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Old 12-27-2007, 08:43 PM   #1
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Default Pinch or no Pinch?

I was wondering what people's opinions are on the mystery of pinch. I understand the mechanics of it. what i'm wondering about is the amount of pinch on a broken in motor. A guy I used to race with that was very fast told me that he likes to break in his motors to the point where they have little pinch left. His thinking was that the friction of "the pinch" robbed power and he liked his motors loose. Like I said, he was always fast so I give considerable merrit to his opinion. But, the mechanics of it lead me to believe that when the pinch is gone, the motor is getting worn out to the point where you may be loosing compression.

The reason I bring this up is because I have a used motor that I got from a friend. The motor ran well the last time he used it but it has no pinch left so he gave me the parts to rebuild it. I think i'm going to try running the motor on a bench before I put these new expensive parts on it because it's my first 1:8 scale and I don't need to start out with a brand new screaming fast JP. Do you think i'll be wasting my time and fighting the tune with this thing? Or do you think it may have some life left for a newbie? BTW, the bearings felt great so i'm not concerned about them.

Sorry for the long post but i'm sure there's a lot of guys out there that are wondering the same as me about pinch.
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Old 12-27-2007, 10:53 PM   #2
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Im no expert, but the way I see it the motor is fully broken in when the pinch is gone. When you have very little or no pinch theres no friction therefore the motor is faster. What you want is a seal between the piston and sleeve when the engine is at running temperature, pinch or no pinch. When the motor is worn you will know, it becomes hard to tune (ie. it wont stay tuned) and you loose bottom end.
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Old 12-28-2007, 12:02 AM   #3
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Pinch is a cold situation and does not tell about the situation on temperature, that is all about the expansion of the chosen materials

But yes, on most engines you need to have a small pinch in cold situation to have a ideal fit on temperature. If the cold pinch is gone mostly there is a small leak on temperature which shows as less bottom power.

Thats why some drivers pre heat the engine so it starts up without pinch which will keep the fitting a longer life.
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Old 12-28-2007, 07:11 AM   #4
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what do you mean by 'pinch'?

the resistance at TDC when u rotate the flywheel?

btw, how do you actually control the amount of pinch during break-in???
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Old 12-28-2007, 08:49 AM   #5
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Quote:
Originally Posted by NitroWD View Post
what do you mean by 'pinch'?

the resistance at TDC when u rotate the flywheel?

btw, how do you actually control the amount of pinch during break-in???
Yes. Pinch is the amount of resistance you feel when turning the crank with the glow plug out at top dead center.

I believe you can control the amount of pinch you will have after breakin with temperature. The guy I mentioned above told me that he broke his motors in on a breakin bench at about 80 F. I know that sounds crazy and that the motor won't even run that cold (which is what I told him) but he said it was true. I have no reason to believe that he would lie to me. I think when you break a motor in at a cold temp. you are actually wearing away whatever pinch that would normally be there after a typical break in.
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Old 12-28-2007, 10:46 AM   #6
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leak down of the piston and sleave is important, you can have real good pinch

but a huge amount of leak down and that would make for a very hard motor

to tune
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Old 12-28-2007, 06:24 PM   #7
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leak down of the piston and sleave is important, you can have real good pinch

but a huge amount of leak down and that would make for a very hard motor

to tune
How do you check for that? Are you talking about the amount of compression with the glow plug in? Basically making sure there is no leak between the piston and sleeve?
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Old 12-30-2007, 03:57 PM   #8
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If the sleeve is a little bit out of round, you can have what feels like a good pinch, but the compression leaks down.
The piston can wear-in to an OOR (out of round) sleeve, and you can have decent performance, but it's obviously not an ideal situation.
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Old 12-31-2007, 09:12 AM   #9
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put the exh port to your ear and turn the motor over slowly if you hear

a lot of hissing that is a leaking sleave
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Old 12-31-2007, 09:29 AM   #10
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.......has anyone here ever done a leak down test on an r/c engine? Can it even be done? Can it be performed on an engine w/o rings? Would it even indicate the condition of the motor? Just wondering out loud.......
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