From: Sanford, NC, USA
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 Hi Dan You have a Zeiss Jena 2.5 cc DK. Although rear rotor, it is a sport, not a performance engine. In order to assess the quality and characteristics of this, and other Zeiss engines, one should consider how things were done in the DDR (East Germany). In that planned economy commercial enterprises were state owned or had state participation. Full employment of the population was mandatory, and a bloated production staff tended not to be highly motivated with respect to quality. So Jena engines tend to be good but not great. Fits tend to be too tight. Access to raw materials was not consistent and a great deal of scrounging (now "recycling") )was necessary. One consequence is that ball bearings are not good quality-and they tend to be fitted too tightly, altogether resulting in a rough feel. At the Jena model airplane plant, response to official directives also led to constant development of new models, which differed cosmetically but did not produce better performance. This makes Jena collecting interesting because of the variety of engines available.
I am interested in engines made by companies that are well known for some other primary type of product so that model airplane engines come as a bit of surprise. E.g., Citizen Watch Company making G-Mark engines and Karl Zeiss, famous for high-quality optical goods, making the Jenas (Jena, DDR, was the city).
I have five Zeiss Jenas, and their portraits are attached below. The wee 1cc engine with the red cylinder muff has piston-port induction, but with the venturi pointing forward, parallel with the crankshaft, making it quite distinctive. It has two races of ball bearings, which seems to be standard for all Jenas, and dates from about 1960. The 2 cc with the blue muff (Jena's head colors indicate capacity) is a Model D RC from about 1961. The two with the smaller diameter black head muffs are 2.5 cc, perhaps from about 1961. One has reed- valve induction , identified as a Model MK and the rotary-valve induction engine, like yours, is a DK. These 2.0 and 2.5 cc models have a long crankcase, whereas the earlier equivalent engines have a shorter crankcase but the same-length crankshaft. The difference is taken up by a black-anodized aluminum spacer between the case and the prop driver. This arrangement is illustrated by the rotary-valved Model DN, below, which has a larger diameter black head muff and dates from about 1960. Note the milled-off bosses below the exhaust ports, which on the Aktivist III (1959) and earlier models were drilled and threaded to receive three head bolts, as on PAWs and Ollies. Note that the box is incorrectly labeled-it says Jena 2,5 DK but this is how it came to me.
These are only a few of the many models of Jena engines from that period. Note how compressed the time line is-lotsa variations in a few short years!
Most of this information is from Selbstzuenderkleinstmotoren und Gluehzuenderkleinstmotoren aus Jena, a self-published book by Eberhard Stenzel, 2007. To my knowledge, it is available only in German.
Now, have I run any of my little Jenas? No, I have not! I always have great intentions to run each new acquisition. But mostly they collect in the cabinet as hanger queens!
As a final item of interest, check out eBay Item No. 300357862286, if you have not already done so. Or if you have, go back and drool over it some more. These boxer-style diesel twins were produced by Rolf Roehner in Western Germany using Zeiss Jena piston-cylinder-head muff sets and sold by the firm Robbe.
Regards. -Gary
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Anyone use a Zeiss Jena 2.5cc diesel? - 
