Cylinders / Pistons

Basics

The 250, 280, and 300 engines all used a common 60mm stroke and the same crankshaft.  Bore sizes, and actual displacement are as follows:

250 (72.5mm bore) 247.7cc

280 (76mm bore) 272.2cc

300 (79mm bore) 294.1cc

Each piston will be available in multiple sizes to make the piston-to-cylinder clearance perfect.

Size A: 72.46mm and 75.96mm and 78.96mm

Size B: 72.47mm and 75.97mm and 78.97mm

Size C: 72.48mm and 75.98mm and 78.98mm

GasGas, Vertex, and S3 all are possible sources for replacement pistons and rings. 

Note that it's possible to remove the cylinder and piston as an assembly without removing the engine from the chassis.  Of course, you must first remove the piston pin.  Reusing piston circlips is a controversial topic.  I'm not advising that you do so, but have personally done it many times without incident. 

Port Timing

These measurements were made with a degree wheel.  They were taken when I first got my TR280i.  I would not bet my life on them, but they should be close.

Exhaust duration: 158 degrees

Transfer duration: 135 degrees

Blowdown duration: 23 degrees

I then back-calculated that the connecting rod needs to be 130mm (center to center) to get dial gauge measurements to agree with the degree wheel.  

Update: The connecting rod center to center length has been measured at approximately 118mm. 

Piston Seizure

I know of two instances of seized OSSAs.  One was a trials model, accidentally run on fuel without oil.  The other was an Explorer that seized when it ran out of fuel while traveling at about 45 mph for several miles.  Although the recommended premix ratio of 110:1 is adequate for trials, it's asking a lot during extended high-speed running.  OSSA anticipated this, and the dual-map bikes have a richer mapping specifically for “road” use. 

After a seizure, the piston probably must be replaced.  If the seizure was very light, it's sometimes possible to sand a damaged piston smooth again.  But if there has been any aluminum transferred to the cylinder, sanding is not advisable.  Furthermore, if any rearranged aluminum has impinged on the ring lands, the piston must be replaced.

If the cylinder damage is just aluminum transfer, it's possible to remove the aluminum with hydrochloric (aka muriatic) acid.  But, if any peeling of the plating has occurred, the cylinder must be re-plated.  Sometimes imperfections in the cylinder plating are tolerated in areas above the ring travel or below the piston travel.

Note that Nikasil plating is very hard, and a diamond hone must be used.  A standard cylinder hone for iron won't work.   

It's probably not possible to determine the extent of seizure damage from photographs.  If in doubt, a local specialist should be consulted.