cutting reed windows

[a.lo.v2]

fitted a RS reed manifold to rapido without modding the piston to try out. more tractable engine but performance has suffered so will cut the piston but where? the opinion seems to be the hole is open at BDC but a trial marking on a piston puts this only a few mm below the rings and not like the ready modified items, is this right? also what is the correct way to cut the power port option on the skirt?

[olliewtf]

From memory I would leave a good 2mm from rings before cutting. remember that the vacuum only occurs as the piston ascends, so basic logic dictates that by the time the mixture is drawn down the manifold and inlet tract, the piston will already have ascended slightly, hence the lower down windows on pre modded pistons.

Be sure also to leave sufficient width either side so the skirt isnt compromised.

When i did mine it was all abit cowboy - i left a good gap either side (See MBs tech page for a figure, i wanna say 4-6mm but dont quote me) and cut one big window. I then cut an arc below, again hesitant of compromising the skirt too much.

I also cut a boost port in the barrel. I would strongly recommend doing so too, its an intrinsic part of the conversion and leaving it out is only going to lead to loss of power compared to your old set up.

[coaster]

I recently put an RS manifold on my Avanti and used an old TS1 piston to take a tracing. I cut out the windows and skirt cut away and then transferred them to the piston with felt tip. I needed to cut 2 rectangular windows so started with drilling a hole in the centre of each window and then increasing the size until I ran out of drill sizes (15mm) and then used the dremel to square them off leaving a radius in each corner. I used an angle grinder to remove most of the cut away on the lower edge of the skirt and finished off with the dremel. All holes need to be radiused to the bore as well to prevent oil being scraped off the bore. If you can wait until the weekend I could do another tracing and or give you accurate measurements.
Re the finger or boost port, I have played with these in the past on an Indian 225 barrel but as my Avanti has a plated bore I decided to see how it was without. I'm still running it in and need to get it on a dyno but up to 60mph it is comparable to the TS1 I was running until recently in terms of acceleration but without the boggy mid throttle and still has a real kick when it hits the power band. I do suspect that it will run out of breath further up the rev range though.

[Fruitjuice]

How important is the size of the piston windows?
How much improvement is found when maximizing the size versus keeping the hole smaller so not to compromize the skirt strength?

[coaster]

How important is the size of the piston windows?
How much improvement is found when maximizing the size versus keeping the hole smaller so not to compromize the skirt strength?

How long is a piece of string?? 2 x 15mm holes will get you close but how much improvement you will see with bigger 'optimised' ports will depend on a lot of factors such as carb size and exhaust and transfer timings and choice of exhaust etc. For a modest state of tune, possibly not worth going mad on the window sizes.