The Stovebolt Site Forums The Shop Area Engine Shop piston clearance

Ok. My first question. While building my first motor (235) I ended up with an over sized ring in the kit. I found that out by swapping pistons with cylinders and rigs and pistons and found that the problem followed the compression ring or rings. But I found the problem by putting a pretty good scratch about an inch long down one cylinder with the top compression ring. So I found the problem, but the scratch I was told to try to hone. I honed it not too much because I didn't want to mess any clearances up. Just enough to not feel the scratch with my fingernail. I can still see it but not feel it. But when I do the test with the .002 feeler gauge and fish scale, I get 5# sometimes, and 8-12# other times. It kind of changes so I'm not really trusting that test. I was also told that honing won't change the clearances enough to worry about it. Should I just get the new rings and go? or should I try to find the exact clearance to ensure it is right on?

Did you use a Flex-Hone in a hand drill to do your repair? If so, & you just honed the top area of the bore, the taper you have put in there may be hard on the top ring. I've never used a fish scale, I just try it using various feeler gages & get to where one is a 'Go' & the next one up is a 'No Go'. In other words, just find the feeler gage that gives you a pretty tight drag thru the bore & that's your clearance. I've run some pretty loose due to the climate here & the way I run them on the interstate with no problem. But, I would highly recommend having a staight bore, means a power hone like Sunnen. Ring end gap is another subject & should be checked seperately.
Hope this helps.
Doug

a rigid hone with stones will remove enough material to make a diff. A flexhone should not. I doubt the fish scale will give an accurate assesment. A feeler gauge should let you know how much room there is betwixt the bore and the piston, if it's in spec per the manual, you can run it.

I used a flex hone that goes in a drill. I did not just hone the top area where the scratch was. I moved the hone back and forth through the piston. It wasn't as nice and pretty as when the shop did it, but I kept it moving. I think I had the drill going too fast to make the lines as angled as the shops hone. But I don't think it would be tapered. I didn't let it spin in one spot. But if a flex hone won't do too much to change the clearance, then I should be good. I just need to get a new top ring. For some reason it didn't want to fit in any of the cylinders, and various pistons. And it came out of the same kit. Something is wrong with that ring.

You do know that you need to check/adjust the ring end gaps, right? an oversized ring can be made to fit a standard piston via filing or grinding the gap until it has appropriate clearnce ( usually somewhere around .004" per inch of bore).

If I had a ring set like that, I would assume that someone has tampered with it and I would use a whole new set. If the rings are the correct size, you should not need to adjust the gap at all.

I wouldn't worry about your scratch and honing etc.

Even if the end gap is filed to bring it into specification, the ring won't be round afterwards. Return that ring set. Somebody has mixed up sizes when the set was boxed at the factory.
Jerry

I went to the shop and asked about all of this. He said not to worry about the scratch and honing. It doesn't change the clearance enough to make a diff. He let me borrow a tool that will file both ends of the ring at the same time, so it will stay round. told me what clearance I need, which came out to be about .004 for the bore size, and showed me how to check that. I checked a few other pistons which came out ok. and when the main and rod bearings are torqued to speck, I can barely move the crank with the counterweights. But that is barely with all my weight pulling/pushing on a couple of counterweights. So I think I'm good to go on until the next road block I get to.

Back in the mid-1950's, Some Buick V-8's came from the factory with blocks that were .005" oversize. Apparently, there was a mistake in the honing process for a run of several hundred blocks. No problem- - - - -a "standard" ring set is supposed to handle an overbore of up to .010", right? Wrong! Those engines used oil from day one! The factory fix was to install a set of .010" oversize rings, and file the end gaps to fit. The oil consumption got worse! The only way those engines ever got better was to rebore them, and install a ring set that matched the bore size.

Rings are ground perfectly round to fit a specific bore size, and then an end gap is cut to allow them to be installed on the piston. If the ring is so big the ends butt together as it goes into the cylinder, filing the ends will allow it to be installed, but it will never fit the cylinder properly. Changing the circumference of the ring causes it to create gaps where it meets the cylinder wall. The only exception I can think of to that rule are "file-fit" rings for racing engines, and they aren't expected to run more than a few miles before the engine wears out or blows up. A round track engine with 1,000 miles on it is an antique, and some drag race engines get torn down and re-ringed after 3 or 4 1/4 mile runs, maximum.
Jerry

I beg to differ! ALL piston rings should be checked and/or adjusted for end gap. All spark plugs should be checked and/or adjusted for gap! A ring that is .010 oversized will not appreciably alter the roundness when spread over a 4" or so bore. Now .030 or .040" may place undo tension on the ring ( as it has to compress more to be installed), but .010, or even .020 should'nt matter much, and too tight of a gap will result in significant cyl. damage!

Re: Buicks, excessive ring end gap will increase oil consumption. Proper fitment of end gaps should have fixed it. I have to think that the .005 was in taper or out-of-round.

A lot of our customers back about 1954/55 would disagree with you. After they were given the runaround by their dealers until the warranty ran out, we finally fixed several of them with a rebore job and rings that fit. There was a special stamped symbol on those blocks that indicated they were .005" oversize, top to bottom. Yes, end gap is important, but adjusting the gap on an out-of-round ring is an exercise in futility!

