Hoiw to check crank seals on chainsaw??

fergus7(NS)February 4, 2006

I have a jonsered 520 saw it ran ok when I got it but the saw shop that I had it to said the seals were bad how do I check the seals to see if what they say is true. I think the shop is a bit untrustworthy as now it won't even start I would like to check it for myself if possible.



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Remove everything until you just have the crank shaft sticking out of the crank case.

Plug the exhaust port.

Install the spark plug.

Make a mixture of 75% dish soap, 25% warm water.

Chuck the start/flywheel end of the crankshaft into a heavy duty drill.

Brush the soap around the crankshaft seals and all places that have a gasket.

Turn the crankshaft with the drill for a few seconds (15).

Look for bubbles.

Have bubbles.. Have leak..

    Bookmark   February 4, 2006 at 7:30PM
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Hippy made a good suggestion.Bad crank seals on a chainsaw dont leak anything out to amount to anything,its the air that gets sucked in that keeps the saw from running right,although bad crank seals shouldnt keep the saw from starting.If your saw was running lean and wouldnt idle and kept dying out,its very possible the seals are bad.One thing to try is to spray some ether around the head gasket and carb boot while the saw is running.If the motor speeds up,you have a leak in those areas.Its all but impossible to get anything sprayed around the crank seals while the saw is running tho.By the way,Ive worked on lots of saws,and while Im sure theres been several,thats the first Jonny Ive heard of blowing the crank seals.Doug

    Bookmark   February 4, 2006 at 10:19PM
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nevada_walrus(Boulder City)

Proper method is to block off intake and exhaust ports. Remove spark plug and move piston to bottom of cylinder. An air adaptor is then installed in the plug hole and a hand pump that will go both pressure and vacuum is used to check for leaks. 5 psi is all that the case needs to hold either pressure or vacuum.

If leak is determined, compressed air regulated to less then 5 psi can be used with soapy water to look for pressure leaks, bubbles. Bad gaskets will almost always leak both directions but cranks seals can leak in only one direction. If the hand pump test shows no leaks but vacuum shows leak you can count on a bad seal. Which is a good thing since you can do a bubble test for vacuum.

The compressed air test is done because when doing a pressure bubble test it almost requires 2 people, one to keep hand pumping while the other sprays soapy water on suspect areas. The compressed air allows just one person to do it. The shop may have done the compressed air bubble test and pumped it up to high, easy to do, and actually blown a seal right out of the block. This often will prevent the engine from starting at all which would explain why it doesn't run after it came back from the shop.

A seal completely blown out of the block is self evident after removing the clutch and flywheel so the seals are visable.

    Bookmark   February 5, 2006 at 9:21AM
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canguy(British Columbia)

I have found grass, string, and other debris caught under the clutch side seal. It seems to be the one that fails first. Sounds like the guy in the saw shop might be fishing in the dark.

    Bookmark   February 5, 2006 at 9:05PM
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Hi and thanks for all the info. I got this saw from a friend that was hard up for cash. I was going to give it to another buddy and he took it to the saw shop and they said that put a carb kit in it charged us 55.00 for work and the saw won't even run now. It did start and run when I gave it to him and why he took it to the shop I have no idea. I think it would make a good saw if it is worth working on it.

    Bookmark   February 6, 2006 at 12:30PM
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