Exposure I just finished putting the lower end of the engine back together - the Yamabond is setting up as I write this. I lapped the cases to make them flat again. While I was at it I pulled the cylinder base studs and lapped the lower deck of the block as well. I'll lap other engine components as I need them for final assembly.
This is where I was when I stopped today.	This engine is just about together. I left the clutch cover off because I wanted to have a way to turn the engine over by hand - it's hard to make small movements using the kick starter. I replaced all the seals of the engine since I had it apart. Though some of them didn't look very worn, it would be a shame to have to replace a leaky seal after opening up the engine this far and they're cheap enough to do it every 10 years or so if a failure like this occurs. The Blaster engine allows most of the seals to be replaced from the outside without taking the engine apart. Just the right side crankshaft seal (clutch side) presents any kind of trouble to get at (it is buried deep in the engine) but it can be replaced without opening the center cases. Some engines are not that easy to work on. Before I put the degree wheel on I located TDC using a piston stop. I also checked it with a dial indicator. The top end looks like it's more together than it is. Under the head is a piston without rings - it's just there so I can discover the port timing and measure the volume above TDC. I need to be able to measure those things so I can determine this engines state of tune.
The porting looks just about like this. Pretty healthy for an air cooled single. There are 5 transfer ports. The roof angle of all the ports are very flat. There is quite a bit of mismatch between the aluminum and the iron.	When I looked inside the exhaust port I could see its traditional shape. It's oval with a generous bevel on the top edge to prevent any ring damage during use. The port is 43 mm wide when measured correctly.
By looking inside the intake port I can see the sharp edges, flat spots and lips getting in the way. Notice there's no auxiliary intake ports in this barrel (yet).	When it's viewed this way several things are easy to see. There are sharp edges on the transfer port dividers and the cylinder wall that separates the transfer ports also show some liner mismatch.
Turns Mixture that comes from under the piston has to make a 180 degree turn to arrive in the transfer port. Sharp edges and squared off port dividers act like barriers and create eddies that the mixture must overcome in order to arrive in the port tunnel. From there, the mixture must turn again - sometimes as much as 90 degrees to find its way into the cylinder where it gets its opportunity to be put to work. Even after it finds its way there, some of it will escape the engine through the exhaust port and some will get diluted due to short circuiting (mixing the fresh with the spent gasses) and pipe affects that are not tuned for all (every) operating speeds. If lucky enough most of the mixture will be burned when everything is working together well. Above and below the ideal speed range, there's a lot of gasses making the trip and not helping with the work. When the engine is put together with more careful attention spent on detail there is a better chance that more mixture will find its way to the combustion chamber to be burned as part of the power making process.
In this picture the hard edges, obstructions and sloppy factory cylinder grinding is shown. As bad as it looks, these engines run very well just the same - though they run much better when these problem areas are fixed. Time spent here smoothing and adding a radius to divider walls will help a lot.	This engine case shows casting flaws that can be improved upon. The bearings are out so it's easy to work this area with porting tools. The idea is to round off the edges without removing more material than is necessary. Sharp edges of extra casting material make waves of flow instead of streams of it.