Piston rings for MK25

[Charles Byron]

I just ordered and got new rings for a MK25 I am working on.

I’ve set the gap on car engine rings before so I know how to do it. But on car engines the rings have square ends.

On these Mercury rings the ends are not square. My question is are they pre gapped. They are standard size rings for a MK25 and genuine Mercury rings part number  39-20058.

The Mercury service manual doesn’t mention anything about ring gap.

Does anyone know? I’d like to know…Charlie

 

 

 

[Tubs]

The end gap needs to be checked and the
notch would need to be filed accordingly
if necessary. .
Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[labrador-guy]

Charles a general rule of thumb is .004 for every inch of bore, so 3inch bore about 10 to 12 thou.  The biggest thing to watch is they need some end gap.   Mercury rings that I have bought were about 2 thou and that is asking for trouble.  I clamp a point file in the vice and squeeze the ends of the rings on the file.  It doesn’t take long to open them up some.   Elmer K did not want the back yard mechanic to have important information about his motors.  Hope this helps

dale

[Pondrocket]

Charlie,

we 1st need numbers on current condition of bores per factory spec then we can advise from there..

 

 

Travis
AOMCI VP Communications
AOMCI Webmaster
webmaster@aomci.org

[Charles Byron]

I just checked the bores. I don’t have a dial bore gage so I had to use a snap gage and a mike. I got 2.439 pretty much everywhere I checked. Now my technique probably isn’t 100 percent accurate but I think it’s close enough for a home workshop. Once I hone them maybe 1 or 2 thousandths more. I’m guessing that’s OK. The Mercury service manual says the bore is 2 and 7 sixteenths. I believe that works out to be 2.435 so I think I’m in the ballpark. And I guess 10 thousandths gap is what I need…Charlie

 

[Tubs]

Most of these old motors that people kept around haven’t
seen anywhere near enough use to need rebuilding and
honing the cylinder to replace rings it doesn’t need in the
first place is more likely to result in a loss of compression
rather than an increase.
I don’t mean to beat you up but if you don’t have
any ware or taper in the cylinder there wasn’t anything
that needed fixing. Honing the cylinder out .002 will
increase the potential movement of the piston by
.004 side to side, front to back. Then there is trying
to get the new rings to seat as well as the ones that
were taken out.
I would suggest knocking the glaze off with some 400
sand paper, gap the rings and put it back together.

Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[Pondrocket]

Charles,

you’re on your way finish cylinder hone for standard bore is 2.441  as Tubs stated you want to deglaze only use a ball hone say 240 grit then clean bores correct oil and good to go

 

Travis
AOMCI VP Communications
AOMCI Webmaster
webmaster@aomci.org

[Charles Byron]

A little background and the reason I’m doing this. The powerhead in this case is from A MK25E that obviously was overheated, burnt paint on the outside and damaged pistons and broken rings on the inside. But I have quite a few parts that I picked up at meets and other places. So I have a good cylinder block and 2 good pistons from different motors. So I’m putting together a motor with parts from a few different motors. And the need for new rings.

Thanks for the help. Now I know what I need to know…Charlie

[Tubs]

My apologizes. The situation you have described
justifies your effort and some expense. Finding used
cylinders in that good condition doesn’t happen often.
Expect to have a strong runner when its done.
Tubs

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[AOMCI Webmaster]

Charles,

Hopefully crank checks out per spec … Also if you have genuine Merc rings and your bores are correct rings are already gapped.

Travis
VP Communications AOMCI
webmaster@aomci.org

[Author]

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