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Old 04-26-2010, 03:41 PM   #1
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Default Engine Pinch Vs Compression

Hey guys I have been looking around for a manual or some Tech tips on Nitro Engines. I currently am new to the Off road nitro scene. Thank you guys.

What I would like to know is what is Pinch and how is it Lost?

What is or how do you check your compression?

What should an engine feel like after a gallon of fuel thru her?

Is it that bad to run an Engine Below 180F?

Is it worth sending an engine out to be race tuned/ Broken in?

Is it worth getting a Hand Fitted Piston?
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Old 04-26-2010, 03:58 PM   #2
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Originally Posted by PsycoB4 View Post
Hey guys I have been looking around for a manual or some Tech tips on Nitro Engines. I currently am new to the Off road nitro scene. Thank you guys.

What I would like to know is what is Pinch and how is it Lost?

What is or how do you check your compression?

What should an engine feel like after a gallon of fuel thru her?

Is it that bad to run an Engine Below 180F?

Is it worth sending an engine out to be race tuned/ Broken in?

Is it worth getting a Hand Fitted Piston?
Pinch is usually referred to the engines mechanical pinch when it is new, you can check this by taking out your glow plug and turning over your engine by hand, any resistance around the top of the piston stroke is pinch. Pinch is lost as the engine breaks in. Compression is lost as the engine wears out.

You can purchase a compression gauge from Dynamite to check your engines compression, I've never worried about it, when an engine starts getting hard to start when hot and doesn't hold a tune is when I know it's in need of repair or replacement.

What an engine feels like after a gallon is hard to say, it will depend on how patient you were with the break in, how well you tune and how high the quality the engine is.

I wouldn't worry about temps, the first mistake we all make is to assume our temp gauges are reading right to begin with. Always tune to keep your engine happy with a good smoke trail and a steady tune. Also read the tuning guide at the top of this forum, it will help you immensely.

If you don't feel comfortable with breaking in you're engine then you could purchase an already modded and broken in engine, like the EB mod's JS.21 for instance.

Hand fitted pistons are intended to eliminate engine break in. for you just getting into nitro, don't worry about getting a high dollar modded, hand fitted piston engine. You will most likely smoke that motor in 3 gallons just learning to tune, we all have.
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Old 04-26-2010, 04:33 PM   #3
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Go here and read what Ray wrote (and what Slider wrote above)...and the pictures and it will all become clear to you ;-)

http://www.rayaracing.com
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Old 04-26-2010, 04:49 PM   #4
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Originally Posted by Slider30250 View Post

Hand fitted pistons are intended to eliminate engine break in.

I don't agree with this, but it does help that a piston isn't overly tight in the sleeve which can cause havoc during the first running. Break in is needed to lap the piston to the liner to obtain a perfect seal. There is no amount of hand fitting that can do this.
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Old 04-26-2010, 04:55 PM   #5
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I don't agree with this, but it does help that a piston isn't overly tight in the sleeve which can cause havoc during the first running. Break in is needed to lap the piston to the liner to obtain a perfect seal. There is no amount of hand fitting that can do this.
I guess my explanation is kinda over simplified, but with a hand fitted piston you don't really have to do a traditional break in. Just tune for a good smoke trail and a happy engine and leave it at that. The piston will still do it's final fitting to the sleeve without the the extra stress.
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Old 04-27-2010, 08:58 PM   #6
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Thanks guys, any more info and advice will be helpful.
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Old 04-27-2010, 09:09 PM   #7
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I guess my explanation is kinda over simplified, but with a hand fitted piston you don't really have to do a traditional break in. Just tune for a good smoke trail and a happy engine and leave it at that. The piston will still do it's final fitting to the sleeve without the the extra stress.
Sometimes you are not able to tune by smoke. Some fuels dont use a lot of smoking agent in their fuel, such as Nitro Juice. Reducing the amount of smoking agent keeps the engine cleaner on the internals in my experience.
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Old 04-28-2010, 12:40 PM   #8
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Thanks guys, any more info and advice will be helpful.
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Old 04-28-2010, 01:09 PM   #9
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Pinch is the force you feel when you turn the crankshaft all arround without a glowplug mounted. Compression is the extra force on the pinch you feel when you turn the crankshaft arround with a plug mounted.


Testing the compression can be done by holding the engine at the flywheel and let it hang on the compression.



- when it stays hanging it is as new.
- when it takes a few seconds to fall through it is OK
- when it falls through within a second the engine is gone....
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Old 04-28-2010, 03:00 PM   #10
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Originally Posted by Roelof View Post
Pinch is the force you feel when you turn the crankshaft all arround without a glowplug mounted. Compression is the extra force on the pinch you feel when you turn the crankshaft arround with a plug mounted.


Testing the compression can be done by holding the engine at the flywheel and let it hang on the compression.



- when it stays hanging it is as new.
- when it takes a few seconds to fall through it is OK
- when it falls through within a second the engine is gone....

Thats a interesting way to test. I've never heard of that before. It makes me want to go take the motors ofut of my cars and see what they do.
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Old 04-28-2010, 06:30 PM   #11
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Originally Posted by Roelof View Post
Pinch is the force you feel when you turn the crankshaft all arround without a glowplug mounted. Compression is the extra force on the pinch you feel when you turn the crankshaft arround with a plug mounted.


Testing the compression can be done by holding the engine at the flywheel and let it hang on the compression.

- when it stays hanging it is as new.
- when it takes a few seconds to fall through it is OK
- when it falls through within a second the engine is gone....
Thank you Sir. You and another person have told me this. I just wanted to clear things up. So if I take the plug out and My engine holds itself on TDC then it is as good as new, Sweet> thanks buddy

Now also lets clear it up, You loose your pinch with how many tanks or quarts of fuel thru the engine???

When should Compression start loosing or how is it lost after the pinch is lost??

How does one keep the Compression strong with out damaging the Engine??

Thank you again for your help.
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Old 04-28-2010, 06:35 PM   #12
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Originally Posted by PsycoB4 View Post
Thank you Sir. You and another person have told me this. I just wanted to clear things up. So if I take the plug out and My engine holds itself on TDC then it is as good as new, Sweet> thanks buddy

Now also lets clear it up, You loose your pinch with how many tanks or quarts of fuel thru the engine???

When should Compression start loosing or how is it lost after the pinch is lost??

How does one keep the Compression strong with out damaging the Engine??

Thank you again for your help.
You want to do that test with the glow plug in. However, i'm not sure if that test is relevent, because the compression is different when the motor is up to temp.
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Old 04-28-2010, 07:13 PM   #13
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You have to take in consideration that not all engines weight the same. So i dont think that compression test is too accurate.
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Old 04-28-2010, 07:14 PM   #14
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You want to do that test with the glow plug in. However, i'm not sure if that test is relevent, because the compression is different when the motor is up to temp.
If you do it with the plug out and it holds up you have mechanical pinch. Which means it's not fully really broke in.

If you still have this it's always best to preheat the engine before starting. I always practice using a heat gun to preheat my engine before starting to ease any stress.
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Old 04-28-2010, 07:26 PM   #15
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So is this wrong? Or is he wrong? Can someone elaborate more on this? I understand a engine at 200 temp is what we need to talk about but I am talking when the engine is cold and you have it in your hands.
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