Piston/Bore Clearance - K241

[ol'George]

Originally Posted by RustyShackleford

I had the machinist widen the bore just a hair to hit .007 clearance, since this is what every Kohler manual and resource says to go with, as well as Norman over at iSaveTractors (who was very helpful via email, by the way).

All well and good, until I go to check the piston ring end gap last night...and find that the top ring has a .022 gap and the second ring has .021. Spec is .010-.020 for new bores, up to .030 for used bores. Dang! Not sure what the deal is. Using a Kohler piston and Kohler rings.

At this point, it seems like the only options would be

• Buy another set of STD rings and see if the gaps are a little tighter
• Buy a .010 over piston/ring set and try again (yuck)
• Slap it together and run'er

At this point, I am leaning towards option 3, and don't think I could stomach option 2

Edit: Quick update, I emailed Norman again asking him what his thoughts were. He suggested trying another ring set, so I figured what the heck. Ordered a set of rings from him. We shall see what the gap is like once they arrive. Worst case, I have an extra set of rings, and Norm has a few bucks in return for his advice

Well lets see:
3.141 x .007 piston clearance =.021991 ring gap, yep figgers right.
and 3.141 x .003 clearance =.0094. ring gap.
if yall gonna insist on .007 piss ton clearance yall gonna git a larger ring gap.
@ this point run what ya got, no other options, it might have a little piston rattle.
Only udder real option, is custom file a set of ,010 oversize rings to .010 end clearance.
wouldn't be the first time that is done.
I just carefully use a file backed with 50 years of experience.
Or one could use a piston ring filer ( google it)
FWIW: it only takes a few file strokes, then check them, very easy to go too much and you got what clearance you have now, or more.

[RustyShackleford]

That’s a good idea! I’ll have to think on that’n.

What I don’t understand is… .007” is the minimum clearance that Kohler calls for! And yet, by the math you posted, that leads to more ring gap than they call for right there. Makes no sense to me. Maybe the Kohler engineers were doing a bit of when they wrote the rebuild manuals?

I should add that I very much appreciate all the advice y’all have given me over the last 9 months. It’s been a good learning experience, fer sure

[ol'George]

Only thing I can think of is the old pistons expanded more than modern aftermarket pistons.
Back in the early days ( like pre WW2) or earlier, pistons were cast iron with a different clearance requirement.
Then engines went to aluminum pistons, still later years pistons were cam ground, in as much as being larger in skirt diameter than when measured below the wrist pin area.
Best I can do as far as theory.
FWIW: they don't recommend filing a 3 piece oil ring style ( 2 scrapers and an expander)
Single piece cast iron rings can be custom fitted.
Hope this helps.

Edit: also there are forged pistons and cast pistons, forged (high performance) pistons need a couple of tho' more clearance than forged.
But I believe kohler pistons are cast, not forged.
A forged piston will have some rattle till it warms up.
All irrelevant in this case I guess.

[green407]

Originally Posted by RustyShackleford

That’s a good idea! I’ll have to think on that’n.

What I don’t understand is… .007” is the minimum clearance that Kohler calls for! And yet, by the math you posted, that leads to more ring gap than they call for right there. Makes no sense to me. Maybe the Kohler engineers were doing a bit of when they wrote the rebuild manuals?

I should add that I very much appreciate all the advice y’all have given me over the last 9 months. It’s been a good learning experience, fer sure

You need the ring gap and the piston clearance because the ring and piston expand more then the cylinder walls of the engine.

If you went with a ring gap of less then 0.009" then the rings would expand closing the gap and pushing the outer edges of the rings into the sidewall of the cylinder causing excessive wear.

Similarly the piston needs room to expand, but we can't change the size of the piston so we adjust the cylinder.

[RustyShackleford]

That all makes sense, thanks guys. I am sure these engines do run hot.

Just seems very odd that an in-spec piston clearance would result in an out-of-spec ring gap. Unless the rings themselves are out of whack.

[RustyShackleford]

Well I got the rings in from iSaveTractors. Ring gap actually increased to 26 thousandths or so. Hmf.

Figuring on just running what I had. I have a feeling it will work fine, and I’m not in the mood for further dinkin’ around. Just want to putt putt around the yard already

[ol'George]

A lot of quality engines produced pistons of Standard, .003,010,.020,.030 etc.
The Std and .003 oversize pistons used standard ring sets sometimes.
Yes, some manufactures would sell .003 oversize rings, usually aftermarket sellers, but not always, and the demand for these . The .003 sizes just disappeared over the years.
In the 50's they were available, that is 2 generations ago.

They would state "for worn bores that are still within specks" but would caution to check end gap.
in today's world very few re-builders are around and they just go to .010. or more oversize because it is less costly/easier.
Times change, it is a throw away world.
If a customer had a cylinder that was in a nearly new engine that was scuffed or scratched for various reasons it was common to lightly hone it and put in a .003 oversize piston/ring set. easy peasy warranty repair.
those days are long gone.

So a .003 oversize bore would result in approx. 0.0095 additional ring gap.
I don't see a problem using the lesser of ring gap clearance of rings you have.
In the big picture of things, maybe many many hours of operating time it might consume some oil than a tight fitted ring would not.
I'm betting you will never see it live to that point, maybe your Gr children will.
It is not like this engine is operating a irrigation pump or a lighting plant,
used on a daily basis.
Just my take.

[RustyShackleford]

I agree George. Thanks for the history, and the reassurance

Hopefully will have a first start video and some rebuild pictures for you guys soon.

[dodge trucker]

Something here don't make sense to me. You were questioned about running a new piston in a used/worn bore.
You came back and said the block was bored/honed.
How much was it bored?
You have to use the size piston that goes with the overbore. So if it was bored I'd guess it was at least 0.010. means that you can't use a std bore piston.

If everything else is in fact OK I wouldn't worry about an extra thou or 2 on ring end gap of all the machine work is where it should be.
Air cooled engines do tend to run a little bit more piston to bore clearance than water cooled ones.

And 1 more thought/ I have had engines that were borderline on needing a bore or not.... Kohler used to offer a 0.003 over piston, meant to be used with a STD size ring pack. WHEN YOU CAN FIND them (they have not been made in ages, and are getting scarce) and IF your engine can be honed and maintain a non tapered/non egg shaped bore, then if consider setting piston/bore clearance as you are after.
But otherwise no.

And if you use the std rings on one of those NOS/OEM 0.003 pistons then yeah your ring end gap maybe a thou or 2 over "expected".

[ol'George]

Originally Posted by dodge trucker

Something here don't make sense to me. You were questioned about running a new piston in a used/worn bore.
You came back and said the block was bored/honed.
How much was it bored?
You have to use the size piston that goes with the overbore. So if it was bored I'd guess it was at least 0.010. means that you can't use a std bore piston.

If everything else is in fact OK I wouldn't worry about an extra thou or 2 on ring end gap of all the machine work is where it should be.
Air cooled engines do tend to run a little bit more piston to bore clearance than water cooled ones.

And 1 more thought/ I have had engines that were borderline on needing a bore or not.... Kohler used to offer a 0.003 over piston, meant to be used with a STD size ring pack. WHEN YOU CAN FIND them (they have not been made in ages, and are getting scarce) and IF your engine can be honed and maintain a non tapered/non egg shaped bore, then if consider setting piston/bore clearance as you are after.
But otherwise no.

And if you use the std rings on one of those NOS/OEM 0.003 pistons then yeah your ring end gap maybe a thou or 2 over "expected".

Possibly you didn't real all of my replies??