Consensus on piston lightening

YesImLDS

YesImLDS

Member
I have spent the last couple days really looking into make the stock piston lighter. Some recommend taking 1/4 inch off the intake side and calling it a day, some day taking a couple mm off the exhaust and some even say to make a big window on the intake side. Some of the things that I realized came up in every thread was port timing. Some even go as far as making massive still holes throughout the piston. You are messing with port timing and you can cause ill effects on your few grams you have taken off the stock piston.

I guess what I am trying to figure out is there a right way to do this mod to lighten the unsprung mass of the engine? Something else that came up is that between the 2 stroke 49cc and the 66cc the crank (or it might be the rod, please correct me if I am completely wrong) is the same on both models making especially the 66cc quite unbalanced. By taking away mass on top it should function more like a 49cc would and stop as much vibrations as higher rpms. As long as you aren't opening up the exhaust during compression and you aren't over exposing the intake side so the gas going into the engine doesn't come back out through the carburetor you should be in good shape.

So is there a right way to lighten the piston safely without any bad effects? Is there any standard measurements that people go by on places to trim and lighten?
 
Lifting the inlet skirt is common practice, it has more to do with correcting the inlet timing than it does piston lightening however and can 'ruin' your low down power if an excessive amount is taken. But also increases potential power in the higher revs.
The only time windows should be cut is for reed valve conversions.
There is very little that can be taken from the exhaust skirt before the exhaust port becomes open to the crankcase, however depending on base gasket a mm or so can be taken.
The primary way to reduce weight in a piston is to make its skirts thinner by removing metal from the inside of the piston, some pistons are lighter, but most are 84g.
My lightest modified piston, which has windows, full skirt lift and lots of metal removed from anywhere I could, is 68g.
Piston lightening is unlikely to cure an unbalanced crank, however can lift the point of unbalance, but in my experience only by a few hundred RPM.
 
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So would you say 1/4 inch would be a safe amount to remove from the intake side? As well as a couple mm on the exhaust side as long as it's not opening during compression?
 
1/4 inch depends on a few things, I never cut a piston skirt without checking the intake duration with a timing wheel, 120 deg of intake duration is ideal for low down grunt, but it depends on the 'tune' of the rest of the engine, different barrels and strokes will give different port timing, some people like to lift the inlet skirt so that the inlet port is fully open at TDC, but this is likely to give poor low end, 3mm cut on the inlet skirt is often enough to improve performance while maintaining good bottom end, I would include some photos, but both camera batteries are flat, and I have not worked out how to repost old photos.
Lifting the barrel will decrease inlet duration, so it depends on base gasket thickness also as to how much cut you take from the piston, as the higher the barrel, the more inlet duration you will want to complement the increase in transfer and exhaust duration.
 
Theon has done more than I but what I do is look in the intake port at the piston skirt at tdc, some of the newer designed pistons totally clear the intake port. Two motors I have that are from eBay both needed piston trimming on the intake. My grubee gt2b 48cc totally clears at tdc, I did clean it up a bit and ramped the top of the piston at the transfer ports. The grubee is the smoothest running with little to no vibration. One thing to remember, if you take one mm off the skirt you are in effect adding twice that in duration because the skirts starts opening sooner and stays open longer.
 
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Theon said:
1/4 inch depends on a few things, I never cut a piston skirt without checking the intake duration with a timing wheel, 120 deg of intake duration is ideal for low down grunt, but it depends on the 'tune' of the rest of the engine, different barrels and strokes will give different port timing, some people like to lift the inlet skirt so that the inlet port is fully open at TDC, but this is likely to give poor low end, 3mm cut on the inlet skirt is often enough to improve performance while maintaining good bottom end, I would include some photos, but both camera batteries are flat, and I have not worked out how to repost old photos.
Lifting the barrel will decrease inlet duration, so it depends on base gasket thickness also as to how much cut you take from the piston, as the higher the barrel, the more inlet duration you will want to complement the increase in transfer and exhaust duration.
Click to expand...

