hey guys i was reading about porting and skirt trimming on here the other day it seemed like a good idea to me to trim the skirt on my piston on the intake side. so i put my piston at top dead center and marked it out and trimmed it and now when my engine is running at high rpms it feels like the piston is going to shoot out the top of my engine it just vibrates way to much. i dont know if i did it wrong or what. what do you guys think?
porting and piston skirt trimming
- Thread starter tantheman
- Start date
I've never tried what you did, but I've been thinking about doing the same thing. Other than more vibration, what else do you notice?
Not really knowing anything about the effects this would have on the tuning, I'm guessing it would tend to move the power band higher in rpm, maybe at the expense of bottom end.
As far as making the piston slightly lighter weight, from my experience with balancing these motors, I would guess it should be an improvment in balance if anything. This would be based on the fact that most of the cranks I've looked at were too light of counter weight. So reducing the weight of the piston assembly would at least be in the right direction.
The piston skirt does help control rocking couple movement of the piston in the cylinder though. It's possible if the piston is a loose fit in the cylinder to start with, cutting the skirt might not be good.
What's the chance of a china girl motor have a loose fitting piston to cylinder?
Oo
Not really knowing anything about the effects this would have on the tuning, I'm guessing it would tend to move the power band higher in rpm, maybe at the expense of bottom end.
As far as making the piston slightly lighter weight, from my experience with balancing these motors, I would guess it should be an improvment in balance if anything. This would be based on the fact that most of the cranks I've looked at were too light of counter weight. So reducing the weight of the piston assembly would at least be in the right direction.
The piston skirt does help control rocking couple movement of the piston in the cylinder though. It's possible if the piston is a loose fit in the cylinder to start with, cutting the skirt might not be good.
What's the chance of a china girl motor have a loose fitting piston to cylinder?
Oo
im not to sure about the piston being loose, but it does have more power in the higher rpms thats for sure i noticed a big difference this is only the third china girl ive tinkerd with tho so im still learning alot about them. i also only took about an 8th" by 3/4" wide notch out of the skirt just barrely enough to open the intake all the way at top dead center
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so id did help with the vibration on your motor then? cuz it made mine vibrate so much that its not even very enjoyable to ride anymore. and it sounds like its going to blow up haha i dont know what to do about it i think i might just buy a new piston or something.
How did you notch the piston without removing it from the cylinder?
This is a modification that I do to every engine I build and I've never had a vibration problem. I can't imagine why you're having this situation.
Start checking the usual things like engine mounts, chain tension anmd alignment, especially the rear sprocket for concentricity to the hub. Trimming the piston skirt should not have caused the vibrations you describe.
Tom
i did remove the piston before i trimmed it and i just trimmed it with my dremel and i just looked at my engine mounts and i noticed that the welds have vibrated loose. i just installed boost bottles and nos also and that made it not vibrate near as much as it was.
AMAZING!! I have a couple bikes that were (and I say WERE) absolute dogs. no, snails. bog central, no performance at all. turns out those cheap zminglu (ebay seller) engines are the old russian ones with center plug low compression heads, and pistons that only opened 1/2 of the intake port!!
Pistonbikes Rob showed me how to do it. remove manifold, scribe piston skirt at TDC, remove the jug, remove the piston. (waay easier than trying to clean shavings out the case-) and easier to make a good grind.
cover the piston in your hand with a rag, so as not to fill it with shavings. use an air grinder to match the grind to the scribe you made. make it smooth, and dont go deeper than the scribe or you'll change the port timing. BAD! (you can go wider, that dont really hurt). but not much wider.
for balance I chose to grind the other side of the piston skirt about 2 mm evenly to compensate for the weight loss on the intake side. If you do this MAKE SURE NOT TO GRIND ENOUGH THAT IT OPENS THE EXHAUST PORT WHEN CYCLING!!!
