So I opened up the ports on my motor (dont recall exactly what the dimensions are now) and the bike sounds very distinct, more powerful. However I read the piston at TDC covered the intake port by 2mm, so I decided to check mine. Sure enough, the piston covered the port, but by a LOT more than 2mm (looked more like 5 to 7 mm), essentially half the intake height now. My question is, would it be safe to cut off the bottom 5mm of the piston skirt to open up the port more? or would that just be not recommended? I would assume the lighter piston would also allow for higher revs, having less inertial weight. Can anyone confirm? I want to make the motor run as good as I possibly can for cheap or free, I plan on buying new crank and needle bearings (high end ones, rated to 33,000 RPM) so those wont be an issue. I also planned on buying (or fabricating) a reed valve so I wont be balancing the crank for a while yet. Anything I do on the intake side, would it be recommended to do on the exhaust side as well? or can the piston be lopsided in weight?
Shaving piston skirt
- Thread starter mew905
- Start date
Hi ... Are you aware that you need to use a reed valve piston with a reed valve induction...the port piston you have now will not opperate very good...have a read up on how the different 2 strokes intakes opperate....1 piston port...2...reed valve...3...rotary valve......you need a different piston for each of these setups.....
i trim the skirt to match the intake port completely. the exhaust is fine, i leave it alone. don't think it can be lopsided in weight.
works great i do this to every motor runs way smoother..also tried a head off a honda qr 50 drilled out the stud holes to fit works great
I think you misunderstand the concept. I said I wanted to add a reed valve eventually, which means I'll add the boost ports and such when I get it (and I'm aware adding the ports without reeds would be counter-productive), I want to open up the intake a bit more by shaving the piston skirt and I was wondering if in the future it would hinder my ability to add a reed valve boost port.
However thank you for your input, everyone, I just wanted to know if it was safe to do and if there was anything else I had to do with it.
Like Bairdco said trim the intake side only, I have only done one so I'm no expert but its not hard. It seemed to balance mine a little better.
so, I shaved the chain case to prevent the chain from hitting it, runs way smoother and quieter now, and bike actually glides like its supposed to when clutch is in. shaved the piston skirt too, well... actually cut it so the WHOLE intake port is open when the piston is at TDC and made sure it would close completely when it was BDC. Sadly, however I lost all low end power, and only gained 1 mph top speed, played with the needle and jets to see if I could get some of that power back. The motor sounds beastly now, however, now that it can breathe. Any way of getting back some of that low end power? I can swap the piston for an unmodified one however I'd prefer not to. I have a Dax RT carb on the way I hope will help, and perhaps an SBP tunable exhaust would help? I also cut the exhaust and put a tip on it to make it quieter again (my roommate lost mine when it fell off, tried to use his but the dome shape was too big). Suggestions would be awesome. Bike doesnt vibrate at all until going around 30 mph now though, and I'm unsure if it can rev higher as I'm afraid to push it that hard, but it never gained much and I think its because of all the power I lost. perhaps I should get a new piston and cut less of it off? basically I cut the piston only so the whole port is open at TDC, so it still has some closed time. I'm thinking with how I cut it, reeds may be beneficial.
EDIT: I'm thinking what happened is all my mods (porting and such) moved the powerband out of reach for this motor, so a long expansion pipe and a good carb I'm hoping will help.
EDIT: I'm thinking what happened is all my mods (porting and such) moved the powerband out of reach for this motor, so a long expansion pipe and a good carb I'm hoping will help.
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yea i would def try the sbp pipe and tune ur powerband in, also the dax carb is great good luck.
I love the sound it makes now, though, pretty deep and throaty, just no low end power. I can try to record it at some point if you like, stock muffler too, sounds close to a high-revving four stroker now (without four stroking of course), but yeah, carbs on its way, and I'll be ordering the SBP pipe too, I'd custom fabricate one but I dont have that set of skills, nor can I find a premade online. I'd love to have jag's pipe but I dont know if hed be willing to sell them, $75 plus shipping sounds pretty fair, but hes also pretty far.
hey i like this thread, i was thinking about doing the skirt mod to my bike, but im not sure if i want to give up all my low end power, theres a pretty big hill in my neighborhood, and it happens to be the only way in or out.
i think the reason you lost all your low end power because that changes the port timing for the intake therefor leaving it open longer on that down stroke and losing some of your a/f mix back out the intake, a reed valve should solve this and dramatically increase the power a tuned pipe will then just be icing on the cake 
Yeah, but ill get the pipe before the reeds, but i noticed sometimes (particularly under 15mph) if i go WOT the motor bogs down, is there a way to fix that? And yeah im pretty excited for reeds though, but imma get some jb weld and stuff the case, and correct the transfers shape.
i've never lost low end power on any of the pistons i've trimmed.
from the picture, it looks like you cut out way more than necessary. mine is always a little half-circle.
is the trimmed part overlapping into the transfer ports? that would explain a loss of power.
from the picture, it looks like you cut out way more than necessary. mine is always a little half-circle.
is the trimmed part overlapping into the transfer ports? that would explain a loss of power.
No, it almost exactly opens the port 100% at tdc, you can see the cut edges through the intake port. As i said before i think the port timing is just way too wide, i have a spare piston i can swap out however but i want to try port matching and the new pipe/carb first to see if the gains will make it 100% usable again, otherwise im going to get reeds (or a boost bottle if someone thinks it may actually help), and go that route. My goal is to have a bike that can touch 70 or 80k without a shift kit.
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Ive have never done the skirt trim yet but from the pics Ive seen you made that way to big. that said since you plan on using reeds when you get the reeds you might not get 100% of the reeds benefit with just that big skirt trim but it will not hurt how the reeds work by no means.
if I was a betting man Id say when you get the reeds on you will not only get your low end power back but gain some over what you had.
if I was a betting man Id say when you get the reeds on you will not only get your low end power back but gain some over what you had.
I'm a betting man. You look silly in that Skirt Scott.
I go as to Yur looks like a a Scottish skirt Scott .
WILL THE REAL SCOTT D. LANG PLEASE STAND UP.
I go as to Yur looks like a a Scottish skirt Scott .
WILL THE REAL SCOTT D. LANG PLEASE STAND UP.
