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Old 08-26-2010, 12:44 PM   #1
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Default When is an engine dead?

I have an old trx .15 that looks new, and has good compression when the head is on, but when I remove the head, the piston slides up and down the sleeve without any resistance, even at the top of the stroke. Is the P/S finished or is this normal?
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Old 08-26-2010, 12:46 PM   #2
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it could be because u took away the compression by taking off the head there is no where for the air compress
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Old 08-26-2010, 01:04 PM   #3
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It is perfectly normal for a broken in motor to NOT have any metal pinch and still have plenty of compression. So yes a normal broken in motor with the head off should be easy and smooth when you turn the flywheel.

People get pinch and compression mixed up way too much on here.
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Old 08-26-2010, 01:10 PM   #4
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Try taking putting the head back on and doing the "flop" test. Hold the engine by the flywheel with the head at a 90 degree angle. If it takes more than about 5-6 seconds to roll past compression, you might need a new piston and sleeve. The best test is just to see how it runs. If it won't stay running when its hot but the tune is good, than it's probably done.

I've had motors that felt like they were worn out, but would still run all day.
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Old 08-26-2010, 01:11 PM   #5
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bacchus is right.. I understand no compression because the head is off, but I was wondering if it's normal for the sleeve to have no pinch. With my other engines, the piston had resistance when it reaches the top of the sleeve (at least enough for it to push the sleeve out of the crankcase) but this one has none at all. It was also difficult to tune, but maybe that's just cause it's a pro .15 So as long as there is compression (not necessarily pinch) then the engine is still good?
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Old 08-26-2010, 01:17 PM   #6
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Pinch is bad (ok not BAD, but not ideal), compression is good. Just think of it that way ;-)

If there is mechanical pinch left in the sleeve, then every time the piston is on the upstroke it wastes energy trying to get through that mechanical pinch and it wears away a little material. This takes away from power output and generates heat.

When the mechanical pinch is gone...the piston and sleeve are sized perfect together, creating GOBS of compression, and no pinch holding it back. So every upstroke that piston makes has little resistance (compared to that mechanical pinch) and it can rev faster in a shorter amount of time.

As time goes on, if you are not pre-heating your engine...the piston and sleeve wear more when they are cold and warming up...and eventually they wear away enough material so that the air is no longer compressing when the engine is warm, but just simply moving around the piston. This is when you flameout and can't tune....and generally the frustrating part of these little engines.

If that makes sense anyway....

Oh and someone correct me if I'm wrong here....my experience with Nitro is limited, but I raced 2 stroke engines for 14+ years attached to 2 wheels ;-)
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Old 08-26-2010, 01:35 PM   #7
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Quote:
Originally Posted by Miniracer23 View Post
Try taking putting the head back on and doing the "flop" test. Hold the engine by the flywheel with the head at a 90 degree angle. If it takes more than about 5-6 seconds to roll past compression, you might need a new piston and sleeve. The best test is just to see how it runs. If it won't stay running when its hot but the tune is good, than it's probably done.

I've had motors that felt like they were worn out, but would still run all day.
I hate the flop test, I think its a bad test, a lot of engines won't pass once the pinch is gone from my experience
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Old 08-26-2010, 01:37 PM   #8
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Quote:
Originally Posted by MotoGod View Post
Pinch is bad (ok not BAD, but not ideal), compression is good. Just think of it that way ;-)

If there is mechanical pinch left in the sleeve, then every time the piston is on the upstroke it wastes energy trying to get through that mechanical pinch and it wears away a little material. This takes away from power output and generates heat.

When the mechanical pinch is gone...the piston and sleeve are sized perfect together, creating GOBS of compression, and no pinch holding it back. So every upstroke that piston makes has little resistance (compared to that mechanical pinch) and it can rev faster in a shorter amount of time.

As time goes on, if you are not pre-heating your engine...the piston and sleeve wear more when they are cold and warming up...and eventually they wear away enough material so that the air is no longer compressing when the engine is warm, but just simply moving around the piston. This is when you flameout and can't tune....and generally the frustrating part of these little engines.

If that makes sense anyway....

Oh and someone correct me if I'm wrong here....my experience with Nitro is limited, but I raced 2 stroke engines for 14+ years attached to 2 wheels ;-)
Well said. I just didn't feel like typing it.
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Old 08-26-2010, 01:40 PM   #9
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Originally Posted by losi_racer View Post
I hate the flop test, I think its a bad test, a lot of engines won't pass once the pinch is gone from my experience
Thats true, thats why I added that the best test is engine preformance....
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Old 08-26-2010, 01:45 PM   #10
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The pinch in cold condition says nothing, due different expansion rates of the piston and sleeve you can never tell how the fit will be at operating temperature.
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