Having decided over a few beers that we should do another cast iron
cylinder I ordered a Wiseco TS250 piston, MB manifold and a new cast
head from MB.  The cylinder was bored locally.  Having established that the rod wasn't bent following the accident at the last sprint, I re-aligned it and welded it up (all of my AF 62mm cranks were loose on the pin on one side, yet tight on the other so this was essential).

The problem with running a 62mm crank is that there needs to be a
balance between the port timings and the constraints of the cylinder, as
the exhaust port is quite limiting and there is only so far you can reliably put the piston over the top of the cylinder at TDC.  The problem with the exhaust port is that to get the transfer timing right, the exhaust port tends to need grinding a fair bit and this makes it
weak.  The last cylinder had to have the gasket face built up with weld after it cracked.

As you can see from the photos the entry to the transfers is not huge, simply nicely angled, but at the exit into the chamber it is much bigger, and more importantly angled more so at each other.

The inlet port is slightly smaller than I wanted.  I decided to not raise the port to take advantage of gas flow, this means it has a longer duration fully open.  The floor cannot go any lower as it just breaks through into the spigot.  It is nicely flowed, and I tend to think a smaller port will allow a higher timing anyhow, with less gas escaping at lower revs (not that this is too important for sprinting).

I did think that I would be better off with an H2750 piston, but they are about $180 before post, so this is expensive, especially if it blows up after 100 yards !  The reason for this is that they have a lower compression height, and would sit on an RD400 nicely, which is 1mm
shorter.  This all adds up, when you factor in the height of the piston in the bore for port timing purposes.  This is the set up I ran on the Rapido motor.

I fixed a pack plate to the base of the cylinder, and then machined it to the desired thickness, taking around 0.5mm from it.

Port timings are quite high, and I wanted lower transfers.  There is
still scope to reduce this without badly affecting port flow at BDC, but I think I would need a new piston, given the inlet timing.  We shall see.  I will provide another update, once the motor is more complete.

Click here to see more details on the forum.

Cheers.
Jez

The Imperials scooter club is based in Guernsey and has a strong membership, we meet every Wednesday to get the spanners out and Sunday for a spin and an ale.