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  1. #1

    Pressure Testing Crankcase on 787 Questions...

    I'm trying to pressure test my crankcase on a '96 GSX 787. I have the top-end (cylinders and pistons) off and shipped out to be repaired. I have the carbs off also.

    I have a junk cylinder which I bolted on the rear (pto) position with a sheet of rubber under it to seal it off, then used an old piece of inner tube with the valve, positioned it under the rotary valve cover so I could use a pump to pressurize just the rear half of the crankcase.

    Will this work? I tried it and it seemed like it started to hold pressure but then when I got it to about 6 psi, it started hissing, like it is leaking over to the front half of the crankcase. Is there a seal between the two crank bores? If I put pressure into one intake port, will that pressurize the entire crankcase? I've looked at the manual and the parts diagram and just can't figure it out...

    Thanks in Advance...
    The Greenhorn

  2. #2
    You really need to have the whole engine reassembled, good gaskets and seals to make sure there is no leakage, you want to remove the exhaust manifold and carbs, and make blank off plates to seal off those parts. I drilled and tapped the plates for air supply fittings and a hose with a gage to read the pressure while testing. I have a 717 rotax and did this on mine. I have a hobby air compressor with a small pressure regulator and an air hose where it connects to the block off plates with a gage that reads 15 psi, so I can increase the air pressure slowly while spraying soap suds on the engine looking for air bubbles indicating leaks. so, just placing sheet rubber may not do the trick to get an accurate test. If you have the old gaskets, you can use them as templates to make block off plates. Hope this helps.
    Last edited by Snipescastle2; 08-16-2010 at 10:19 PM. Reason: adding

  3. #3
    I wouldn't trust that set-up for conclusive results.

    Yes you have Inner Crank Seals between the PTO and MAG
    Cylinders which seal the Rotary Valve Oil cavity
    in the middle.


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