So I'm usually working on 4 stroke engines and new to this 2 stroke game. I've got a few ideas that I would like some opinions on. First has anyone had one of these cylinders decked or possible gotten a longer rod? 2nd is it possible to bore these little motors out abit? One question I have is about the pistons with the cut skirt. Whats does that actually do?
Mod Ideas and seeking advice if good or not
- Thread starter Sparky75
- Start date
Street Ryderz
Well-Known Member
You should try the search and you will find that all of these topic have been covered!mean while yes you can deck the jug,there are longer rods by 5mm,there is not enough plating to bore the cylinder and the cut skirt adds intake duration and allows the full volume of the port witch is other wise blocked!
2StrokeMama
Member
What he said^^^
- Why are you asking about the mechanics of the piston skirt?
- Why are you asking about the mechanics of the piston skirt?
Steve Best
Well-Known Member
Some guys trim the bottom of the intake side skirt because it doesn't quite clear the port.
I tried it but it affects the port timing and loses torque.
If you use a reed valve, you have to cut a hole in the piston skirt to gain full advantage of the reed.
I've decked cylinders and heads to change timing and increase compression, it works.
Compression helps low rpm torque at the expense of engine longevity. Be careful here.
Raising the ports (by decking and adding more basegaskets) gives more rpm and top power at the expense mid- torque.
Longer rods improve the rod/stroke ratio and reduce piston side loading. Improves high rpm power.
The only loss is if the rod is heavier, and it usually is...
Boring is not possible unless you want to re-plate ($250) or re-sleeve ($250) the engine.
An odd sized piston is worth more than a new engine.
The real gains are:
1) Improved combustion chamber shape. 2-3mm squish band, 0.7mm squish.
2) More rpm. HP=RPM x Torque so raise RPM = more HP. Generally about port timing.
3) More torque, generally done by better cylinder filling. Open ports at the bottleneck.
4) Tuned exhaust pipe. Big gain if the tuning is right.
5) Perfect jetting and timing. Both have to be adjusted for the motor.
I tried it but it affects the port timing and loses torque.
If you use a reed valve, you have to cut a hole in the piston skirt to gain full advantage of the reed.
I've decked cylinders and heads to change timing and increase compression, it works.
Compression helps low rpm torque at the expense of engine longevity. Be careful here.
Raising the ports (by decking and adding more basegaskets) gives more rpm and top power at the expense mid- torque.
Longer rods improve the rod/stroke ratio and reduce piston side loading. Improves high rpm power.
The only loss is if the rod is heavier, and it usually is...
Boring is not possible unless you want to re-plate ($250) or re-sleeve ($250) the engine.
An odd sized piston is worth more than a new engine.
The real gains are:
1) Improved combustion chamber shape. 2-3mm squish band, 0.7mm squish.
2) More rpm. HP=RPM x Torque so raise RPM = more HP. Generally about port timing.
3) More torque, generally done by better cylinder filling. Open ports at the bottleneck.
4) Tuned exhaust pipe. Big gain if the tuning is right.
5) Perfect jetting and timing. Both have to be adjusted for the motor.
2StrokeMama
Member
Thank you for the info 
!!! But I said why is HE asking about the piston skirt. LMAO
I just think he should figure out what he's doing with his cylinder before he bothers doing stuff in the unknown.
!!! But I said why is HE asking about the piston skirt. LMAOI just think he should figure out what he's doing with his cylinder before he bothers doing stuff in the unknown.
2StrokeMama
Member
And honestly, I didn't think it would be such a hassle to just purchase a $15 dollar piston set? Hmm. Interesting to know.
Steve Best
Well-Known Member
I am guessing he did read some of the back posts. A couple years ago trimming the bottom of the intake side skirt was all the rage. It does raise rpm through port timing but makes starting difficult and kills low rpm torque. Guys notice that the skirt doesn't completely clear the port so they trimmed it. One guy brags about it and a dozen others jump on the bandwagon. Jaguar, myself and others tried it and saw it for what it was. Yup, buy a $15 piston and try it if you really want to find the answer.
I worked with V8's for so many years. I should have started with these motors. So simple, so cheap, so easy to work on. I cut cardboard or paper gaskets all the time. I use teflon tape or aluminum cans or sheet for headgaskets. A completely destroyed engine is $140 to start over again. that is less than a piston in my KTM380. I can swap a complete top end in 15 minutes, or port a cylinder a bit at a time to see what works and what doesn't. What a learning tool.
I worked with V8's for so many years. I should have started with these motors. So simple, so cheap, so easy to work on. I cut cardboard or paper gaskets all the time. I use teflon tape or aluminum cans or sheet for headgaskets. A completely destroyed engine is $140 to start over again. that is less than a piston in my KTM380. I can swap a complete top end in 15 minutes, or port a cylinder a bit at a time to see what works and what doesn't. What a learning tool.
2StrokeMama
Member
Wow! That's insane. That does sound like a hassle lol. I honestly thought it would be fine to bore out the stock cylinder to fit an oversized piston. I've done it before and multiple times... but not on the assisted motor engines. I've never heard of replating or re sleeving the cylinder... why would you do that if your going after a larger piston?
Steve Best
Well-Known Member
These cylinders are soft aluminum with a thin flash of chrome for the iron rings to run on. If we bore the aluminum, we have to provide a hard surface for the rings to run on. Briggs and Stratton made aluminum cylinders for years, but chromed the piston to run in the bore and used special rings and aluminum to stand the wear. So for these engines we would have to re-plate with chrome or nickle/silicon or press in an iron sleeve for the rings to run on because we would have bored through the thin chrome plating.
I also have Yamaha Blasters which have iron sleeved cylinders. When worn out you just hone them to the next oversize and buy a larger piston. These engines don't have a selection of oversized pistons available. There are iron cylinders available from the Minarelli 50cc vertical engine that will fit, and they are available in a 46mm size as well. These could probably be bored up to 48 or 50mm. If you were to stroke the crank to 44mm you could get as much as 85cc. Not a great gain for all the work, it would probably make about the same HP but at lower RPM. That was my experience when I bored my Blaster 10mm and stroked it 3mm. Not much faster but more grunt, for a lot of work.
The stock Grubee cylinder it top center. Cast iron Minarelli is to the left, lower center is an aluminum Airsal Minarelli.
I also have Yamaha Blasters which have iron sleeved cylinders. When worn out you just hone them to the next oversize and buy a larger piston. These engines don't have a selection of oversized pistons available. There are iron cylinders available from the Minarelli 50cc vertical engine that will fit, and they are available in a 46mm size as well. These could probably be bored up to 48 or 50mm. If you were to stroke the crank to 44mm you could get as much as 85cc. Not a great gain for all the work, it would probably make about the same HP but at lower RPM. That was my experience when I bored my Blaster 10mm and stroked it 3mm. Not much faster but more grunt, for a lot of work.
The stock Grubee cylinder it top center. Cast iron Minarelli is to the left, lower center is an aluminum Airsal Minarelli.
2StrokeMama
Member
Very familiar with the minarellis I suppose the aluminum jugs are for cooling?
PS- Do you put those Mina's on your bicycle!? lol that'd be crazy, if possible.
-But to use those cylinders is very clever. Have you tried with the horizontal? Probably doesn't work, who knows tho... I never tried.
I suppose the aluminum jugs are for cooling?PS- Do you put those Mina's on your bicycle!? lol that'd be crazy, if possible.
-But to use those cylinders is very clever. Have you tried with the horizontal? Probably doesn't work, who knows tho... I never tried.
Last edited:
