Merc KD4

[Boonie]

installing new rings  in a KD4. What is acceptable ring gap?

[Buccaneer]

The old rule I remember, but from “automotive”, was .003 per 1″ of piston diameter.
Hence, a two inch piston, .006.
That would be acceptable minimum gap though……. How large a gap is another question.

Prepare to be boarded!

[fifty20ne]

 

Everything You Need to Know About Ring Gap! (wiseco.com)

• This reply was modified 2 years, 2 months ago by fifty20ne.

[joecb]

Careful, with that wiseco formulation, something is “fishy”, or my math is way off.

0.04 %  of say 2 inch bore is only 0.0008 inch, way too small

and in the other interpretation of the formula …  0.04 X 2 inch = 0.080 inch , way too big

Joe B

PS, I just looked at air  cooled small engine repair manual , typical ring end gap listed is 0.010 to 0.025 … and that is for air cooled engines, I think that on water cooled engines the tighter end of the range would be OK

• This reply was modified 2 years, 2 months ago by joecb.

[Buccaneer]

Not Mercury, but I’m sure the general specs per engine size are similar.
From the Johnson 3rd Edition manual……..

Prepare to be boarded!

[Tubs]

joecb wrote:

Careful, with that wiseco formulation, something is “fishy”, or my math is way off.

0.04 %  of say 2 inch bore is only 0.0008 inch, way too small

and in the other interpretation of the formula …  0.04 X 2 inch = 0.080 inch , way too big

Joe B

PS, I just looked at air  cooled small engine repair manual , typical ring end gap listed is 0.010 to 0.025 … and that is for air cooled engines, I think that on water cooled engines the tighter end of the range would be OK

• This reply was modified 2 years, 2 months ago by joecb.

My rule of thumb has always been .003 to .004 per. inch
when not having a factory spec.
I believe 1% of an inch in .000 is .010
I interpret fifty20ne’s speck. to mean 4/10 of 1 % to
5/10 of 1 % which would be .004 to .005 end gap per. inch.
After seeing Bucks chart I will now use this as a guide
for 2 strokes when not having a factory spec.
“Typical ring end gap listed is 0.010 to 0.025 for air
cooled engines”. ??????
I can imagine what a 10/1000 to 25/1000 end gap would
apply to.
Tubs.

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

• This reply was modified 2 years, 2 months ago by Tubs.

[Boonie]

My minimum gap is .008. To tight?

[joecb]

Ref the chart that Bucc posted, your 0.008 looks just right

Joe B

 

 

[joecb]

not to beat the dead horse… Tubs’ calculations are correct.  4/10 of 1% is 0.004. But. look again at the post in question, it shows 0.04%  or 4/100 of 1% = 0.0004 ( for a 1 inch piston)

So,  It looks like the “wesco” post just had the got the decimal point in the wrong place…typo!

Joe B

1 user thanked author for this post.

Tubs

[green-thumbs]

All good to know and useful information, however one has to start with a good cylinder.  All wasted if cylinder is excessively worn  or scored. Fortunately, the modular design of the Early K Model Kiekhaefers  cylinders can be removed and replace with better cylinders from a parts motor,  Boring to an oversized is not an option as oversize, rings and pistons were not offered and were not needed.  Mercury did it their way.

Once a good cylinder AND piston are in hand then it is time to set ring end gap.

Louis

[Author]

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