will diffrent oil weights effect compression numbers?

Hi guys,

if 10w40 motorcycle oil is used in replacement of 20w50 will it have very much effect on the results of a compression test? I wouldnt think it would change much in my case

I have 2 cylinders that are showing lower than spec after doing a compression test. wot all plugs out etc. 90 120 135 140. I beleive the lest allowed is 125 psi?

after a couple squirts of oil in cylinder on it started showing 120psi. It also burns oil though I never see any.

any idea what a usual ring job costs if I do the labour?

ive had a problem with a miss in high rpm. would low compression of 90psi possibly be causing this?


Sorry about the book

 

oil wouldnt cause low compression. have you checked your valve clearances?

 

-different weight oil will not make a big difference.
-The lower two #'s are a concern especially compared with the higher ones.
-The wet test does indicate worn or stuck rings, possibly broken but I doubt it since it came up like it did. Sometimes being low on adjacent cylinders and good on the others is a blown head gasket, but a positive wet test is usually rings.
-if you do the labor the ring job will be fairly cheap. You can rent most of the tools needed. You will need to pull the motor, and check the cylinder bore with a micrometer, or a caliper and bore gauge to see if it will need a re-bore. If so it will cost a bit more, and you will need to get new rings, and pistons then have the shop bore based on the new + size rings. If your bore is still within spec you can just hone and replace the rings.
-Yes, low compression and especially when it's that different will do exactly that.

 

-check your valve clearances; adjust any that are seriously out of spec, and re-test.

-generally speaking, your "wet" results would indeed indicate a ring issue in #1.

-you DO NOT have to pull the motor to pull the head/cylinders.

 

well I guess I know what Ill be doing this winter when the weather turns. Thanks guys

 

Squirting oil in a cylinder will almost always bring up compression because it's an incompressible fluid that effectively increases the compression ratio until it's blown out.

To precisely compare the effect of oil sealing the rings, you need to test all of the cylinders with the same amount of oil, and don't use too much (with a CR of 9.0, there is only 20mL of volume at TDC). 5mL is enough.

I'd agree with everyone else's assessment, unless you tried it with too much oil. If you saw ALL of the cylinders jump by 30 psi, then that's a different story.