Ring end gap

[voodoo]

I am boring my 3 cylinder Johnson.
It is around a '89, but has been worked on before so I am not sure what to believe.
It cracked the # 3 piston after maybe 20 hours of use on a "rebuild", (Not me) possibly incorrectly as it appears to me that a ring turned on the piston and caught a port.

Seloc lists a ring gap of .07-.17 for 88 and prior, and a ring gap of .19-.30 for 89 and up.
How do you tell which it is?
Is it in the piston design?
Mine appear to be the newer pistons without the extra skirting I have seen in the book.
Correct?
I have new GLM .30" + pistons for it now.
As well piston clearance is not listed, what should it be?
Do I trust that the GLM pistons are .30" larger than oem and bore to 3.187 + .030"?
Thanks.

 

[Texasmark]

Re: Ring end gap

The .03 overbore is very slight when compared to the circumference of the ring (diameter x3.14).

You will have serious problems if ring gap is too small (where the ring can't expand as much as it wants to when heated) but slight if any if too large, considering you put your gaps on opposite sides of the pistons. Any pressure loss thru one slot would have to go half way around the piston to get out through the other, and by then the piston would be going the other way, especially where you need the pressure the most at WOT.

So if you don't get any accurate technical answers, I'd opt for the wider gap.

Mark

 

[Dhadley]

Re: Ring end gap

Ring end gap has more to do with ring material than anything. With the old material you'd look for a .007" to .017" gap. With todays material it's not unusual to see .018" to .022" on a fresh bore.

If you cracked a cast skirt that's a real good sign the bore was too big. measure it with a dial bore gauge and let us know what you have.

What do you mean by "the extra skirting"? Which blok do you have? the 3.000" std bore or the 3.1875" std bore? (How many plates on the exhaust side of the block?)

 

[voodoo]

Re: Ring end gap

3.1875 bore.
I have seen pistons with "holes in the sides. Maybe for loop charging?
My pistons do not have skirting below the openings for the transfer port.
I took the block to my mechanic today and he agrees that there is probably too much clearance.
He is going to measure it.
After measuring the new pistons, how much larger should the bore be?
The old pistons are new std. bore oem. I think maybe someone honed it out too much.
I also had a stalling issue and am going to replace the vro pump with a different pump.
I mix the fuel anyway.
What should I use?

Thanks for the help.

 

[Dhadley]

Re: Ring end gap

OMC never made a 3 cylinder motor that wasn't a looper. You have the 56 cid motor, good motor. That piston design is the only design that will work in that motor.

Don't try to measure the piston, it's cam ground, barrel shaped and tapered. Go strickly by the bore size + .0005".

First things first -- is the shop doing the boring an automotive or marine machine shop?

 

[voodoo]

Re: Ring end gap

OMC never made a 3 cylinder motor that wasn't a looper. You have the 56 cid motor, good motor. That piston design is the only design that will work in that motor.

Don't try to measure the piston, it's cam ground, barrel shaped and tapered. Go strickly by the bore size + .0005".

First things first -- is the shop doing the boring an automotive or marine machine shop?

It's a good friend of mine who has repaired bikes, sleds, etc for twenty years.
However he doesn't have much experience with OMC.

I am going with .030 oversize, what should it be bored to if you don't measure the pistons?