I am not an expert at all in any of this so bare with me. How much lifting the inlet skirt can effect performance? If you were to lift it so it matches your inlet port before compressing completely wouldn't that just give the base more gas/oil mixture, or would the force of the exhaust stroke push that full back through the carburetor? I guess I don't quite under stand. I do understand the exhaust side though. As long as it's not opening when at TDC you should be good to trim whatever is below it correct? As long as you don't go overboard. This should make these engines "more balanced"
 
Too much inlet duration will let charge back through the carb at low revs, as the revs increase, momentum of charge will help 'hold it in'.
Where is your out of balance?, As I explained, piston lightening may lift your out of balance point a little, it may make the vibrations a little less?
Leave enough piston skirt on the exhaust to give a good contact and seal.
What power curve are you after? There is no point in lifting the power curve if it puts your max power in your out of balance zone. These motors can give excellent low down and mid range with conservative inlet duration, giving a more usable power curve, think V8 vs Rotary, The rotary may be fast, but the V8 is nicer to drive.
With 4 bikes, two are made for revs, with properly balanced cranks, and extended port duration, The other two are built for grunt, pulling strong from just above idle. Depends what your after, and how well balanced your crank is. An out of balance crank will be a real pig in a highly tuned motor, but may be bearable in a more conservative motor.
How fast do you need to go? Or do you want a bike with good street manners?
 
I'm more looking for mid range to high range. The bike is usually cruised around 25-35 with a 36tooth sprocket. I was more so hoping with lightening the piston for less rotating mass or unsprung mass therefore needing less fuel to turn it over and to smooth it out in higher rpms, should also increase longevity of the engine itself by not shaking itself to death as much. I don't specifically want to go any faster, I just would like some better acceleration to my top speed, but with lightening the piston more revs would be available and in turn more speed. I don't want to really get into the crank as this isn't really going to be a really powerful motor, more so a fun project to keep me busy instead of sinking thousands into another project car. I guess what really sparked this rebuild of the top end is the fact that I bought a buddy the same engine and he seems to smoke me in the low end and I barely catch up in the high end and I wanted some more free power and this was one of the few options with a dremel!
 
you can run the long ZAF 60 rod and grubee gt5 type high wrist pin piston , this setup is lighter than all the others out the box by a few g..... then mod to your liking

i am running this setup in my stufffed and balanced 38mm stroke hybrid motor and its the smoothest motor i have had yet !!!
 
Sounds a little more pricy then I want to shell out. More looking on the free side, like how porting helps quite a bit
 
To get the most from these motors I recommend using a timing wheel to get an accurate idea of your port durations, doing a squish test to get an idea of how much compression ratio you have, when done, do a compression test to get an idea of how much actual compression you have.
Good gains will be had from port matching and improving the transfers, getting your compression ratio up and also making sure that you have adequate ignition retard, as some engines do not have enough.
I can get a motor to be strong all the way through the rev range to 8000+ RPM with only 120 deg of inlet duration. But to get max power above 8000 if that is what you want, it is a matter of sacrificing low down, and then I recommend a good pipe to help complement a lifted barrel, wider ports, longer inlet duration and a better balanced crank.
 
the piston skirt is actually wider than the piston dome, they do this because the dome feels most of the heat of the combustion s iy expands more thus to have the piston expand evenly the skirt at a lower temp must be larger in diamiter than the hot dome so they expand to match and fit the bore... if you trim 1/4 in off the entire intake side of the piston the skirt will fit real loose and there will be a bit more piston slap on the exhaust side and it will where a lot faster... the top diamiter of the dome of a 70cc,... 47mm bore x 40mm stroke dax low wrist pin piston is 1.837 in but the skirt at the lowest point is 1.850 in in diamiter thats a .013 of an inch differance !!!! its made this way so the hot dome can expand more than the skirt thus making a perfect match at operating temp...

