Two into One

[Puffs]

Alan, can't you just press the 2 cranks together, joined via a straight pin? That is the method I know...

[Guesi]

Here is another way to do it. As you see there is only ONE crankcase, which makes the whole engine smaller...

[breakwellmz]

Nice!

[Puffs]

Well, isn't that the sealing separation, pretty much as used in the Jawas? Something like that I referred to in viewtopic.php?f=19&t=11636&start=15#p54863

Jawa, who designed this probably in the 30's or so, had one extra trick up their sleeve: they rotated the porting so that the barrels better fit together. The advantage of bringing the pistons closer together, in a 180° configuration, is that you minimise the so-called rocking couple vibrations, and can achieve an engine running with very low vibration. They also used just a single carb, for simplicity & cost.

The 634 and earlier models had a narrow separation, holding just a seal and 1 bearing, but in later years they accepted slightly more vibrations to gain extra reliability, and added a second central bearing on that (then wider) joining pin. Not sure exactly when that happened, but the 640 has 2 bearings and the barrels farther apart compared to the 634. In addition to a (presumably) longer service life, the wider separation might also be more desirable from a thermal view.

Some pics from when I replaced the central bearing & big ends in my 634. I was so fortunate that I could do the pressing at my brother's, the balancing I did at home. Pulling said separation:

A pic of measuring for balancing:

Note a 640 has a stroke of 65mm, just as the ETZ 250cc, and I think new complete 640 crankshafts are readily available. Many other parameters should be verified, but maybe that's another option? Then you'd only need another barrel.
Thinking a bit more about it, you probably won't be able to fit the bigger bored ETZ barrels in that spacing. Anyway, the construction has been tried & tested (for decades), and looking at it may serve as a guidance.

[AlanJ]

Hi Guesi,
Great picture, it surprises me though that the wider cranks are only running on two bearings, it would have to be very well balanced, that's quite a
large heavy lump to go spinning at 4k=. I am looking at a complete bike, but I suspect it will be too high priced, Thanks for all your info on this subject,
very good.
Stay safe Alan.

[Puffs]

I don't think it's running on only the outside bearings. I think that's a separation that's fixated in the crankcase, with a bearing & seal in it - it's a 180° configuration, so it needs pressure isolation. And now looking at the text, I think it says so: mittig abgedichtet = centrally sealed.

For comparison a clearer view of the 634 crankshaft, without that fixated separation:

As said, later ones even had 2 bearings in the middle.

[dave47]

Would it have been too hard to acquire a Jawa 350 engine and fit it into the Trophy chassis, rather than constructing essentially the same thing by destroying a pair of ES175 engines? Or am I missing the point?

[AlanJ]

Surely if one has a centre bearing the normal system is to have horizontally split crankcases, other than that that centre bearing would have to be in a housing
that was very tightly controlled to the cranks, not ideal really. To be honest, I don't think the joining of the cronks is too much of a problem, it's the extended
final drive shaft and bearing housing that will require most of the work. I must say though, I am surprised about the amount of info on this subject, very good.
I take my hat off to the guy who wrote how to do the addition in a few sentences, brilliant.
Stay safe all Alan.

[Guesi]

Would it have been too hard to acquire a Jawa 350 engine and fit it into the Trophy chassis, rather than constructing essentially the same thing by destroying a pair of ES175 engines? Or am I missing the point?

This engine was made in the 70s in the GDR. For them it was not that easy to go to the shop and buy a Jawa engine....

And some people do things just because they CAN do it

[dave47]

Thanks for the info Guesi. That sounds reasonable.

[Puffs]

some people do things just because they CAN do it

Exactly. That's why Mount Everest was climbed. And a project like this you'd also do just for the journey.

[DAVID THOMPSON]

it would improve your machine shop skills a bunch
and the mistakes would fill a scrap tub

[AlanJ]

Hi All,
As I mentioned earlier, I was following a complete bike on ebay, then 2 days to go and 2 newbies raised the bid by over £250. I mailed him for a buy i now
he said no, and then I guess someone made him an offer he could not refuse, cos it disappeared, bit of a bummer really, it was a good one, some you win
some you don't. I think the best way to go though would be another engine, which takes up the least room.
Take care Alan.

[AlanJ]

Hi All,
I think, thanks to all your input on this project, I now have very good information on which way to go, and as I already have one half of the project
and as the right-hand engine has the most amount of work, I will make a start on the RHS. Thanks again, everyone.
Stay safe Alan.

[Puffs]

Alan, for your convenience an automatic (computer) translation of the earlier document in German:
"
Until his serious sports accident in 1972, Thomas Heuschkel was one of the best road racing drivers in the GDR for a long time.

He has now transferred his experience and knowledge gained with MZ racing machines to the field of used machines when building an MZ ES with a two-cylinder engine. However, it is not intended to mass-produce the Heuschkel-MZ, and our idea of ​​the machine is by no means intended to encourage replication, it is only intended to convey the acquaintance of an unusual and apparently very demanding self-construction. Cylinders from the Trophy model, housing from the TS 250 and numerous in-house production parts form the basis for the two-gas engine. A through hole with a diameter of 125 mm was drilled into the TS housing in the horizontal crankshaft plane, which enables new housings to be shrunk in from both sides and screwed to the original housing that has been drilled out. A crankshaft pressed together using changed serial part numbers is sealed in the center. This narrow housing part - mounted on the crankshaft - is shrunk into the housing. On the alternator side, another cylindrical roller bearing takes up the crankshaft, while a ring groove bearing 6009 is used on the gearbox side.

The original bore for the cylinder neck is closed with a filler piece. By milling off the entire upper part of the housing in the plane of the original cylinder mounting surface, the plane surface is created with which the 30 mm thick base plate for the current mounting of the cylinder is screwed. The lower inlets of the overflow channels are also incorporated into this base plate. New centrifugal cast liners are shrunk in with an oversize of 0.24 mm. The two BVF carburettors of the TS 250 (intake diameter 30 mm each) sit on reworked intake ports. A straight-toothed pair of spur gears transfers the torque from the crankshaft to a generously dimensioned eleven-disc oil bath clutch. The clutch basket is mounted on the clutch shaft (25 mm diameter) with two needle cages; the shaft, in turn, is supported on the drive side with two annular groove bearings 6005 in the modified housing. The idler gears of the first and fourth gears were mounted on needle cages. The drive shaft and 3-speed wheel have been strengthened or changed. The automatic gearshift and gear ratios of the individual gears have not changed.

The engine has a compression of 9 and can be driven with VK 88. At speeds around 100 km / h, just under 5 l / 100 km are consumed. Consumption should be able to be reduced even further through better flushing in particular. Further motor data: H / W = 65 mm / 58 mm; Cubic capacity 344 cm3. The maximum power is delivered in the speed range around 5000 rpm. Control times: E = 140 °, O = 110 °, A = 160 °. Measured against the scope of the work on the engine, the changes to the chassis appear minimal: both swing arms are mounted in newly manufactured gunmetal bushings, the frame in the steering head area is reinforced, the wheel drive has been converted to a TS version. According to Thomasl Heuschkel, the full swing machine can be driven very easily and safely - even in fast corners.
"
Source: https://www.ets250.com/?page_id=511 , which also contains other pics. How he gets to just 344cc may not be entirely clear, but he probably used 175cc barrels (which I suppose use the same 65mm stroke?), in order to keep the pistons closer together. Incidentally, Heuschkel's machine did have road approval (in the DDR, in 1976).

More akin to your plans might be and
Good luck!