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Old 03-10-2008, 08:24 AM   #1
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Default Pinch...

A newbie to the hobby and I am wondering if you guys can share with me how one defines an engine having enough pinch/compression?

did a google but was unable to find any answers. How do i know if an engine has no more pinch? Is there a simple test for me to do?

having this knowledge will aid me in determining when my engine is worn and also help prevent me from buying a dud second hand engine.
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Old 03-10-2008, 06:26 PM   #2
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A newbie to the hobby and I am wondering if you guys can share with me how one defines an engine having enough pinch/compression?

did a google but was unable to find any answers. How do i know if an engine has no more pinch? Is there a simple test for me to do?

having this knowledge will aid me in determining when my engine is worn and also help prevent me from buying a dud second hand engine.
Donít confuse compression with pinch; Sleeve Pinch is not that important contrary to popular belief. The question is how well it seals as soon as the piston starts to close the exhaust port and if it holds compression after you roll it to top dead center.
Place a few drops of after run oil trough the top (removing plug), then place plug in place, then roll piston to top dead center and listen and look for air bubbles through exhaust port. If thereís none, then your engine still has compression, even if it has no pinch.

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Old 03-10-2008, 07:13 PM   #3
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Any engine will be fully broken in only when it looses the pinch, at this point it will also get faster. The pinch is there so that while breaking in the engine, the piston copies the shape of the sleeve, thus allowing a better seal and retaining compression.
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Old 03-11-2008, 07:38 AM   #4
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that clarifies on things...

so as long as my engine can retain the pressure at TDC then its ok?
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Old 03-11-2008, 09:31 AM   #5
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Most engines will keep a slight pinch in cold conditions, it has to do with expansion of the piston and sleeve on worktemperature and helps to have a cold start. When the pinch is gone and the engine runs fine then there is nothing to worry about.
When the pinch is gone and you have running problems then yes, the set must be gone....

A nice test is to grab the engine at its flywheel and let it hang on its compression:



If it stays hanging the engine is very good
if it goes through its compression within a view seconds it must be fine
if is falls directly through its compression and the engine runs bad then you need a new p/s set.
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Old 03-11-2008, 07:30 PM   #6
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Most engines will keep a slight pinch in cold conditions, it has to do with expansion of the piston and sleeve on worktemperature and helps to have a cold start. When the pinch is gone and the engine runs fine then there is nothing to worry about.
When the pinch is gone and you have running problems then yes, the set must be gone....

A nice test is to grab the engine at its flywheel and let it hang on its compression:



If it stays hanging the engine is very good
if it goes through its compression within a view seconds it must be fine
if is falls directly through its compression and the engine runs bad then you need a new p/s set.

Great test Roelof!

Manufacturers need to back off the pinch a bit. They are getting a bit out of hand on the required pinch needed to make a good running engine. All they are doing is causing rods and crankpins to wear prematurely. Maybe they think the customer expects lots of pinch so thats what they make?

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Old 03-11-2008, 11:35 PM   #7
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Originally Posted by mxwrench View Post
Great test Roelof!

Manufacturers need to back off the pinch a bit. They are getting a bit out of hand on the required pinch needed to make a good running engine. All they are doing is causing rods and crankpins to wear prematurely. Maybe they think the customer expects lots of pinch so thats what they make?

BK
It seems that Picco thinks the same way. the new generation Picco's have much less pinch than in the past. Break in is a breeze and the rods last much longer.
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Old 03-11-2008, 11:42 PM   #8
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I would say with mass production it will be very costly to make better fit of piston to sleeve-to catch 2-3 microns when pistons made by thousands is very hard, so manufacturers decided to stay on high side of tollerances to make sure that customer get "not loose" engine.
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Old 03-12-2008, 01:20 AM   #9
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I would say with mass production it will be very costly to make better fit of piston to sleeve-to catch 2-3 microns when pistons made by thousands is very hard, so manufacturers decided to stay on high side of tollerances to make sure that customer get "not loose" engine.
Indeed that is the reason.
I am involved with the develloping of an engine and we tryed everything. Producing at lower tolerances means you have to start with a tight fitting to get the piston and sleeve fit to each other.

We also experimented with precise grinding after producing and indeed the results are much better without needing a tight fitting but with higher costs because the extra work.
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Old 03-25-2008, 10:25 PM   #10
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Default I Agree...

Rather the pinch in the motor, than in on my wallet. Closer tolerances means higher manufacturing costs, and guess who's gonna cover that.....................US.
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