2-stroke Case Splitting and Reassembly Jig for 80cc Happy Time Engine

The materials used are:

Four M4 x 25 bolts with nuts & washers.
One M12 x 50mm bolt (2mm pitch) which is drilled and tapped for an internal M8 x 1mm pitch thread, together with suitable washers and nut.
One M8 (1mm pitch) x 20mm bolt with washers.
5mm thick steel plate which won't bend under stress and allows sufficient thickness to tap a thread for the 2mm pitch M12 x 50mm bolt.

The four M4 bolts screw into the magneto side of the case halve and are used to anchor the jig when the M12 bolt is screwed through the plate; attaching itself to the M8 thread on the magneto side of the crankshaft. This allows the crankshaft and ball bearing race to be either pressed out or reinstalled into the magneto side case half.

The (1mm pitch) M8 bolt screws into the clutch side of the crankshaft and is used to either press out or reinstall the crankshaft and ball bearing race on the clutch side case half.

The black Delrin tool is used to press out or reinstall the clutch shaft bearing race (can also be used to press out a crankshaft bearing race that's detached itself from the crankshaft; remaining fixed in the case half) - note the protruded nose and recessed section which allows the tool to centre itself inside the roller bearing, applying force only to the outside bearing shell.

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I made one that's the same idea, but somewhat simpler. I've used it many times now with great success.

How does your jig attach onto the crankshaft ends to pull the crankshaft and main bearings into the bearing receiver in each of the case halves?

My jig was designed to avoid using a compressive force on the crankshaft when M6 case halve bolts would otherwise be required to screw the case halves together; driving the main bearings into the receivers; distorting the crankshaft and sending it out of true.

Fabian wrote:How does your jig attach onto the crankshaft ends to pull the crankshaft and main bearings into the bearing receiver in each of the case halves?

My jig was designed to avoid using a compressive force on the crankshaft when M6 case halve bolts would otherwise be required to screw the case halves together; driving the main bearings into the receivers; distorting the crankshaft and sending it out of true.

My tool is only used as a press to get the carnkcase halfs apart.

When installing the crank into the case, I first drive the bearing into the case half using a socket to drive the outer race in the carrier. It helps to warm up the case a little bit with a propane torch first, but not really necessary. Then I use a deep socket and washers, with appropriate size bolt to pull the crank through the case half, and into the bearing.

How do you prevent the bearing inner race from galling the machined shafts as they are pulled through the inner race, because the friction will cause fine galling that will act as tiny scraping faces on the crankcase seals?

Fabian wrote:How do you prevent the bearing inner race from galling the machined shafts as they are pulled through the inner race, because the friction will cause fine galling that will act as tiny scraping faces on the crankcase seals?

Galling the shaft isn't much of a concern really, because the bearing race usually isn't that tight of a fit on the shaft. You can almost slide some of them on by hand. Still you should be able to prevent galling by lubing the shaft a little before installation. You could also cool the crankshaft in the freezer first.

A much bigger problem is trying to find a crankshaft that's even close to true. Same goes for balance.

Truing up the crankshaft isn't a problem with a set of V-blocks, a dial-indicator and a copper hammer.

Balance isn't really an issue either as a single cylinder engine cannot be perfectly balanced; it can only be balanced for a narrow rpm range, set by the chosen balance specification.

My experience with Chinese 2-stroke bicycle engines (all 8 of them) is that the main bearings are a very tight press fit on the crankshaft, and can't be press fitted at room temperature without galling the shafts.
The only method that worked adequately for me was to heat the main bearings in an oven and to cool the crankshaft down to minus 86 degrees Celsius (minus 123 Fahrenheit) with dry ice, allowing the bearings to slip onto the shafts, and even then, some were a snug fit.

The differences in the complexity of our case splitting tools is that mine has the been designed with the dual purpose of not only pressing out the crankshaft, but also enabling the crankshaft fitted with main bearings to be pulled into the bearing receivers.