cutting piston skirt... even more??

[mew905]

I was curious, after cutting my piston I felt like I lost alot of power so I ditched it in favor of a stock ported GT5 jug and piston. After doing that, I noticed... I still havent gained any of the power back. Maybe its because the intake port is so small compared to the GT5-a, but I'm considering the 5a jug making a return to the bike, I thought blowby was an issue but after checking out the piston in the stock jug, it appears to have the same markings as my old piston did.

Here's my proposition: seeing as how my old jug and piston were faster, and both lack any sense of low end power, perhaps I could make up for it with even higher RPM's (dont worry about the bearings, I found an SKF dealer nearby where I'll be grabbing some better ones). My motor is already ported for higher RPM's, mostly because I screwed up with it, but if I make the piston even lighter, I should be able to raise the peak RPM and reduce overall vibrations anyway... cutting the piston window helped alot with them already, so... what if...

Noone saw an issue with cutting the window in the skirt, and it was only as high as the top of the intake port at TDC, so what if I just made it completely flat? I may lose compression (less material) but replacing the bearings I'll "stuff the crank case" as recommended by jag, as well as I'm going to try and get some reeds. Already have the Dax RT carb, SBP expansion pipe, and jag CDI, so reeds and a higher compression head I think would complete the package and perhaps get me my 70km/hr target

 

[Ron344]

You have to watch out that you don't cut the exhaust side of skirt too high becuse the bottom of piston skirt covers that port.On the intake side the skirt goes above the port.

 

[mew905]

alright, I'll probably mark it at TDC so I dont cut it too high and allow the intake charge out the exhaust, but I think being perfectly flat shouldnt be an issue. My goal is to make the piston lighter and allow higher RPM's and reduce vibrations without having to balance the crank (which I lack the tools to do). Not to mention balancing the crank will reduce vibrations at a specific RPM, whereas lightening the load ON the crank should reduce vibrations throughout the whole RPM range. Counterbalancing vs a lighter load, a lighter spinning object will weigh exponentially less than a heavier object at the same RPM.

According to a piston inertia calculator, a 55g piston at 1200 RPM (roughly idle):
Your Upward Inertia Force at TDC is 4.54 pounds.
Your Downward Inertia Force at BDC is 2.88 pounds.

45g piston at 1200 RPM (idle, cut skirt):
Your Upward Inertia Force at TDC is 3.71 pounds.
Your Downward Inertia Force at BDC is 2.36 pounds.

55g piston at 6500 RPM (redline):
Your Upward Inertia Force at TDC is 133.19 pounds.
Your Downward Inertia Force at BDC is 84.52 pounds.

45g piston at 6500 RPM (redline):
Your Upward Inertia Force at TDC is 108.97 pounds.
Your Downward Inertia Force at BDC is 69.16 pounds.

So with a 10g weight reduction, you're losing 25 pounds of inertial forces (assuming this calculator is accurate)

 

[ivan H]

Hi, I'm not sure its such a good idea to remove the skirt, its there as a means of supporting the piston in the bore, so it doent rock on the gudgeon axis. Check out Hi Po light weight pistons, they have a low profile on the sides but still have a skirt fore & aft @ 90 degrees to the pin for this reason. Probly wont break anything but it'll cause piston & bore wear (out of round) a lot quicker, & u'd only b guessing at a balance factor. Cheers

 

[mew905]

This piston is a full 10mm shorter than ours, doesnt have much of a skirt. Our pistons are a stock 48mm from dome to skirt end.

http://www.ebay.com/itm/Piston-Set-...3847&pt=Motorcycles_Parts_Accessories&vxp=mtr

Anyways, just for fun, I measured my piston (without the intake window, completely stock measurements, as far as I know, the only difference between the pistons of each kit is the dome size and the wrist pin hole height).

each ring is 2mm, the space between the rings is 2mm. the 22.5mm window is the chord length (straight line), not the arc length, arc length (curved line along the circumference) is 22.721mm if you really want to know. distance between wrist pin hole and window is 7mm, distance between wrist pin hole and bottom ring is 5mm, making the dome 5mm (rather hard to measure)

 

[ivan H]

Hi, its its not gunna break anything by cutting the skirt down, & piston r cheap, so give it a whirl, hey. Cheers

 

[mew905]

I appreciate your input and I didnt mean to sound condescending, That's what I was worried about when I first cut the piston: is the imbalance and it rocking around inside the cylinder. But after seeing such a short cylinder with a longer stroke (being advertised for 80cc motors and being so short), I think a short piston would be just fine, just much, much lighter. Only thing that worries me is the relatively soft metal the piston is made of.

 

[Tool Maker]

Moving the bottom edge of the skirt in relation to the intake port changes the intake timing. This is much more important for performance than piston weight.

Be careful how much you change port timing with any modifications to the skirt or port location. You can lose a lot of power by going too far.

.

 

[mew905]

Done. Cut the skirt. Now its 37mm on the exhaust side, 36mm on the intake side. Also added 1mm transfer ramps on each side of the piston, and rounded the edges slightly so they're not so blade-like. Lookin forward to seein how it runs once I do the crank case mods.

EDIT: and I'm aware changing the port timing can ruin the power, this piston has the port timing made huge, I wont be using it until I can confirm not only the exhaust port will be closed at TDC (if not, oh well, I'll just use my stock piston), AND until I get reed valves... otherwise this piston's pretty useless, though it's very rev happy and almost no vibration

 

[Ron344]

I hope it works, but I'm still concerened about exhaust port. I would look for a lighter racing piston with thiner rings to get more preformance.

 

[mew905]

Well, failure, 37mm is too short for the exhaust port, looks like its around 4 or 5 mm too short, and uncovers the port at tdc. Oh well, still got a spare piston. Then ill shorten it to 43mm, and chop off everything on the skirt except the port areas. Still no word on the weight of a stock 48cc piston + rings. But im going to cut the skirt and then drill holes as recommended on jag's site until i get reeds, after which i'll cut an intake window, rebalance the crank, and cut boost ports and auxillary transfers, as well as transfer ramps on the piston itself. My goal is to have enough power at the low-mid range to actually make you "hang on", with enough high end to pull up to safe-ish highway speeds (about 70-80kph). With 4 1/4" holes drilled into the muffler baffle and no cap (only because it fell off) theres a noticable boost in power around 40kph+

 

[mew905]

humm, well even after the failure I still went ahead and weighed out the piston. turns out it's actually a LOT heavier than I thought, at a whopping 75.4 grams AFTER cutting, putting the useless piston at 182.59 pounds upward force and 115.87 pounds downward. No wonder the 66cc motors are so unbalanced.

 

[Scott.D.Lang]

Id like a little more info in this. For one the jeg site you keep mentioning and anything about porting the jug and piston skirts for a reed set up. Ive ported many heads on cars for turbo and NA but this porting a piston and porting for a 2 stroke is all new to me.