Just about exactly 50 years later, I was called in as a consultant by a Cadillac dealer who had several Northstar engines that started burning oil about the 35,000 mile range. We discovered the low-tension rings Cadillac was using, combined with cylinders that were honed .0005" oversize at the factory was the cause of the problem. About 15 engines in a 3-state area were exhibiting the same symptoms. All those engines got replaced under warranty.
Jerry

I'm not saying that a straight true bore is unimportant. I'm also not saying that ring tension is unimportant. What I'm saying is that an oversized ring fitted properly (end gap wise) to a mildly smaller bore will actually increase ring tension. It was designed to fit into a larger hole! when you put the same size circle in a smaller hole you have to compress it more, thereby increasing the tension. I was thinking he may have neglected to properly size the end gap, causing the ring to stick because of binding rings. I have used .010" over rings in standard bores probably 4-5 times, once on my own motor and run them hard for thousands of miles without problems. Granted this was in Harley motors, but the rings don't care what motor they're in! I can however se where a .040 or so difference could cause loss of circularity ( is that my own personal word?)affecting ability to seal, just not .010 spread over an approximately 4" bore! I have to believe that other factors led to this oil consumption, not the "loss of roundness" All rings are made into bigger circles than they are made to fit into. All piston manufacturers recommend a method of adjusting endgaps on their rings! Pistons, bores and all other engine parts can vary wildly from their manufacturing specs, therefore a wise mech. always checks them!

What's the reccomended piston skirt clearance on an air-cooled Harley engine? If it's anything like the Lycoming aircraft engines I've built, it's several times that of an automotive engine. The pistons flop around like a BB in a boxcar until the engine reaches normal operating temperature, and the 65 HP Continental in the Aeronca Champ had a nasty habit of seizing up on takeoff in hot weather unless the pistons had very sloppy clearances. Yes, ring end gap is important, but the ring has to be round to start with, and it's got to match the cylinder bore before the end gap is cut during the manufacturing process. At least that's what Perfect Circle told me when I attended, and aced their 3-week Doctor of Motors training school with a 96% grade average.
Jerry

Btw I have built hundreds of motors, and installed pistons from most manufacturers (wiseco, J&E, mahle, Kieth black, etc.) I would never in a million years assume that .010" rings will not have tight gaps and install them into a .010" bore without checking. It is easy to do and I have seen the results of tight rings on a motor. That motor ( built by a new mechanic) ran fine until it was broken in, but it's first trip on the freeway (first time it attained that high of a temperature) it siezed in the bore and galled the cylinder. The mechanic was shocked that you have to check the ring endgap! He said " it's said on the box it was standard!"

To check end gap you place the ring in the bore. Using either the piston, or a ring squaring tool you square the ring (make sure it is perpendicular to the bore) Then you use a feeler gauge to determine the gap. If it is not at least .004" per inch of bore you MUST file it to attain this clearance. Equally important is the piston to cylinder clearance. The optimum means of checking this is with a dial bore gauge and a micrometer. A quick and dirty way is with long feeler gauges. The clearance will vary with the material ( forged,hyperuectic,cast, etc.)The intended usage ( low rpm street, high rpm racing, etc.) And the thermal range expected. Typically a tighter clearance will be less oil consumptive, quieter, and longer lasting, when used within it's rpm and heat range. The trade off with looser pistons is noise and oil consumption for the ability to survive higher temps ( racing?) and more abuse. As an aside the reason forged pistons need more clearance is that they are denser, and therefore expand more when heated. Also the construcion (design) affects clearances. The more material in the piston crown, the more it will expand. If you could look at a pistons profile you would see that it is substantially smaller in diameter at its crown than it is at it's skirt. This is because of the amount of material, and it's coefficient of expansion. Bottom line, check everything while your motor is being built, instead of wondering if you should have when it is not running right after it is assembled!

I'm not looking for a [censored] contest, here. I'm just expressing my opinion. Harley motors have pretty tight clearances. Mostly around .001" or .0015" I would guess that since most aircraft have fuel mixture adjusters to compensate for altitude changes. Any time you run lean you create more heat. Low altitude ( taxiing, takeoff, etc.) will cause thee mixture to be dramatically leaner than the same setting at high altitudes. Airplanes remedy this by having a mixture adjusting control, so you can set the mixture appropriately for a given altitude. I would speculate that the motor you are refering to is siezing because of the mixture being lean, coupled with the power level being elevated because of take off. At take off the drag is very high and the power requirements are extreme. simultaneously the air is more dense, and that makes the mixture leaner. Lean mixture, coupled with high power requirements is a recipe for siezed pistons this would require large piston clearances.

I congratulate you on your success with your class, and I don't mean to detract from your knowledge. I'm sure you are quite competent and capable. We are debating a small yet significant
aspect of engines. A difference of opinion about ring end gaps, does'nt mean I'm not a good mechanic or that you're not a good mechanic. Lets offer both of our opinions and let everyone who reads it decide thier own course.

The comments about the Cadillac Northstar engine are interesting. A friend of mine has had one for a number of years. He gets about 500 miles to one litre of oil. GM has told him that this isn't a problem!

Thanks for the advice. That makes sense that once things get to the operating temp that the clearences will change. So even though the pistons slide in the cylinders fine right now, doesn't mean they will after things get hot.

I am waiting for my head to get done at the shop. I had to find another one since I found out mine was cracked. So in the mean time I have plenty of time to take the pistons out, rings off and check them all. I don't expect them to be bad, but it would give me a peace of mind. especially since this is my first time.

He just needs to stop buying metric oil. Cadillacs are made in the USA, man!(And Mexico maybe.)