I did this to the first bike, and combined with a newer HC slant head and b6hs, the thing runs like a champ. so I did it to my red bike. took 1/2 the time. maybe 30 min total, and same results. and no vibration problems. micargi or electra. awesome solution. now the dogs are runnin strong.
then for S & G's I peeked at the new Silver Jet on the race bike I'm building. the piston totally clears the port. no need. so if you have an older motor and its an unexplainable pig, check the piston skirt in the window.
Very worthwile and easy Mod.!
Pistonbikes Rob showed me how to do it. remove manifold, scribe piston skirt at TDC, remove the jug, remove the piston. (waay easier than trying to clean shavings out the case-) and easier to make a good grind.
cover the piston in your hand with a rag, so as not to fill it with shavings. use an air grinder to match the grind to the scribe you made. make it smooth, and dont go deeper than the scribe or you'll change the port timing. BAD! (you can go wider, that dont really hurt). but not much wider.
for balance I chose to grind the other side of the piston skirt about 2 mm evenly to compensate for the weight loss on the intake side. If you do this MAKE SURE NOT TO GRIND ENOUGH THAT IT OPENS THE EXHAUST PORT WHEN CYCLING!!!
I did this to the first bike, and combined with a newer HC slant head and b6hs, the thing runs like a champ. so I did it to my red bike. took 1/2 the time. maybe 30 min total, and same results. and no vibration problems. micargi or electra. awesome solution. now the dogs are runnin strong.
then for S & G's I peeked at the new Silver Jet on the race bike I'm building. the piston totally clears the port. no need. so if you have an older motor and its an unexplainable pig, check the piston skirt in the window.
Very worthwile and easy Mod.!
Seems to me that ANY trimming on a piston skirt could easily throw an otherwise well balanced engine off-balance.
Crank will need to be lightened a bit to compensate.
MSRFAN posted a thread on chinagirl crank balancing... looks good to me!
Try it!
...don't trim front of piston skirt to compensate for what you have done to rear (intake) side!!! Does not equate!
More study is required before undertaking such a venture.
But I'm all for it!
rc
Crank will need to be lightened a bit to compensate.
MSRFAN posted a thread on chinagirl crank balancing... looks good to me!
Try it!
...don't trim front of piston skirt to compensate for what you have done to rear (intake) side!!! Does not equate!
More study is required before undertaking such a venture.
But I'm all for it!
rc
I have done it to four of my motors with no ill affects . I have never noticed any worse vibrations from doing this and have spun my motor past 8000 rpm . And yes this motor is still running .
Well that sure sounds impressive HT!
If it works, it works!
I wasn't up to speed for a race, anyway... lol
My little 48 just plugs along.
I gotta find the wrist pin bushing size again so I can buy one before it cuts loose. That's all the work this one will get.
Best
rc
If it works, it works!
I wasn't up to speed for a race, anyway... lol
My little 48 just plugs along.
I gotta find the wrist pin bushing size again so I can buy one before it cuts loose. That's all the work this one will get.
Best
rc
I did this mod. after reading this thread and checking the piston on my BGF 48cc. My piston needed about .250 removed to open the port fully, also found casting burrs in the intake port near the piston. After trimming the piston and using a chain saw file to deburr the port the results were impressive, three [3]miles per hour increase up the road that is uphill from my drive for about 1/8 mile. Strange that vibration seems less after this mod.
The concept of removing a like amount from the other side of the piston to 'compensate' for material loss/balance is wrong thinking. In effect you're removing twice the amount of material you need to from the total piston mass which might have an effect on balance. There is no need to match the sides of the piston. Remove only enough to clear the intake port then carefully smooth and polish the grind area to reduce the possibility of stress cracks. I go so far as to polish the grind area on a bench buffer so there is no grind marks.
Whatever you do don't try this modification with the piston in the cylinder and the cylinder on the case (block). There's too much chance of getting filings into the lower end no matter how well you clean afterwards. Take the piston out of the engine.
Tom
Whatever you do don't try this modification with the piston in the cylinder and the cylinder on the case (block). There's too much chance of getting filings into the lower end no matter how well you clean afterwards. Take the piston out of the engine.
Tom
