wedgedaddy
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thank you.
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what is pinch???
- Thread starter wedgedaddy
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Metalhead Matt
RCTalk VIP
How tight the piston fits the sleeve.
I will elaborate a little bit on what metalhead matt said. . .its esentially the compression that the engine has.
If you have a micrometer, you can measure the "pinch" of an engine by taking the sleeve out and measureing the inner diameter (ID) of the bottom and top of the sleeve.
As you go from bottom to top, the ID of the sleeve tapers, this gives you your "pinch". These small engines don't have rings on the piston to seal the combustion chamber. They rely solely on the tight fit of the piston/sleeve. As the sleeve heats up, it expands, so the pinch you feel when cold, you don't feel as much when it's at 220F.
If you don't feel any pinch at all when it is cold and the glowplug is out, it's highly likely that it is time for a re-pinch or a new piston/sleeve as it is too worn to make good compression when it heats up.
I had some sleeves re-pinched quite some time ago:
https://www.rcnitrotalk.com/forum/showthread.php?t=56420&highlight=pinch
You can see in the photo's that I was easily able to push the piston through the bottom and out the top of the sleeve. Short of using a hammer, you should not be able to do this if the pinch is new. This is a tell tale sign that the pinch is gone.
You can also see how far I could push the piston by hand after the pinch was restored.
As you go from bottom to top, the ID of the sleeve tapers, this gives you your "pinch". These small engines don't have rings on the piston to seal the combustion chamber. They rely solely on the tight fit of the piston/sleeve. As the sleeve heats up, it expands, so the pinch you feel when cold, you don't feel as much when it's at 220F.
If you don't feel any pinch at all when it is cold and the glowplug is out, it's highly likely that it is time for a re-pinch or a new piston/sleeve as it is too worn to make good compression when it heats up.
I had some sleeves re-pinched quite some time ago:
https://www.rcnitrotalk.com/forum/showthread.php?t=56420&highlight=pinch
You can see in the photo's that I was easily able to push the piston through the bottom and out the top of the sleeve. Short of using a hammer, you should not be able to do this if the pinch is new. This is a tell tale sign that the pinch is gone.
You can also see how far I could push the piston by hand after the pinch was restored.
