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Sunday, January 15, 2017

HRC piston and ring prep info

This is some information on piston and ring preparation for the Honda RS125/RS250. These steps can be used on any high RPM 2 stroke used for racing. Small detailed tedious things like this can be the difference between failure and reliable use. HRC knows a thing or two about building GP 2 stroke engines. This is mainly used to prevent rings sticking, catching in ext port, and ring stopper pin damage. 

Cylinder
With increased power, the 2001 model year RS125R and RS250R have cylinder bore shape and timing similar to the manufacturer's kit engine.
Accordingly, cylinder maintenance has become more important than before.
Basically, it is recommended that the items described in the manual be followed. In particular, mentioned below is the description on the necessity for chamfering.
Picture
The chamfer on the upper surface of the exhaust port may be glittering when obliquely viewed from the skirt side of the cylinder taken from engines which have travelled to some degree.

The figure above shows a magnified view of the cross section of the port chamfer. The piston ring slightly protrudes into the exhaust port when the piston goes up and down.
This chamfer again rests in the bore surface. If there is, however, an angular (even obtuse-angle) section at the cylinder chamfer interface section, the piston ring movement gradually becomes bad, resulting in strengthened contact by the ring on chamfers.
For this reason, chamfer should be made. When chamfering is made for upper and lower ports, wet abrasive papers will be applied to the interface of the bore surface and the port chamfer at small angles (10 to 15 degrees) to make the interface round.
Chamfer maintenance would avoid stuck rings or peeled plate at the cylinder port.
Piston
•Piston contact
Piston contact should not usually be removed. Severe contact with the cylinder is attributed to some other reasons. Pistons have special profile (out of round), which is designed to produce heat emission, providing proper contact when pistons and cylinders are subject to deformation.
Accordingly, careless piston modifications in profile will degrade the ideal profile, resulting in more contact with other portions.
Ring sticking
With the increase of service engine speeds year after year, even if keystone rings are used, sticking may result.
If rings and pistons are new, ring groove modifications are not required against sticking. However, if stuck rings are found at the time of maintenance, the modifications must be done. Stuck rings may cause engine troubles as well as a significant power drop.
Stuck rings can be identified by signs of a failure exhibited at the engine start, such as unwillingness to start engines or abnormal engine noise emitted from the silencer. 
Piston rings
The abutment of piston rings should be chamfered at the gap using a round file. This is intended to prevent stopper pins from being cut by rotational force generated when pistons are moving up and down.
Picture 

All this info and more can be found at
http://www.sp125racing.com/rs125r--rs250r-notes-on-engine-maintenance.html

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