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Crank compression leak testing
USA PA17601
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May 5, 2019 - 11:51 pm
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Any body with some facts/data about acceptable crankcase compression leak down?

These crankcases can hold 50 psi with slow leak down to 40psi over 5 minutes. 65psi to 50psi in three minutes. Max pressure obtainable is 80psi before bungs blow out. No sound/sight of any leaks. Leak candidates are the sealing bungs or the crank seals.

At what point does pressure leak down become a problem upsetting fuel/air mixture and cause of overheating/piston seizure?

richard hyatt
Shaftesbury, Dorset.
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May 6, 2019 - 8:05 am
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Good morning to you,

Primary crankcase compression on a 2 stroke is usually very low compared to atmospheric pressure.

There are some 2 strokes that have plain bronze main bearings with no oil seals 

These rely upon the molecules of oil along the main bearing/crankshaft to give the sealing process.

So , you can see the pressure levels are minimal inside the crankcase , just enough to cause a vacuum sufficient to draw the incoming charge of new fuel and air into the area below the piston.

Then , of course , consider the downwards path of the piston "compressing" this new charge upwards through the transfer ports into the combustion chamber -ably assisted by the exhaust vacuum created in the exhaust. 

I have never seen a definitive psi figure for the crankcase , but rely upon ensuring the principles above are all in place.

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May 6, 2019 - 8:25 am
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In Paul Dempsey's book on Two Stroke Repair and Maintenance he recommends testing at much lower pressures:-

Vacuum test (which replicates conditions during the intake phase when atmospheric pressure forces seal lips out of contact with the crankshaft) - Draw the crankcase pressure down to 7psi.  If pressure remains constant or increases by no more than 4.5psi over 20 seconds the seals should be OK.

Pressure test - Use bike pump or similar to pressurise crankcase to 7psi.  Pressure should hold steady for 20 seconds.

The pressure test is really just a double check of the vacuum test.  I did these tests on a small cyclemotor engine and got it sealed OK on the second attempt.  It is difficult to know where any crankcase leak is on the vacuum test but it is much more audible on the compression test.  I did my test with the barrel and head fitted, sealed across the exhaust flange using rubber innertube and a metal plate, then used a spare inlet manifold with plastic tube sealed inside for vacuum gauge and bicycle pump.  So this also tested the integrity of the head and base gaskets.  

Cheers.  Bruce.

Central France
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May 6, 2019 - 8:28 am
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There are several previous post which may be of help. ** Please log in to view ** helps.

1956 D3 awaiting a new magneto. 7 other bikes in the Barn, 6 runners and a still in progress Morini 250

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May 6, 2019 - 9:33 pm
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thonebt's post quoting Paul Dempsey is the standard method for testing chainsaws and strimmers etc but I've never heard of anyone testing at the sort of pressures the OP referred to.

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