Piston/Bore Clearance - K241

[RustyShackleford]

Hi all,

Back with another engine question. I’m rebuilding a K241 and am using a NOS piston, style A specifically. When I dropped it all off at the machinist I brought the Kohler rebuild manual, but read it incorrectly when I told him what clearance to go with. The manual lists one clearance for the K91 (3.5-6 thou) and 7-10 thou for all the other K-series engines. Well, in my rush to get it all dropped off I mistook the K91 clearance for what I needed, and ended up with about 4 thousands clearance (measured with a feeler gauge).

Now, obviously I need to have that widened up some. My question is, I have read on here (from J-Mech, I believe) that he usually shoots for 5 thousands clearance on Kohler K’s. That’s not spec, so I just want to verify…should I stick with the Kohler rebuild manual recommendation of 7-10, or is 5 better for some reason? And if so, why?

Thank you!

[ol'George]

Put that feeler gauge away and use a dial bore gauge or at least a mike with telescoping gauge.
Better yet, take it to the machinist that did the work, if you don't have the precision tools.

You just might find you have the correct clearance.
Guessing is not what you want to do.

[RustyShackleford]

Will do! I’ve read conflicting things on using feeler gauges before. Some say they do it, others say to never do it. Figured it’s better’n nothin’. I was planning on taking it into the machinist tomorrow either way, so I’ll have him check it out.

At least it’s not too loose!

[finsruskw]

You are going to put an NOS piston in a hole that has not been bored??
In effect a worn cylinder bore.

Best to let your machinist tell you whether that is a good idea or not.
My guess is that he will advise against that depending on his findings as to the taper and/or out of round current bore.

May I ask as to just why you are considering that?
To save a buck or two?

[Billy-O]

Originally Posted by RustyShackleford

Will do! I’ve read conflicting things on using feeler gauges before. Some say they do it, others say to never do it. Figured it’s better’n nothin’. I was planning on taking it into the machinist tomorrow either way, so I’ll have him check it out.

At least it’s not too loose!

I agree with ol'George.... Another thing about pistons is that it is often slightly barrel shaped at the skirt... That would make the using the feeler gauges difficult and inaccurate.

[RustyShackleford]

Originally Posted by finsruskw

You are going to put an NOS piston in a hole that has not been bored??
In effect a worn cylinder bore.

Best to let your machinist tell you whether that is a good idea or not.
My guess is that he will advise against that depending on his findings as to the taper and/or out of round current bore.

May I ask as to just why you are considering that?
To save a buck or two?

Sorry I should have been clearer. The block was gone through already. Bored & honed, valve seats ground and valves lapped, and all that. I was just mistaken in telling the machinist the wrong piston/bore clearance to use. Totally my mistake. I actually have darn near NOS everything for this rebuild, even a NOS crank! Just want to make sure it’s all as “correct” as it can be while I’m rebuilding it. Part of the fun, I suppose. I’ll actually be taking your advice from an older thread where you suggested cutting a 1” nipple in half and welding it onto the exhaust elbow in place of the worn out threads that are currently on the elbow. Hoping it will work nicely!

Originally Posted by Billy-O

I agree with ol'George.... Another thing about pistons is that it is often slightly barrel shaped at the skirt... That would make the using the feeler gauges difficult and inaccurate.

Gotcha, that makes sense.

Thank you guys! I appreciate the help, as usual

[RustyShackleford]

I’ll be taking it by the machinist’s place after work today, hopefully. Should we shoot for .007-.010 clearance as per the manual?

[ol'George]

Originally Posted by RustyShackleford

I’ll be taking it by the machinist’s place after work today, hopefully. Should we shoot for .007-.010 clearance as per the manual?

Most engines of that bore size use less than 1/2 of that clearance, of course it depends where one measures the piston.
I prefer to measure the piston skirt at right angle to the pin, and below the pin at the biggest part of the skirt.
If one truly has .007-.010 measured in that situation, it will rattle like a marble in a bottle.

[RustyShackleford]

Originally Posted by ol'George

Most engines of that bore size use less than 1/2 of that clearance, of course it depends where one measures the piston.
I prefer to measure the piston skirt at right angle to the pin, and below the pin at the biggest part of the skirt.
If one truly has .007-.010 measured in that situation, it will rattle like a marble in a bottle.

I was thinking the same, but...all of the Kohler manuals say .007-.010? Perhaps these old Style A pistons really expand that much when they run? I would hate to lock up a shiny "new" engine

I certainly trust the collective experience of everyone on here - amd I do find it odd that Kohler would spec such a loose tolerance. Maybe these engines just run stinkin' hot

Edit: Here is the post/thread I was referring to regarding reading on here about using feeler gauges: https://www.onlycubcadets.net/forum/...d.php?p=464530

[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