piston swap

O

oylavabeer

New Member
today i tried to to swap pistons and bore into a good botom end and i tried to swap it with an old grubee piston, but the wrist pin is in a diferent hieght to the top of the piston meaning a lot more compession can be achieved. If u take some meat off the botom of the piston so that it will clear the crank saft this wont alter any valve timing issues. I dont have a grubeee botom end or con rod to test this out but looks like a big gain in compressionfor verry litle has anyone done this or will it stop it reaching top dead centre anh thaugth or experiences would be welcome
 
Removing the base (skirt) of the piston won't change the timing. Years ago when I worked on cars, we saw re-builders put a engine together as cheap as they could. I had one come into us that needed rebuilding. Upon tearing the engine down I discovered that the pistons had NO skirts. Valve or timing has nothing to do with the piston, in fact the timing will be the same without a piston (plug fireing) TDC of the piston (weather there is enough room or not) is TDC. If the piston travel is too short then you won't have enough compression to ignite the fuel. Too high..excessive compression... within reason. And the piston can hit the plug. Remember if the compression is too high, added head gaskets might help. How far above the top of the cylinder does the piston protrude?
 

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Changing the wrist pin to piston crown height in a 2-stroke motor will change the port timing. Port timing is the "valve" timing in 2-stroke operation principals.
 
I am not explaining properly .If Grubee botom end had one of the Motawissa piston in it top dead centre would see the piston height be increased by 4-5 mm, as the con rod must be longer ( forget about trimming the skirt that would be needed iff doing this in reverse, as I learnt) proably hit the head. I hill try to get some pics. And maybee a thicker base gasket will help with timing.
 
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use playdough on top of the piston with the spark plug pulled.
rotate the engine then pull the head to see how much room there is.
take an old spark plug & grind off the threads & stick it in the plug hole if worried about spark plug depth.
 
I always wanted to learn more practical information about cutting windows into the piston skirt for transfer... tricky stuff !
rc
 
"Most of that stuff needs a intake reed valve to work properly."

Tnx VMB.
...Makes sense... start messing with the timing of the flow of the charge and there should be a corresponding control for it... a reed valve !

A reed valve is most effective at bottom end, right?

Over the years I've never had the opportunity to modify an engine that was fully thought out in advance... and usually ended up with a bike that screamed at top end and was doggy where I really wanted the power.

Tnx
rc
 
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