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Piston Preparation

Pistons nowadays are coming out of the factory ready to be installed. Though, there are still some things that can be done to improve reliability and here are a few of them. Following examples & photos are for Honda nsr250 pistons but most principles apply on all two stroke bikes. 

a) Oil holes. 
If you are going to fit the pistons on a cylinder with a bridged exhaust port, you might want to consider adding a couple oil holes on the exhaust side if they are not already there by the factory. 
Diameter of the oil holes is dependent to exhaust bridge width but most of the time 1mm holes will be adequate. 
Dykem or a permanent pen becomes handy here again. Paint the exhaust side of the piston and temporarily install it on the engine. You should be able to mark the piston from the exhaust outlet by using a machinist scriber and this will reveal the available space to drill the oil holes. 
b) Ring grooves preparation. 
Exhaust side ring grooves should be filed with a triangular file or oil stone. Then they should be chamfered with #800 grit. This will give the piston rings a better life while the piston and exhaust bridge are both expanded. 
c) Piston skirt Chamfering. 
Chamfering the piston skirt edge will help on trapping oil between the piston and the cylinder wall and hence reliability. Chamfering can be done with metal files, wet & dry sandpaper or rubberized abrasives e.g Cratex as per the below picture. 
d) Rind ends preparation. 
Dress the ring ends with a diamond file to prevent scuffing. This should be combined with ring gapping, consult your service manual for correct clearances. 
Last but not least, thoroughly clean all the parts with warm soapy water, dry them up, lubricate them with your favorite 2T oil and they are now ready for installation. 


  1. Verry good write !

    I usually preparating the piston inside too . When molding piston , factory left some burr inside piston .If You polish correctly , the piston will be easyer .
    I like this side .


  2. Is this a cast piston ?

    I always read that you should only drill forged pistons, never cast ones, as they could later crack .... Now I see that you drilled a cast Vertex and I'm confused ...
    I have a Honda CR250 cylinder with a exhaust bridge and I am not sure if I can/should drill the cast Vertex piston I have already bought or just install it as it.

    I searched the web and got across this site from a cylinder repair shop from NZ: bridges.html

    A Note about OEM and cast pistons… The same cooling effect would be benefited from an OEM or any other piston. It’s not often recommended for cast pistons because the “sand-cast” piston will weaken at the metallurgical level when you drill holes in it unless you stress relieve and re-harden it.

    Cheers !


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