Mariner 25 HP - Bad compression (under 50psi), bad piston rings?

[Oversea]

Hi

I bought a 25 HP Mariner from 1982 (25E, model: 695) a year ago as a spare engine.
The seller said that it run bad and that is has low comp one cylinder.
I did a dry compression test on the sparkplugs and both cylinder gave me about 50psi.
With a bit of oil in the cylinders this bumped up to about 105 to 111 psi, fairly OK.
I've found out that this i most likly because of a bad piston rings.
Im now thinking about taking of of the headcylinder to check if its a broken piston ring och or a bad cylinder.


Any thoughts what it might be?
Also, If I need to change the piston rings... Do I need to change all the gaskets for the cylinder?

A long, long time since i've changed piston rings and gaskets on a engine...Don't remember much... All answers are welcomed!

 

[quicktach]

Re: Mariner 25 HP - Bad compression (under 50psi), bad piston rings?

Without squirting oil in, you should be over 100psig with no more than 10-15% difference between readings. Pull cylinder head and check rings - wear, plugged with crud, broken....check for scoring on piston walls, cylinder walls.... Best to replace with a piston kit that has rings. May be able to get by with polishing cylinder walls lightly and not turning.
And yes - change gaskets.

 

[Oversea]

Re: Mariner 25 HP - Bad compression (under 50psi), bad piston rings?

Thank you for the information!

I have not been able to find the pistons for this engine. I did however find the piston rings made by Sierra (part: 18-3985).
If I cant source the pistons is it still worth just changing the gaskets and piston rings? Or is it likely that i will get the same problem again even if the pistons looks OK?

 

[fatheadred]

Re: Mariner 25 HP - Bad compression (under 50psi), bad piston rings?

Thank you for the information!

I have not been able to find the pistons for this engine. I did however find the piston rings made by Sierra (part: 18-3985).
If I cant source the pistons is it still worth just changing the gaskets and piston rings? Or is it likely that i will get the same problem again even if the pistons looks OK?

What was the outcome on your motor? I am actually getting ready to tear into mine. I have an 81 mercury 25hp and it calls for piston 767-8511A3, mercury advised that 767-9104A 3 was a replacement of it where a few were still floating around but not from Merc. I'll let you know how it works out.