Honda Spree and Elite 50 Forums

How much can a leaky oil seal effect compression.

if your talking about crankshaft seals on a 2 stroke, you won't see a difference in compression. Compression only checks piston rings, cylinder condition and head gasket.

To properly check if an oil seal is bad you need to perform a leak down test. This test is very cumbersome on a 2 stroke. There are other tests which people claim work to test crank seals which I have little to no faith in. One is spraying carb cleaner around the suspect seal while the engine is running and listen for a change in rpm. Personally I think the seal has to be real bad for this to work.

If you do have a bad crank seal your engine will run lean . You may notice eratic or high idle, bogging, white looking spark plug, gas/oil seapage from the seal area.

I mean oil seals not crank case seal. If they do not effect compression then I guess something is bad in my top end. Will this http://cgi.ebay.com/ebaymotors/REPLACEM ... arQ5fParts fit my 86 Honda aero50?


Oh and my engine does not run I am working on getting it running has very little compression.

That linked piston fits your motor. The oil seals will not affect compression, most likely rings are bad.

evilone wrote:That linked piston fits your motor. The oil seals will not affect compression, most likely rings are bad.

Well time to take that head off then.

Good idea, hopefully you won't need a piston.

evilone wrote:Good idea, hopefully you won't need a piston.

Fingers crossed.

mopar4u wrote:if your talking about crankshaft seals on a 2 stroke, you won't see a difference in compression. Compression only checks piston rings, cylinder condition and head gasket.

To properly check if an oil seal is bad you need to perform a leak down test. This test is very cumbersome on a 2 stroke. There are other tests which people claim work to test crank seals which I have little to no faith in. One is spraying carb cleaner around the suspect seal while the engine is running and listen for a change in rpm. Personally I think the seal has to be real bad for this to work.

If you do have a bad crank seal your engine will run lean . You may notice eratic or high idle, bogging, white looking spark plug, gas/oil seapage from the seal area.

You need to bone up on your 2 stroke theory. A 2 stroke pressurize the mixture in the crankcase and then feed the mixture into the cylinder UNDER PRESSURE. So a bad crank seal will effect compression. How much depends on how bad the seal is, but it will lower it for sure.

You also do not need to preform a leak down test to check the case seals, a leak down test only checks the cylinder and rings. You pressure and vacuum test a two stroke crankcase to check the seals. You car guys need to stop applying 4 stroke stuff to these 2 strokes.

Bear45-70 wrote:

mopar4u wrote:if your talking about crankshaft seals on a 2 stroke, you won't see a difference in compression. Compression only checks piston rings, cylinder condition and head gasket.

To properly check if an oil seal is bad you need to perform a leak down test. This test is very cumbersome on a 2 stroke. There are other tests which people claim work to test crank seals which I have little to no faith in. One is spraying carb cleaner around the suspect seal while the engine is running and listen for a change in rpm. Personally I think the seal has to be real bad for this to work.

If you do have a bad crank seal your engine will run lean . You may notice eratic or high idle, bogging, white looking spark plug, gas/oil seapage from the seal area.

You need to bone up on your 2 stroke theory. A 2 stroke pressurize the mixture in the crankcase and then feed the mixture into the cylinder UNDER PRESSURE. So a bad crank seal will effect compression. How much depends on how bad the seal is, but it will lower it for sure.

You also do not need to preform a leak down test to check the case seals, a leak down test only checks the cylinder and rings. You pressure and vacuum test a two stroke crankcase to check the seals. You car guys need to stop applying 4 stroke stuff to these 2 strokes.

Very rarely does a bad seal show up on a compression test. Compression tests are done to check the top end.
I'm not a car guy, I work and ride on snowmobiles all winter.
Perform a leakdown test/pressure test (whatever you prefer to call it) from the intake, not from the spark plug hole. Block off the exhaust. http://www.duncanracing.com/TechCenter/ ... sstest.pdf

mopar4u wrote:Very rarely does a bad seal show up on a compression test. Compression tests are done to check the top end.
I'm not a car guy, I work and ride on snowmobiles all winter.
Perform a leakdown test/pressure test (whatever you prefer to call it) from the intake, not from the spark plug hole. Block off the exhaust. http://www.duncanracing.com/TechCenter/ ... sstest.pdf

I beg to differ with you. After 25 years as a 2 stoke mechanic, I can tell you that a bad crank seal sure as * will show up on a compression test. But then having been a factory certified 2 stroke mechanic makes my experience null and void according to you. I was doing crankcase vacuum and pressure test 20+ years ago, what were you doing?

Well I will be taking them engine off the scooter tomorrow and bringing it down onto the work bench so I can work on it easier.

The piston is toast and I don't know about the cylinder it doesn't have any gouges out of it but it does have like a bump on it in one spot looking form the bottom or the top you can see it sticking up a little bit. Will post pics later.

Making new thread with pics

Bear45-70 wrote:A 2 stroke pressurize the mixture in the crankcase and then feed the mixture into the cylinder UNDER PRESSURE. So a bad crank seal will effect compression. How much depends on how bad the seal is, but it will lower it for sure.

Makes sense.