pinch help

[hardhead]

ok I'm haveing trouble understandig what pinch is can somebody tell me what pinch stands for "good or bad" how it works and how to do it . thanks in advance harhead

 

[Amoeba]

ok I'm haveing trouble understandig what pinch is can somebody tell me what pinch stands for "good or bad" how it works and how to do it . thanks in advance harhead

There are 2 ways "pinch" is used in RC terms that I'm aware of, the "pinch test" where you pinch your fuel line and listen/time the revs before engine dies out to determine mix settings & possible airleaks or the "pinch" in the piston/sleeve. I assume you mean the latter.

Unlike gas engines which use piston rings and even cylinders, nitro engines use a lapped piston and sleeve design. This means that the cylinder bore becomes gradually smaller as the piston reaches the top of the sleeve. This gives you "pinch" when piston is at TDC (top dead center). This pinch is required to keep the piston and sleeve sealed for good compression when the engine gets up to temp.

Over time, the piston will end up wearing down as a result of friction which tapers the head of the piston to match the size of the sleeve when cold. When the engine gets up to operating temp the piston sleeve naturally expands. A bad pinch/worn piston head = the piston is now too small, allowing for blow-by and overall compression loss. Basically the cylinder pressure blows by the piston down to the crankcase, and it will cause the engine to overheat and stall. Unfortunately, the only way to remedy this is a complete rebuild with new piston, sleeve and connecting rod. You could do just a new piston or a new sleeve but because of the tight tolerances needed for good engine performance (and the existing wear on non-replaced piston/sleeve/rod) it's not recommended. Get them all at once and replace them as a unit.

Does that help?

An easy to use rule of thumb to determine if your pinch is still good on engines with a pullstart: Put car on the ground and pull the pullstart without holding the car down. If you can pull the cord up most of the way and your car rear end isn't lifting off the ground, your pinch is going bad.

 

[hardhead]

There are 2 ways "pinch" is used in RC terms that I'm aware of, the "pinch test" where you pinch your fuel line and listen/time the revs before engine dies out to determine mix settings & possible airleaks or the "pinch" in the piston/sleeve. I assume you mean the latter.

Unlike gas engines which use piston rings and even cylinders, nitro engines use a lapped piston and sleeve design. This means that the cylinder bore becomes gradually smaller as the piston reaches the top of the sleeve. This gives you "pinch" when piston is at TDC (top dead center). This pinch is required to keep the piston and sleeve sealed for good compression when the engine gets up to temp.

Over time, the piston will end up wearing down as a result of friction which tapers the head of the piston to match the size of the sleeve when cold. When the engine gets up to operating temp the piston sleeve naturally expands. A bad pinch/worn piston head = the piston is now too small, allowing for blow-by and overall compression loss. Basically the cylinder pressure blows by the piston down to the crankcase, and it will cause the engine to overheat and stall. Unfortunately, the only way to remedy this is a complete rebuild with new piston, sleeve and connecting rod. You could do just a new piston or a new sleeve but because of the tight tolerances needed for good engine performance (and the existing wear on non-replaced piston/sleeve/rod) it's not recommended. Get them all at once and replace them as a unit.

Does that help?

An easy to use rule of thumb to determine if your pinch is still good on engines with a pullstart: Put car on the ground and pull the pullstart without holding the car down. If you can pull the cord up most of the way and your car rear end isn't lifting off the ground, your pinch is going bad.

yes it does. How long should a motor run if it has good pinch? So when you pinch the fuel off its runing on whats in the line and the cylinder, a wore motor will let the compresion buy the sleeve which would cause the motor to die sooner . Am i right? And once the motor gets up to normal temps and is adjusted properlly it would crank when it first turns over,and if it has bad pinch when its hot it would be hard to start because of wear in the sleeve. I think i under stand pinch now. Thanks hardhead

 

[Amoeba]

yes it does. How long should a motor run if it has good pinch? So when you pinch the fuel off its runing on whats in the line and the cylinder, a wore motor will let the compresion buy the sleeve which would cause the motor to die sooner . Am i right? And once the motor gets up to normal temps and is adjusted properlly it would crank when it first turns over,and if it has bad pinch when its hot it would be hard to start because of wear in the sleeve. I think i under stand pinch now. Thanks hardhead

Proper motor maintenance (and good initial break-in) along with putting the piston at BDC when done running will help extend the functional life of the motor but eventually will have to be replaced since all mechanical things wear out. The BDC trick is because the sleeve will cool faster than the piston and if the piston is at TDC you are again increasing the wear on piston head and taper for good pinch. Some motors last for years of use, some not so good... it really depends on the manufacturer and maintenance. OS and SH engines tend to have good longevity whereas some racing-oriented engines (like Picco's or Novarossi's) tend to have a shorter lives.

