The Chronicles of Sled

Part 1 of 2

Well back in the original update I alluded to the fact that there was quite a difference in weight between the old 44mm 125cc pistons and the new 47mm 142cc pistons. At the time I decided to try the lighter pistons without balancing the entire reciprocating assembly. The results were more than satisfactory with no appreciable difference in vibration.

Now that I'm back in my trade I have access to very accurate electronic scales used for colour matching so I decided to find out what the actual difference in mass between the two pistons actually was.

I was quite staggered to find out that the 47mm pistons are over 10 grams lighter the the 44's. The full 44mm assembly including the gudgeon pin, rings and circlips is 93.82g and the same assembly with the 47mm items is 83.55g.

The next big step to be done here is to start lightening the crank, the 11% lighter pistons should allow good scope to remove a significant amount of metal from that large fast spinning mass that is the CB125T crankshaft.

Another necessary modification to the new bigbore pistons is to cut new recesses into the crowns. The pistons I've selected are actually from the Chinese copy of the 70cc version of the old Honda Cub motor, to the younguns that would be more familiar as a pit bike engine.

The standard recesses are quite shallow and the spacing between the valves on the pit bike engine is quite narrow. The CB125T combustion chamber is a bigger diameter and the valves are set a a wider included angle. This means that without some form of modification the valve heads touch the crown if the piston even with plenty of piston to head clearance and the subsequent lowly compression ratio (can't have that now can we?)

I've contemplated setting up a mill at work to do this, it would be the best way to get the accuracy required. It would also require quite a bit of measurement and set up time and maybe even construction of something to hold the piston at the correct angle for cutting whilst keeping the whole thing rigid.

I ran this past Koba's dad who sub contracts to our company and he suggested using some of the old bits of engine to build the jig to do it. Upon further discussion we decided the it could be done using an assembled engine with a cutting or grinding tool mounted to the valve head and the valve being spun in the guide with the pistons inside the bore at TDC.


I have lots of old bits, including many unservicable valves and cylinder heads
so I got to work and here are the results...


Before and nearly after.
The piston inside the bore has had the first roughing cut done.



This is the 'grinding tool' valve 40 grit emery cloth super glued to the valve face, for the initial roughing cut.

To get the correct depth of the recess I set the chuck of the electric drill at the same distance from the top of the valve guide and ground the crown untill the chuck hit the guide.


The electric drill installed on the engine jig.