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Old 06-24-2009, 03:27 PM   #1
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Default Difference between Pinch and Compression ?

I have a motor that I am starting to question it being worn out. I have read some other post that people say if itís easy to turn the flywheel then itís worn out. But doesnít that mean the pinch is gone right, not necessarily the compression. Or are they same? If different how to you test them to see if they are still good?
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Old 06-24-2009, 03:37 PM   #2
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Pinch is the physical tolerance between the piston and sleeve. The sleeve tapers towards the top of the cylinder, and when an engine is new, the piston and sleeve are usually tight enough that the piston gets somewhat "stuck" at the top of the cylinder - some more so than others. That's pinch.

Compression is simply what it implies, the amount of compression the engine is able to produce during the compression stroke. A fresh engine will have a tight seal between the piston and sleeve, and therefore hold more compression, while a worn engine will have wear and more clearance between the piston and sleeve, and some of the compressed air/fuel mixture can leak past the piston. The amount of compression or pressure that stays above the piston is compression.

If you remove the glowplug and crank the engine, any resistance you feel is pinch. Re-install the glowplug, and even if there's no "pinch" left, the resistance you're feeling is compression.
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Old 06-24-2009, 04:10 PM   #3
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... as long as there is oil in there. while the plug i out put a couple drops of aro or wd-40 in there then test the compression. i like to turn the flywheel fast a couple times and listen to the pop, then turn it over slowly and listen to any farting between the piston and sleeve as it goes up. heh heh, i said farting.
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Old 06-24-2009, 04:16 PM   #4
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Not a nitro guy but does someone make a compression gauge that threads into the glow plug hole? Seems that would be the easiest way to tell the relative health of the engine.
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Old 06-24-2009, 04:34 PM   #5
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Quote:
Originally Posted by SteveP View Post
Pinch is the physical tolerance between the piston and sleeve. The sleeve tapers towards the top of the cylinder, and when an engine is new, the piston and sleeve are usually tight enough that the piston gets somewhat "stuck" at the top of the cylinder - some more so than others. That's pinch.

Compression is simply what it implies, the amount of compression the engine is able to produce during the compression stroke. A fresh engine will have a tight seal between the piston and sleeve, and therefore hold more compression, while a worn engine will have wear and more clearance between the piston and sleeve, and some of the compressed air/fuel mixture can leak past the piston. The amount of compression or pressure that stays above the piston is compression.

If you remove the glowplug and crank the engine, any resistance you feel is pinch. Re-install the glowplug, and even if there's no "pinch" left, the resistance you're feeling is compression.
Very well put!!
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Old 06-24-2009, 11:53 PM   #6
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good question, should help clear up stuff for some people.

I always hear of engines with 5 or 6 gallons and "tons of pinch left" obviously they're talking about compression.
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Old 06-25-2009, 12:02 AM   #7
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Dont use wd40 its bad on your engine!!!! Don't Believe me read the massive modds web page on engine care. You will see!
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Old 06-25-2009, 12:03 AM   #8
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The way I was taught is a very simple way to check your compression. take off the cooling head and turn the flywheel, if the piston comes all the way to the top of the sleeve, you have lost compression
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Old 06-25-2009, 10:30 AM   #9
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The way I was taught is a very simple way to check your compression. take off the cooling head and turn the flywheel, if the piston comes all the way to the top of the sleeve, you have lost compression
seriously? or are you being sarcastic?
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Old 06-25-2009, 10:51 AM   #10
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You could also check with the flop test.

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Do the flop test. Hold onto the flywheeland let the weight of the engine fall to the side. If the compression is enough it will not flop over, or turn over. It will stick and hold the weight of the engine.

Note: Some engines (V-Specs mostly) will not pass the flop test but still run great.
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Old 06-25-2009, 11:00 AM   #11
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If you take the head button and cooling head off the motor and it turns over you have lost your mechanical pinch not your compression, you can take a engine that has low mechanical pinch and that engine can still have compression. Mechanical pinch is just the taper on your sleeve in comparision to your piston. compression is formed by mechanical pinch on the engine, aswell as the the fact that you are compressing air and fuel with the piston untill the point of explosion, with the help of a glow plug, spark plug ect.

air + fuel + compression + ignition = combustion engine
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Old 06-25-2009, 11:14 AM   #12
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If I take care of my engine and tune to smoke and performance and use temp as a guide to when I am off I find my engines will last well into the 7-8+ gallon mark. I have abused engines running them too lean and way hot when I had no idea how to tune a nitro engine and they blew up within a gallon. Once the engine fully breaks in after a gallon or so the majority of the metal pinch will be gone and you are left with compression. When you have the engine in your hand and rotate the fly wheel feel for the nice thump the piston makes. If there is still metal pinch present then the fly wheel will get stuck and become hard or impossible to move by hand. When the engine is cold if you can turn the fly wheel over through the full stroke of the piston and sleeve then the engine is broke in. That thump will remain there until the engine wears out and needs a new piston and sleeve. The thump is the compression. There is no way I am aware of to measure the value of the compression, just pay attention to performance. When an engine looses compression the performance will suffer. It will no longer wind up to the RPM range it use to, therefore top end will suffer drastically. Also the low end will suffer because the compression will not let the engine build up torque. The best test is on the track. If the engine stops performing the car will suffer and struggle to get around the track, it will feel sluggish and slow. I hope that helps.
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Old 06-25-2009, 11:23 AM   #13
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Quote:
Originally Posted by Jaz240 View Post
The way I was taught is a very simple way to check your compression. take off the cooling head and turn the flywheel, if the piston comes all the way to the top of the sleeve, you have lost compression
Um no. The con rod will stop the piston at the same height on a new or worn engine assuming the rod isn't completely worn or streached. I think I know he method you are talking about, but you have to remove the piston and sleeve from the engine to test it. With the p/s out if you can push the piston past the wear mark on the sleeve(eg. all the way to the top of the sleeve) your engine is worn out. If that is the theory you are talking about all it really does is test pinch, not compression, and as we know pinch doesn't matter. The best test of compression is the flop test. Although, like pyromania said, some engines will fail the flop test and run perfectly. OS engines are the prime example of this. They appear to have virtually no compression and will run great that way for gallons.
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Old 06-25-2009, 10:27 PM   #14
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I was being serious and apparently 100% wrong. The way it was explained and shown to me made perfect sense that if the sleeve is loose enough to allow the piston all the way to the top you will get way to much blow by to have sufficient compression, besides the fact that the engine will have no performance with the piston pretty much hitting the bottom of the button. i have also noticed that every time one of my engines totally crapped out, the piston reached the top of the sleeve..thanks for clearing it up for me, thats one of the reasons I love this forum so much..apologies for the misinformation..i guess if you are taught wrong you will spread the wrong advice until someone sets you straight
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Old 06-25-2009, 11:18 PM   #15
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Quote:
Originally Posted by Jaz240 View Post
I was being serious and apparently 100% wrong. The way it was explained and shown to me made perfect sense that if the sleeve is loose enough to allow the piston all the way to the top you will get way to much blow by to have sufficient compression, besides the fact that the engine will have no performance with the piston pretty much hitting the bottom of the button. i have also noticed that every time one of my engines totally crapped out, the piston reached the top of the sleeve..thanks for clearing it up for me, thats one of the reasons I love this forum so much..apologies for the misinformation..i guess if you are taught wrong you will spread the wrong advice until someone sets you straight
as what jamminkrazy pointed out; the only time the piston could reach the top of the sleeve is due to a stretched rod, not a loose sleeve...

no offence...
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