You are correct that an engine with failing pinch will be difficult to restart when hot.

As for the pinch test (fuel line test) a properly tuned motor in good shape should run for ~3-4 seconds before increasing RPM's and dying out as the fuel in the line is used up. The increase in revs is due to the lean condition from less and less fuel.

Glad I could help. Rolex may chime in to correct or clarify anything I've said. he's an engine guru

 

[hardhead]

Proper motor maintenance (and good initial break-in) along with putting the piston at BDC when done running will help extend the functional life of the motor but eventually will have to be replaced since all mechanical things wear out. The BDC trick is because the sleeve will cool faster than the piston and if the piston is at TDC you are again increasing the wear on piston head and taper for good pinch. Some motors last for years of use, some not so good... it really depends on the manufacturer and maintenance. OS and SH engines tend to have good longevity whereas some racing-oriented engines (like Picco's or Novarossi's) tend to have a shorter lives.

You are correct that an engine with failing pinch will be difficult to restart when hot.

As for the pinch test (fuel line test) a properly tuned motor in good shape should run for ~3-4 seconds before increasing RPM's and dying out as the fuel in the line is used up. The increase in revs is due to the lean condition from less and less fuel.

Glad I could help. Rolex may chime in to correct or clarify anything I've said. he's an engine guru

ok you throwed somethimg else at me define BDC "before dead center"? I know what TDC is how do i do this pull the exhaust and make sure the piston is at its lowest point or does it have to be at it lowest point. And how soon should this been done after running it ? sry to be such a newb !

 

[MGT8.0owner]

Glad I could help. Rolex may chime in to correct or clarify anything I've said. he's an engine guru

lmao that funny lol rolex is the engine guru but his savy has never even started lol

 

[Amoeba]

ok you throwed somethimg else at me define BDC "before dead center"? I know what TDC is how do i do this pull the exhaust and make sure the piston is at its lowest point or does it have to be at it lowest point. And how soon should this been done after running it ? sry to be such a newb !

Bottom Dead Center (BDC) = when piston is at the bottom of the stroke/cylinder
Top Dead Center (TDC) = When piston is at the top of the stroke/cylinder

The easy way to figure TDC/BDC is to remove the glow plug, shine a light into the hole, and rotate your flywheel. Watch where TDC/BDC are and use a sharpie to mark the positions on your flywheel. Because of the way the con rod rotates there will be some wiggle room at TDC/BDC, it doesn't have to be super exact. I use black sharpie for BDC, Red for TDC on my flywheel so when I'm done running for the day I flip the flywheel with black facing up. Makes setting piston to BDC stupid easy.

Put the piston at BDC any time you turn off the engine just to get into the habit.

lmao that funny lol rolex is the engine guru but his savy has never even started lol

Why would he even start it? Aside from how dirty it'd get from running the engine, I don't think his Savvy would even be able to move under the weight of all that hop-up bling he's got attached.

 

[hardhead]

Man i feel like i took a smart pill thanks for schooling me in nitro engines !

 

[Amoeba]

Man i feel like i took a smart pill thanks for schooling me in nitro engines !

Heh. 3 months ago I didn't know jack squat but hanging out with the RCNT gang schooled me fast, they know their shizznit here.

 

[sweetdiesel]

Another way to see where BDC is (for the depth perception impaired ) is to look through the exhaust port.

 

[Rolex]

Glad I could help. Rolex may chime in to correct or clarify anything I've said. he's an engine guru

Nope, NAILED it!!

Only THIS Savage engine hasn't been started, but I've been dealing with nitro engines for years. Some of those engines are still dirty. :whhooo: True.


Note the 'day glo' yellow dot on the right of the flywheel. When it's on top, the piston is at Bottom Dead Center.
The easy way to find it is to turn the flywheel from side to side till you feel the resistance. Turn left and as the piston enters the cylinder, you'll start to feel the compression. Turn it to the right, same thing. Do this a couple of times and mark the center of those two points and you have BDC.

 

[Suicide]

cmon break it in so you can show it off to everyone at the bash.

 

[Amoeba]

cmon break it in so you can show it off to everyone at the bash.

He can't, it hasn't been dipped in gold yet.

:

 

[Suicide]

golds too soft rolex should it would melt haha

 

[hardhead]

Thanks for the help fellows i hope i'll be able to return the favor soon

 

[Rolex]

Glad we could help. Just send me some gold leaf.