How to tell if you have good pinch?

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Keebibear

Keebibear

Well-Known Member
Joined
Jan 1, 2004
Messages
290
Ok so is there a simple way to tell on a good pinch or if the motors wore out. Just picked up a .21 rigger with a Mac. Previous owner said it was down on power and thinks it needs bearings but i can turn it over by the prop shaft so im thinking needs a pinch. So what is good pinch or whats considered wore out?
 
She slaps your face.
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"Pinch" is over rated. In fact I've come to realize that pinch only serves as a heat generator that produces heat to help burn the mixture that could be more properly burned with more compression,or hotter plug.

A NEW engine with a good "Pinch" runs great, but as the pinch goes away the power fades. This is simply because extra heat caused by the FRICTION producing "Pinch' has been reduced. In reality this engine has a compression ratio that is too low. Increase the CR and the engine will regain it's perfomance and probably run faster than ever.

In our marine engines the piston normally out grows the cylinder. This means that the piston gets tighter in the sleeve once it's running. So an engine that seems to have no compresson seal while it is cold will probably have a good seal when it is hot. This over growing piston is what makes it wear in the first place. I have found that what most call "wore out" is just final ready to run really fast.

I ran a MAC 67 today that was as you describe and could easily be turned over with the prop. I squeezed the head down tight, put some very old 50% fuel in it and it easily pulled a 1667 with 5.5 cup, ran 84 mph and was looking for more.
 
I'm not sure "pinch" is what you're looking for more than just a good seal. Once my motors are broke in (a gallon or more) you can turn them over easily with the prop shaft without the plug in. I mean it's like there's no resistance at all. Put the plug in and try again and it's a whole new deal. Play with the water to the head to max out your performance. If you get it just right you will hear a notable increase in RPM once the engine comes up to temp. Makes a big difference.

My buddy calls "pinch" friction.

You must be doing something wrong Ray. Just the right amount of "pinch" get's you lov'n!
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Ron Jefferson said:
I'm not sure "pinch" is what you're looking for more than just a good seal. Once my motors are broke in (a gallon or more) you can turn them over easily with the prop shaft without the plug in. I mean it's like there's no resistance at all. Put the plug in and try again and it's a whole new deal. Play with the water to the head to max out your performance. If you get it just right you will hear a notable increase in RPM once the engine comes up to temp. Makes a big difference.

My buddy calls "pinch" friction.

You must be doing something wrong Ray. Just the right amount of "pinch" get's you lov'n!
default_tongue.png
default_ohmy.png
Click to expand...
On a serious note, I have had engines that would turn over easy when cold. When fired the heat build would create a great seal. Has a lot to do with proper operating temp. OMHO.
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Andy Brown said:
"Pinch" is over rated. In fact I've come to realize that pinch only serves as a heat generator that produces heat to help burn the mixture that could be more properly burned with more compression,or hotter plug.

A NEW engine with a good "Pinch" runs great, but as the pinch goes away the power fades. This is simply because extra heat caused by the FRICTION producing "Pinch' has been reduced. In reality this engine has a compression ratio that is too low. Increase the CR and the engine will regain it's perfomance and probably run faster than ever.

In our marine engines the piston normally out grows the cylinder. This means that the piston gets tighter in the sleeve once it's running. So an engine that seems to have no compresson seal while it is cold will probably have a good seal when it is hot. This over growing piston is what makes it wear in the first place. I have found that what most call "wore out" is just final ready to run really fast.

I ran a MAC 67 today that was as you describe and could easily be turned over with the prop. I squeezed the head down tight, put some very old 50% fuel in it and it easily pulled a 1667 with 5.5 cup, ran 84 mph and was looking for more.
Click to expand...
Agreed. I actually take some of the "fit" out of the big motors before I even run them. Also helps prevent "sticking" a new piston.
 
Put plug in with everything tight add a little light oil do the engine is not dry. Pass over tdc and see how many times you can go over and still have compression. Maybe 10x is good I think.
 
As it wears turn up the pressure more boost we have no turbo so lower compression
 

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