Cutting Piston for Reedvalve.

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Chris in Margate
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Tue Apr 18, 2017 6:24 pm

Been asked many times I know !

First ever attempt at a reedvalve in nearly half a century of stripping and building Lambrettas so now I need some sound advice.
Piston on con rod no rings and barrel on. Drawn a line to the extremities of the inlet manifold onto the piston at Bottom Dead Centre. Taken it out and permanet marked the area visible at BDC.
Got the tools, ready to cut BUT on this instance measure 3 or 4 times and ASK !
The top of the oval I have drawn is only about 5 or 6 mm from the bottom piston ring.
Is the vertical positioning of the hole or holes absolutely critical. I intend to cut an oval from the skirt also.
Something made me stop. It will take me a couple of minutes to cut and smooth so a short wait is not a problem. Many of the TS1 pistons I have seen have their cut outs level with the gudgeon pin.
In a nutshell, where do I cut ? I have read numerous threads but now I'm blinded by yhe ambiguity of them all.

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garry inglis
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Tue Apr 18, 2017 7:41 pm

have a look on edens gt big block blog he has covered what you want and also pics

eden
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Tue Apr 18, 2017 8:39 pm

you don't want to cut a hole as big as the inlet port,, your piston will fall apart.
you can just cut a 10mm high hole that is nearly as wide as the inlet port, basic rule of thumb is just make sure the window in the piston is in the port, or at least in the boost port slightly before the bottom of the skirt of the piston passes past the bottom of the inlet port.

Chris in Margate
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Tue Apr 18, 2017 9:17 pm

With an oval removed from piston skirt ?
So with the piston rising and the skirt just below the inlet port, looking into the inlet port should be a visible window in the piston?

eden
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Tue Apr 18, 2017 10:44 pm

when the piston it traveling down, as the bottom of the skirt reached the bottom of the port the window should just start to show at the top or in the boost port....

Some pics here
https://www.facebook.com/eden.bakewell/ ... 503&type=3

and

Image

Image

Chris in Margate
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Main scooter: Lambretta 1964 225 Special
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Tue Apr 18, 2017 11:05 pm

This is so very helpful.
It's going in an untuned Rapido so I guess I'll make sure that just before my scalloped skirt drops below the inlet, them my window will appear.
Will add boost port later on before rebore and 71mm piston.
Greatly appreciated.
Chris

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