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Discussion Starter · #1 ·
I have a 2018 XC-W 250 that i would like to verify if it hold pressure in the crankcase.
Would it be possible to let me know if the piston need to be in a special position?
Yesterday i put 5 psi and it was leaking a litle bit around the connection flange (power valve aera i guess).
I checked in the manual and there is no seal for the connection flange.
In fact, when installing it, the manual specify to degrease the sealing surface and coat thinly with sealant. So i am not sure if all this is effectively airtight. I am wondering if blocking the exhaust port with the piston would help in that test. Would the exhaust port be block when piston is at TDC?
 

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You would normally test with piston locked at TDC
 

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You are testing crankcase for leakage. Piston at TDC effectively seals above it and eliminates most leakage above to remove leakdown pressure drop. Also removes some (not all) possible PV leakage. You would normally see PV o ring and cover leakage when performing leakdown test from spark plug hole but pressure cross leak is also possible from PV when crankcase is pressurized because ports bring your pressure to exhaust port and PV through ports when piston is below exhaust port. Hope that helped
 

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Discussion Starter · #5 ·
If the below engine is like our KTM, when piston is at TDC, the exaust port and transfert port would be blocked by the piston. Is this for that reason it must be at TDC?
 

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Yes. You're going to prevent most leakage due to piston covering ports. This prevents pressure loss when testing
 

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Discussion Starter · #7 ·
But one question remain for me. I believe the sealing is done by the ring around the piston and those are above the exhaust port when piston is at TDC. So pressurized air might come under the piston and still exit by the exhaust port.
 

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That is correct. You would seal a portion of exhaust when piston is at BDC but air pressure would push piston upward if your flywheel was not locked.you can lock flywheel by inserting coin or soft copper tubing in primary gear. Be sure not to exceed 10-15 PSI in crankcase.
 

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Discussion Starter · #9 ·
According to the 2T animation above, If piston is at BDC (bottom), wouldn't pressurized air bypass the piston through the transfert port and then find a way to exit from exhaust port and pv?
 

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Check your piston height on your engine ports as it rotates. Port timing in your animation may differ from running engine.
 
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