Early crankcase design flaw

Although my CT90 is a 1969 model, the engine is from a late K0 model, dating from around 1968.

The original kick-starter design fitted to these earlier 90 engines used a permanently engaged pinion gear that was turned by a ratchet and pawl assembly (this design was also used on other 90cc bikes in the late 1960s, including the S90).

old style ratchet and pinion kick starter used on the C90 K0 and other Honda 90s of the era

the main weakness with this design is that it incorporated an aluminium projection inside the right-hand crankcase - intended to prevent the ratchet mechanism from over-rotating and contacting the transmission - which can break off, with predictable results.

The case in my engine shows signs of the damage, although someone has attempted a repair in the past (see the picture below).

As explained in the 1965 service bulletin below, if the broken off part gets loose in the engine then the damage can be severe.

left: service bulletin / middle: tell-tale damage (repaired) / right: unbroken kickstarter stop

if you have an older engine, note the guidance above (left) about how to locate the kick starter to prevent the ratchet colliding with the kickstarter stop.

New crankcase design

In the late 60s Honda introduced a new kick-starter mechnaism that remove the need for an internal stop inside the case

new type kick start mechanism - The kickstart pinion gear fits over helical grooves in the end of the kick starter shaft. A friction spring is fitted into a groove in the back of the pinion gear, and located into a notch in the case to stop it rotating. When the kick starter lever is depressed the drag created by the friction spring causes the gear to move up the helical thread toward the transmission where it meshes with the countershaft low gear.

Surprsingly, David Silvers still had stock of the crankcases and since, I was not sure on the part numbers applied to the early engine, so I ordered a couple of the cases which purported to fit the early CT90s

which in line with David Silvers sometimes random pricing policy was a very reasonable £27.

I returned the "early" version, which had been mislabelled and was actually the same as the version fitted to my 1978 C90z2, and is designed to work with the newer sliding pinion kick starter.

The version intended for the 1969 ratchet kick start was the newer case with various bits machined off to accommodate the older style mechanism
... there had been no attempt to reinstate the stopper - not sure if that was just a case of penny pinching or they had concluded by then that the cure was worse than the disease:

top left: later case modified to fit the old kick start desing / top right: case to fit the later kicks start/ bottom: original CT90 K0 case

looking more closely, the kick-start stopper seems to have been a bit of a white elephant: the kick starter would have to rotate an improbably long way to reach the gears. Admittedly, if it did rotate this far then the small spring that keeps the ratchet against the starter gear would get knocked off and you have to open up the engine case to fix it. By comparison it would only need the kick start leaver to rotate 180 degrees (easily done if it is installed vertically) and the stopper would get broken off with potentially far worse consequences.

It turned out the replacement (modified) RHS crank case does not fit the cylinder. I had not noticed until getting to the point of fitting the piston, but the opening for the cylinder is a couple of millimetres smaller than the other side.

One possible explanation is that the case was intended for the S90 models fitted with thinner alloy cylinders, but at this distance of time it is hard to be sure. Anyhow I ended up fitting the original repaired case and returning the two I ordered from DS.

Crank

I did read somewhere that the s90 cranks have a different taper on the crank shaft, but I've had a few (possibly mislabled) ones from ebay and they have all been identical to the C90/CT90 versions. The big end bearings was a bit graunchy so I replace the connecting rod, pin and big end bearing with replacements from Toko Japan - one of the original manufacturers working for Honda and also replaced the by Naichi main bearings. You can read about the procedure here

Crankcase gaskets

Be careful you get the CT specific gasket that goes between the left crankcase and the alternator cover. The section of the gasket highlighted in red in the picture below (left) covers the oil return from the sub transmission (this part of the gasket is missing from the C90 version).