If the depth of the scratches are minor, and the piston to cylinder clearance is acceptable, you can get away with honing the scratches out. Honing
must be done anytime you install new rings, in order for them to seat.
- Lack of oil on the cylinder walls can cause smooth bores, as is common with fuel washed cylinders.
- Rings not seating correctly due to an improper hone can also cause the cylinders to be smooth.
- Detonation can cause the piston skirts to scratch the cylinder walls.
- A piston installed backwards (420a pistons are 1mm offset) may cause the pistons skirt to scratch the cylinder walls.
- Improper piston to cylinder wall clearance can also smooth the cylinder bore and may scratch it as well.
And of course, any combination of the above mentioned points will both hasten and worsen the problem.