Dr Joe Ehrlich Development 2 stroke Engine rebuild

Published on 1 September 2023 at 18:43
customised motobecane engine crank cases

The Motobecane castings tidied up nicely, although they turned out to not be the original 50cc units I thought they were.

motobecane crankcases with bearings

A pair of new old stock bearings ready for fitting.

motobecane d52 bottom end reassembled

the bottom end all together, everything has gone fairly well except for some unfortunate gasket nastiness.

Motobecane engine with Dr Joe Ehrlich head and cylinder

The barrel and head test fitted, there is no piston in there yet as I don't have the gudgeon pin or little end roller bearing sorted yet.

broken and siezed bearings from the motobecane two stroke engine

After looking through my purchase of the Dr Joe Ehrlich motorcycle parts I had determined that there was the complete engine that came in the bike, the development 75cc engine with the adjustable boost ports and almost enough parts to build a complete second 50cc engine, so that's what I decided to do. It didn't take long before I hit a problem, I wasn't at all surprised. My in built sub optimal scenario sensor soon sensed something sub optimal when I went to put the two halves of the crankcase together.

I had thoroughly cleaned the two crankcase halves and had laid out on the bench all the parts that I needed - the crank assembly, the crank case castings, two new bearings, the gasket, some bolts to hold it all together and the tools I thought I would need to do the job. First thing after cleaning was to put the crank in the freezer so that it contracted - that makes it easy to fit the main bearings without resorting to the use of unreasonable force, which comes with a massive risk of breaking something - not good on unique one off parts.

Once the crank was ice cold I got the two main bearings and heated them up until they were just about too hot to handle and then quickly slid them on t the ends of the crank - they went on very easily but within seconds were absolutely solid as the temperatures equalised, the crank shaft expanded and the bearings contracted. The assembly was returned to the freezer to cool back down, once it was as cold as the chicken it was sat next to it was taken out of the freezer, one crank case half was warmed up and the crank outer bearing was pressed in - all good so far. I then went to fit the gasket that goes between the two halves of the crankcase only to find it didn't fit. I had thought Dr Joe had used standard castings, turns out I was wrong. The gasket did not fit, the crank case was about 1/2 an inch in diameter bigger than the standard item. This was a bit of a problem as by now the crank shaft was warming up and the crank case was cooling down. By the time I had applied some instant gasket the two did not want to join together so I had to separate them and try again. More cooling, more heating and the use of some 6mm bolts to pull the two halves together and the job was a good un. It is vital in a two stroke engine that the crank case is properly sealed as gasses within it are compressed. I am confident that mine is good to go.

the next problem I encountered was the lack of a little end bearing, the gudgeon pin is missing too. This would not normally be a problem except the con rod is non standard - it has a 16mm diameter hole for the needle roller but the gudgeon pin is only 10mm. I have yet to find an off the shelf bearing that will fit. The Raleigh Runabout moped used the same basic engine but the con rod is very different, I notice the piston in my bike is a Mahle racing type, it has a part number so getting a gudgeon pin should not be difficult, I can always make one if needs be. I may have to make a press in insert to make the hole a size where a commercially available needle roller can be used. I'm not too worried about it at the moment, at least it is a bit more together now.

One thing has been bothering me and that is the issue of ignition timing, I have no idea how to set it. I am assuming it was done on some sort of jig as there are no keys or timing marks to either set a predetermined position or to make adjustments. It's all very odd. I shall have a look on youtube, I'm sure somebody will have a vid on runabout timing and it should be the same method even if the degrees of advance are different.

Another thing I noticed when I was putting this one together is that it has a much thicker base gasket  the one that goes between the barrel and the crankcase. Changing the thickness of this alters several things on a two stroke - mainly the port timings and the compression ratio. I am starting to think that the rod is different to replace the compression lost by raising the barrel. Hard to say with me not being able to fit the piston just yet.

Steve, the chap I bought it off, did warn me that this would definitely not be a straightforward project - he was not lying none LOL. I am very much enjoying it but I am concerned that the first engine does not seem to have very much compression at all. The bore is fine, the ring is a new one, which gave slight improvement over the original but I would expect a lot more. O am thinking maybe the Mahle single ring piston does not do a good job of sealing at near zero rpm. It's another thing to research.


Some of the destroyed bearings that came out of the gearbox.


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