this being said i have removed 1/4 in off 3 of my drag bikes and with a lowered intake port it makes for around 125 deg duration and a great top end bikee !!! just dont expect the piston to last that long LOL mine get srs scoring on the exhaust side after this mod. it does lighten the piston a lot by removing the whole intake side though and that can be a benifit if a all out race / drag bike that is going to be rebuilt every other race is what your after

there is a good compramize though and that would be to lower the intake port as far as you can to add duration and widen it as far as you can to increase the amount of flow/lift, then mark a half moon at top dead center where the piston covers the port and remove that part of the skirt making shure to maintain a decent portion of the outer edge of the intake side skirt, if you need more intake duration you can just raise up the half moon cut up the skirt a little bit at a time till you get where you want to be.

also make damn shure your exhaust is free flowing and port matched, if your still running the stock exhaust i sugest cutting off the headder tube and splitting the chamber and gutting all the baffles and catalitic converter from it and paying jiffy lube or whoever 10$ to weld it back together... then drill the stinger out to the desired back preasure and you will have a makeshift expantion chamber !

another neat trick iff you have a steady hand it to hold the heaad on the jug and the jug in your hand and look down ito the cilynder from the bottom and you should see that the stock head combustion chamber is actually too small for the 47mm bore and a 3-4mm flat spot over hangs into the jug before the chamber squish band starts.... if you hold it tightly together and scribe the head around the inside edge of the jug you will now have a guide on your head to expand your squish band ! all you need to do is carfully grind off the extra material with a rough sanding wheel then finnish it off with a beartex wheel and poish the chamber, then you can deck it down to the desired compression ratio.

its veerry easy to widen the squish band on these stock heads simply by marking them on the jug and carfully grinding them out at the same angle as the piston dome.

ohh and if your only getting 25-30 mph on a 36 t rear them my man you have a srs problem and its probably in your carburation !!! tune your carb so it 4 strokes when on a flat and under no load but quickly snaps out to a clean 2 stroke when the throttle is twisted.
 
if all out light top end is what you after i must warn you it will not eliminate the vibratios LOL you see its a single ciylinder so there is no way to harmonize it just move the vibrations around the rpm range by adding or subtracting LBs from certain arias, if you lighten the top the vibrations tend to move up the rpm range, the trick is to lighten it just enough that you dont impact the rotational inertia and cause stalling but move it enough to get it over your max roms so you can never actually get the engine up to an rpm that it ill vibrate at !

i find the best combo for no vibes and super high rpms to be a sinlge piece full circle 38mm crank with the drilled holes stuffed with JB weld to add wieght to the crank then run the long 115mm ZAF 60 con rod and the type b high wrist pin piston, this rod and piston are bolth lighter than any other rod and piston combo by almost 3 gr ! then remove the intake side skirt and grind out the inside of the piston wth a round stone to smooth out the casting flash and also tohten it up a bit more this allong with a titainium wrist pin and a stihl chainsaw bearing or a puch moped needle bearing will make your top end considerably lighter and the extra whieght on the crank will add to its rotational mass and give a stronger launch and more ummmph.

so from my experiance its actually a heavyer crank and a lighter top end with a longer rod that realy gets thease motors ripping up in the 10,000rpm range !
 
Ended up only taking a few grams off between the intake and exhaust side and the inner side of the piston. May drill sine holes in it tomorrow in places that never see any port for a little extra oomph. Surprisingly I probably took only a mm more off the intake side. The exhaust seemed to have quite a long sorry on it. Still should seal perfectly though. I'll post pictures tomorrow
 
Here is the exhaust side. Left enough for a nice seal without threatening any structural aspects.
R5wilhW.jpg


And here is intake side. I didn't grind very much here because I didn't want to mess it up.
EkjzHyM.jpg


After everything I went over it with 2000 grit sandpaper to get a very smooth finish because of a couple oops with the dremel slipping.

Still plan on drilling a few holes that won't every be shown using the drill press tonight
 
I also drilled 4 3/16th holes (2 on each side) and put everything together after a port and polish of the intake and exhaust side. Also threw on a dax slant head with slightly more compression supposedly. I can hit 39 on flat land now instead of around 35/36. I am assuming it's because of a combination of all the things above. Cruising rpm don't shake as bad either. So I say it was worth it. Not exactly a night and day difference, but for a total of maybe $25 invested with the dax head included it was a decent improvement.
 
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