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Crankshaft Lifespan
- Thread starter ranga97
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Sportster-2001-951C-Stock
Well-Known Member
I think that would be the 787RFI, 135psi seems to be getting low. But, confirm your compression gauge is calibrated to be sure.
Eventually maybe you know, a piston will melt from blow-by and let go, spreading bits of aluminum everywhere inside the crankcase contaminating the bearings and ruining the crank.
Otherwise while apart for a top end (before the aluminum explosion occurs of course) if the crank bearings feel and look good you can probably reuse it. The inner seals though, might begin leaking and flood the case with oil from the RV....?
180 hours isn't much really, I think those 787 motors can reach over 600 hours?
Eventually maybe you know, a piston will melt from blow-by and let go, spreading bits of aluminum everywhere inside the crankcase contaminating the bearings and ruining the crank.
Otherwise while apart for a top end (before the aluminum explosion occurs of course) if the crank bearings feel and look good you can probably reuse it. The inner seals though, might begin leaking and flood the case with oil from the RV....?
180 hours isn't much really, I think those 787 motors can reach over 600 hours?
ranga97
Active Member
Thanks heaps, good to know!
Is there a way to inspect the wear without cracking the head bolts? The ski doesn't feel slow enough to rebuild the top end yet, but should I do it anyway after about 225 or so hours?
EDIT - The gauge is on the cheaper end, and uncalibrated, only used to verify both cylinders are close to one another. No access to a well-calibrated gauge...
Is there a way to inspect the wear without cracking the head bolts? The ski doesn't feel slow enough to rebuild the top end yet, but should I do it anyway after about 225 or so hours?
EDIT - The gauge is on the cheaper end, and uncalibrated, only used to verify both cylinders are close to one another. No access to a well-calibrated gauge...
To inspect the crank and check the tolerances the cylinders at a minimum need to be removed, but even the, you close to getting it all torn down. If you think your actually going to tear it all the way down, do yourself a favor and remove the PTO BEFORE you start to tear it apart. I use an impeller removal tool and a 6 foot piece of clothesline and use that to "wedge" the PTO piston into the cylinder head and then just keep good steady pressure until it spins free.
Sportster-2001-951C-Stock
Well-Known Member
You need to clean RAVES periodically so during this time I use a light to inspect the piston skirts looking for scoring and monitor the condition. Look over the ring lands carefully, rock the motor back and forth noting if the rings seem to be stuck in the lands with carbon or from aluminum distortion.
An accurate compression gauge is your friend, check during plug changes if possible. Thankfully, both of your cylinders are blowing the same pressure so that's encouraging but at some point it won't be that way. It's really hard to know if maybe tomorrow you might roast a piston but at some point once wear reaches too much this will happen.
Read this article describing the basic mechanisms:
http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html
An accurate compression gauge is your friend, check during plug changes if possible. Thankfully, both of your cylinders are blowing the same pressure so that's encouraging but at some point it won't be that way. It's really hard to know if maybe tomorrow you might roast a piston but at some point once wear reaches too much this will happen.
Read this article describing the basic mechanisms:
http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html
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Remove the spark plugs. Turn the crank CCW to Top dead center on teh PTO cylinder, then go clock wise an 1/8 of a revolution. Feed as much clothesline down the spark plug hole in the PTO cylinder, the turn the crank CCW again to jamb the piston into the rope. Then just remove the PTO from the crank. I use an impeller removal tool and a breaker bar with a 3' cheater pipe while its still bolted in the ski/boat. Just don't lower the piston too much or you will expose the transfer ports and the rope could go down one the get cut off by the piston.
It's the rear piston, pto= power take off. That's a term used when adapting to a "gear box" of sorts to transmit the power from one unit to another. Meaning the disc or coupling on the crank shaft is the adapter to couple the engine to the drive shaft. So to make a long story short, that's the piston closest to the pto. The front piston is the mag piston, it's closest to the magneto.
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Sent from my SGH-T889 using Tapatalk
Sportster-2001-951C-Stock
Well-Known Member
The idea is to avoid getting the rope caught in one of the port windows. Otherwise damage to the piston ring or ring lands can occur beside nipping off a piece of rope in the port to get caught up later.
Also, this can EASILY bend a valve over in a 4-stroke engine so just use this idea on 2-stroke engines.
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