Cylinder Bleed Down Test & Top End Rebuild - Polaris ATV Forum
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post #1 of 2 (permalink) Old 02-05-2018, 08:01 PM Thread Starter
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Cylinder Bleed Down Test & Top End Rebuild

How does a fella do a bleed down test on my 95 Magnum 425? Trying to see why compression is low. Either worn rings or valves leaking? How hard is a Top End Rebuild and can you do it without removing the engine?
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post #2 of 2 (permalink) Old 02-06-2018, 05:32 AM
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Leak down is easy - screw a compression gauge adapter into the spark plug hole - depending on the style of adapter, you can either connect a compressed air hose directly to the adapter or as I do, use a rubber tipped blow gun to just push into the opening of the adapter and squeeze the trigger.

Now before applying compressed air, get the engine about 10 degrees before or after TDC of the compression stroke and lock the crankshaft - there's as many different ways to lock the crank as there are service technicians - my favorite on a Polaris is to slip a 1/2 or 3/4" pry bar, long bolt, piece of re-bar or big screwdriver through the primary clutch and secure the end against the floorboard - now before applying compressed air at full force, apply air gently to verify the crankshaft is locked and that the locking device will hold and not fly out (which could cause damage or injury) - once satisfied of the method used to lock the crank, increase air pressure - you can easily use over 250 psi of compressed air (Peak cylinder pressure (PCP)is the maximum chamber pressure achieved during the combustion process. This figure would normally be in the 600 to 2000 psi range {source: http://www.sdsefi.com/meltdown.htm}) - you can then listen, feel or otherwise determine where the air escapes to.

Another method is to remove the camshaft or back off the valve tappet adjusters so the piston stops a BDC and with valves closed - only thing wrong with doing it this way is the most wear in a cylinder occurs where the rings stop at the top of the piston travel - on a plated cylinder, you can be worn through the plating at the top of the cylinder but the bottom of the cylinder is fine. If you check at BDC everything might be fine, but if you check at TDC, you could find a tremendous amount of leakage past the rings. About the only thing you can definitively check a BDC is leakage past the valves and head gasket.

You should be able to rebuild the TE with the engine in the frame, but it is easier with the engine out. Engine in, you have to work around cables, wiring and frame - engine out requires removing clutches, clutch covers etc - takes at about the same amount of time either way
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