New Piston, How Tight is too Tight

[BrianS]

Hi, I have a 16 HP Kohler that is going back together. I did everything by the the numbers including boring the block. I put a .020 over piston in and rotated the crank. It's free at both ends and tight but smooth as the piston moves. I put a pair of vicegrips on the output shaft and can turn the crank without too much trouble but really notice the drag of the rings. Can anyone tell me what this is supposed to feel like? Thanks BrianS

[PatRarick]

There should be a noticeable drag from the rings. How much of a drag? I would have to say that depends on experience. Don't know any other way to put it. As to reboring an air cooled engine, I was taught that you do not let the machinist bore the block to fit the piston, as many machinists want to do. You are to have the block bored exactly .010, .020, or .030 over the original bore. The theory is that machinists will bore an air cooled block to water cooled engine tolerances if given the piston. With the higher operating temperatures of an air cooled engine, the tolerances are different than a water cooled engine. I was told this at both Briggs and Kohler seminars. Both claim that you should tell the machinist what to bore the block to, then purchase original factory pistons and you will be OK. Pat

[arjr111]

Hi Brian, redid my 14hp kohler recently, it was still within tolerance so there was no need to bore. Anyhow, as long as you measured the bore and the ring end gap and they are within tolerance (for .020) you should have no problems. See link...Good luck.....Art http://www.yetmans.com/kohler/page4_5/page4_5.html

[arnoldir]

Hello, From my days in the motorcycle buisness we set motors up with no less than 0.004" and considered 0.009" to be worn out. We measured by holding a feeler gauge down the bore and dropping the piston in. A proper reading was a feeler that "just" Lets the piston by with a one fingered push. Measure at least 4 places and take the highest reading, usuallly down where the bottom edge of the skirt runs. The end gap of the rings is really the most important spec to keep an eye on as if to tight they will jam as they expand with the heat. Happy tractoring

[CleanBee]

Hi Brian, With the new bore and cross hash it should have, the resistance will be greater untill the rings are broke in and the cylinder is worn smoother. This added resistance is done purposely by the person who bored the cylinder to cure the rings to the slight taper of the cylinder. I would have checked all specs and with that not be concerned about its feel. If it was bored by a trusted shop and both the rings and piston match, let it fly. I usually changed the oil on rebuilds shortly afterwards to get rid of any fine shavings left over from the break in process. Just my empty pocket change, hope it helps. Dan

[dlcentral]

Right info about Briggs piston fit.Kohler needs .004-.007 skirt wall clearance.A bit more is OK on the 16's,,Briggs piston skirts are not cam ground[oval] like Kohlers therefore more clear is needed.,,With no rings installed.and a long .004 feeler ga. in the bore,the piston should slide thru easilly,,

[acjohn]

I just rebuilt a 10 HP Kohler and bored and honed it to the book specs; .007/.010 clearance on the thrust face of the piston. With new piston and rings, the ring end gap measured .014 with .010/.020 being within spec. If your ring end gap is too tight, the rings will bind as they expand and surely set up the engine. There are a lot of different opinions on bore to piston tolerances depending on the application. If you look at the Yetman's link and go "by the book" you can't go wrong. I took .030 off the head and went .030 over on the bore of mine. I have about 4 hours on the engine after rebuild and the rings have seated and power is really coming on!! Good luck.

[BrianS]

Thanks Guys, Sounds like I was on the right track but the reassuance sure helps my nerves. I bored and honed the block myself to the Kohler specs. I've been a machinist for 20 years but this is my first complete engine rebuild. I finally get to use my skills for something fun. I can make it to the numbers but like Pat said It's hard to judge the fit without experiance. Later Brian

[JP]

I don't agree with the bore it, then find a piston thinking. In a perfect world maybe. There's a few reasons why; Plus - Plus tolerance on the machining of the piston is on the high side (say .010 to .020, your piston is .020), the hole tolerance is the same. 'Louie' bores it .020 smaller (he's within tolerance). NOT good! The same applies with Minus - Minus, of course only opposite. I worked for Kawasaki, in ALL the engines we 'built', every piston had it's own hole. Whether it was a road racer or dirt bike. NEVER, a seizure or premature compression loss from the piston/cylinder tolerance. Water cooled or air, the tolerance is in the engine spec. A number of years back the motor on our clubs (Catamaran Racing Club) threw a piston. The dealer did the hole punching before he got the pistons (he went to the first over size). There was SO much piston slap, I was afraid it would go again. So I took it down for inspection. We had to get Wiesco pistons for it in what would've been the 4th over (factory only offers 3 sizes of overbore). The bottom line; Hmmm, who makes out better? The dealer that sez to do this & ends up with a sloppy job that needs to be re-done (he sells more parts, maybe an engine or shortblock) or You? The first question I always ask; "Do you bore the cylinder to the piston?". If the answer is "No", I keep looking. Even when I find someone who is a "Yes", when picking up the bored cylinder, I ask to shown the 'fit' BEFORE I pay. Of course, that's just my opinion............ JP PS The outboard I mentioned is still strong after 10 years, the first time it was 1.5 years.

[acjohn]

I may not have been too clean in my explanation. I agree completely with JP. Mic the piston, bore and hone to the PROPER clearance. Intall rings in bore (using piston the ensure they are square in the bore, measure end gap and file if necessary. I've done a couple small engines, an inline 4 cyl, 2 V6's and a V8. Been happy with all and you get max engine life. Also, depending on the number of other components that are new, you may want to use assembly lube on some or all components. I use it on all rebuilds since I have completely cleaned all parts and want to ensure good lube (no dry starts) when I first fire the engine.

[Leroy]

Good one John. I was just going to add that. Many people dont think of assembly lube. I dont start a project unless i can finish it. that way i dont fail to often. Leroy

[roma3112]

i would definetly agre with the assembly lube, I myself try to get the best I can find. When i redid the engine on my 3112 i used it through the entire re-assembly process. I am not too up to speed on your engine, but if it is approaching the age of mine, major internal components are expensive, and short blocks are completely outrageous at $900.00 or so. good luck with the re-assembly

[PatRarick]

acjohn and JP, Cannot argue with you as I am not a machinist. I see your points. I don't know the tolerance of air vs. water cooled engines. I do know that I have had problems with aluminum blocks, in having the cylinder bored to fit the piston. Can't say I've had a problem either way with cast iron, though. Pat

[acjohn]

One more note on assembly lube and any else I can ramble on about. I break in an (automotive) engine at 1800 - 2200 RPM for 20 minutes, then change oil. I have never rebuilt a pressure fed small engine, but the same theory would apply as far as oil change interval. In a non-pressure fed engine, the metal particles will remain suspended in the oil as the slinger throws it around. For this reason, theoretically, you should have less wear on the moving parts if you were to use a magnetic drain plug. The steel/iron particles will be attracted to the magnet. In a pressure fed system (with filter), the particles (most) are trapped by the filter. After initial start-up you generate a large pressure differential across the filter due to both the metallic particle buildup, and entrapment of the moly particles used in most assembly lubes. You want to maintain good flow thru the engine so you must change the filter soon. So........if you have a newer pressure lube system, change the filter after about 20 minutes. If you have an older engine, use a magnetic drain plug. Let's face it, the less often we have to rebuild these things, the more time we can spend playing on them!!