Carving Windows in a Piston

[Starwave]

Hello

I've got an LTH reed valve on its way and will be fitting it to a Rapido 225.
I'll be drilling a hole or a couple of holes in the piston but will, for now at least, leave the cylinder as it is - i.e. no boost ports and all the existing ports unaltered.
What I'm after is some advice about the position of the windows. Should I mark the piston through the inlet port whan the piston is at BDC and then remove material from that area? Or should the windows be positioned lower so the duration of some part of the windows lining up with the port is longer? I don't want to drill mahoosive holes (maybe 15mm or so?) and will bear their horizontal position in mind so they will line up with a future boost port and avoid ring pegs.

Hope this makes sense. I'm not convinced it does................

[oldbiker]

If at all possible I would try to keep the holes above the height of gudgen pin boss, only because that`s the way I`ve seen them on many of the jap bikes. Although the likes of the Yam DT and Blazer pistons have the windows quite low on the piston skirt


Try to avoid sharp corners in the piston windows and use a little emery cloth to remove any scratches or cutting marks. As these are all stress raisers.

Also I like to give the piston window a little more area than the boost port in the barrel (25% or so)

[chris2470]

Hi Starwave
TBH I would firstly fit it as it is for now, simply just matching the inlet to the manifold.
Otherwise you will have no base line reference as to what is working as desired and what isn't but you should get a noticeable improvement in bottom to mid range torque.
I would be interested to find out as I too have me eye on one.

[Starwave]

Cheers for your comments.

I suppose the holes are better positioned above the gudgeon pin to help ensure piston rigidity eh?

Probably am best off building the top end as is, like you say, for a baseline. Could then try piston windows only and then piston windows and boost ports. Was going to do the piston alterations because the bits are on my workbench and I don't want to be taking it apart again soon.......lack of time like everyone else. Also thought the reed without piston windows may be a bit of a restriction. That said, it might run sweet as and I leave it unaltered for years.......

Will hopefully post what happens when the scoot's up and running and being ridden with the reed valve fitted.

[sean brady scooters]

http://scooterotica.org/forum/viewtopic.php?f=7&t=6591

[Adspeed]

Think about your positioning first, also the angle of the cut to improve gas flow. Mark your holes in pencil on the piston, drill the four corners out but not too close to your lines. Take your time and use fine emry bands to finish........heres a GS Piston'd Polini 130 I did.

The holes want to be no wider than your inlet for sure, not sure what you mean by line up with future boost port?

[dsc scotty]

Why put holes in if no boost ports added? what is the point?

[Adspeed]

Why put holes in if no boost ports added? what is the point?

Fair point Scotty I missed that one........there is no point Starwave in using the reed, with or without holes unless you work the barrel!!

[Starwave]

I reckon some benefit will be gained from holes in the piston even without a boost port. Should help assist scavenging the cylinder I'd have thought. Probably gunna give it a go anyway.......
What I meant by positioning the piston hole to line up with a future boost port was I will consider the hole position on the piston so it will benefit from a boost port if I have one machined in the future. Hope that makes sense.
The essence of my first query was really where the window is best positioned relative to the inlet port. Not specific to a LTH reed, just a general 2-stroke thing. Should I mark the piston through the inlet when it's at BDC and remove material from that area?
Looks a very tidy job that piston's been treated to. Reckon I could achieve owt like that with a drill and a dremel? Can't see it me self.

[davidblythe]

Why put holes in if no boost ports added? what is the point?

thought the windows had another purpose which was to achieve 360 degree timing for the inlet port otherwise you ended up with a power ported reed valve