Holes in piston for reed conversion

[Dazza]

I'm fitting an atomic mini reed block to my Rapido 225, I have an avanti piston I'm at the moment which I hope to use but need to cut the holes in for the reed conversion.

I'm pretty ok with the practical side of doing it but I'm not sure where to put them exactly or whether to put two small holes or one large one.

I could do with dimensions ideally to transfer to the piston.

Cheers

[Adam_Winstone]

You also don't 'need' to cut holes in the skirt, rather, read up on 'power porting' which is the cutting of a big arc into the bottom of the skirt to greatly increase inlet timing (less than 360 degrees though). Note that the cutting of an arc is better than simply cutting across the entire inlet side of the skirt as it helps to align/support the piston and is less likely to result in piston slap as it crosses the inlet port.

My reason for adding the above into the mix is simply to make you aware that there are a number of options and that the old 'measure twice, cut once' almost applies here... but should be more along the lines of 'read up on your options, then decide what you want to do, then cut once,

Adam

[Adam_Winstone]

PS - If you search on line then you should still be able to find photos of the Avanti pistons that Ron was selling that were already cut for reed use. These had one big oval cut into the inlet side. If you choose to go down this route, make sure that you don't cut the hole so big that it gets too close to the edge of the skirt as it could promote cracking and have the skirt break off in use!

[Eden]

My GT230 last year ran with no window in the piston all year, I just cut a nice arc in the skirt.
The new GT250 I'm currently "running in" has the same arc and a small window, the reason I cut the window this time was because the 230 lost 2 bhp on the dyno with out the window compared to when it ran with a window.

[Dazza]

I think Ron is out of stock of the Pistons Adam, I had a look earlier , how do you determine how much of an arc to cut into the skirt ?

[gaz_powell]

AF do it for £15

when i was looking for a piston for a TS1 200 they offered a standard asso 200 with the arc cut in and not a hole(s)

[Captain Pugwash]

If you've got a high speed die grinder and a decent tungsten cutter and know how to use them, I don't think it will take long to do.
Only it's really easy to make a slip when cutting an arc into the skirt of a piston.
You also need to clamp the piston so it can't move, but without damaging it.
If your not 100% confident and Think you might wreck your piston then give it to AF's or A N Other tuner to cut the arc or windows (or both) into the piston for you.

Cutting piston windows is much easier than cutting the skirt as you can drill the openings.

If you want the windows any bigger or a different shape you can cut some more alloy off with a Tungsten cutter, but now because your cutting into a drilled hole it's much harder to slip than it is when your cutting at the open edge of the piston skirt.

High RPM using very little pressure and the application of some WD40 or paraffin will stop the tungsten cutter from clogging and it also makes the cutting a little smoother and less likely to catch and kick back when your cutting the alloy.

Good luck, Mark.

PS
Diamond burrs could also be used, slower but there much easy to control and they don't try to kick like a tungsten does if it digs in to the alloy, but again spray often with WD40.

[Scooterlam]

AF do it for £15

when i was looking for a piston for a TS1 200 they offered a standard asso 200 with the arc cut in and not a hole(s)

Recently did the same and got one of AF's woosners came cut with arch and small window.

[Dazza]

So something like this

With the window cut into the piston where it would be visible at BDC?

[Captain Pugwash]

Very nicely done
On my old Suzuki piston reed valve engine (this was back in the Mid 80's) I did very similar but I positioned the window about 6mm higher up in line with the boost port and it wasn't as wide as that one.

[Dazza]

Very nicely done

Not my work unfortunately, it's a photo gleamed from AFs website

[Wack]

I've not done one of these for a while but I think the best way to position the windows is to bolt the top end up with spacers instead of the head then with the piston at BDC you can scribe the sides of the inlet onto the piston,to give you an idea how wide to cut the window(s) and skirt .The window would then be cut about central to the inlet for maximum fuel flow thru them at BDC, then turn the crank slowly to TDC and as the windows start to disappear this is where to cut the skirt to open allowing a 360 degree inlet .Remember measure twice,cut once

[Dazza]

Think I've got it now , based on all of the above, going to have a go myself rather than splash out on a new piston ( that's my get out of jail free card)
Thanks for all your help chaps and I will report on the outcome..whatever it may be

[Captain Pugwash]

Think I've got it now , based on all of the above, going to have a go myself rather than splash out on a new piston ( that's my get out of jail free card)
Thanks for all your help chaps and I will report on the outcome..whatever it may be

Using tape to cover the piston might be a help, Mark it all up then only leave it uncovered where your going to remove alloy, the tape can help prevent unnecessary damage to the piston while you working on it.

[hullygully]

Put the piston back on your con-rod (minus the rings), slide your barrel down & mark/scribe the extremities of your inlet width onto your piston.
Take the piston back off & mark the centre line of the 2 scribed lines, then mark 2 more lines, 3mm either side of the centre line.
Then mark 2 more lines, one @ 18mm from the skirt upwards, the other 38mm upwards
You should now have 2 windows which you will have to radia the corners by probably 5mm
good luck

[a-teamlambretta]

Put the piston back on your con-rod (minus the rings), slide your barrel down & mark/scribe the extremities of your inlet width onto your piston.
Take the piston back off & mark the centre line of the 2 scribed lines, then mark 2 more lines, 3mm either side of the centre line.
Then mark 2 more lines, one @ 18mm from the skirt upwards, the other 38mm upwards
You should now have 2 windows which you will have to radia the corners by probably 5mm
good luck

you read that out of a book didn't you mickey love

[hullygully]

[a-teamlambretta]

Only joking mate, when are we dowling your head

[Dazza]

So, I've done the deed, fitted the LTH reed valve and cut the piston but I'm not happy...

There's hell of a lot of induction noise that seems unsettling - it's almost like an exhaust sound , not the usual induction roar but more of a 'crack' not a metallic or mechanical sound at all.

Other than that it starts fine and revs ok but I haven't been out on it yet...

I have a more than likely feeling that I've overdone the window in the piston which is causing the noise , I scribes the inlet onto the piston at BDC and opened it up to that , it was quite high up the piston so felt safe that there was plenty of meat left below the window and around it - have I been overzealous with the Dremel???

[dscscotty]

Reed induction cylinders do have a very different sound to piston port inlet. I wouldn't worry to much, I'm sure it will be fine. Did you cut a boost port from the inlet?

Scotty