piston/ring question

Fri May 10, 2013 6:04 pm

I was wondering about a few things I've noticed on Cub #2 since I've had the head off getting everything cleaned up and valves lapped before I try to start it. The pistons have a little bit of side to side play in them if you press on them and push them back and forth. They aren't really loose like they're flopping around or anything. You have to press firmly to get them to do it. They all seem to move about the same. No more movement side to side than front to back. Does seem looser at the very top than if the piston is 1/2 way down the cylinder. Also, what should be an acceptable gap between the piston and cylinder wall? No doubt it needs a rebuild, but until I hear it running and actually rolling under it's own power, I'm not willing to commit to a full rebuild. Once the bad surprises stop popping up, then I'll start giving her the royal treatment. Thanks!

Re: piston/ring question

Fri May 10, 2013 6:09 pm

Maximum ring end gap: .017"

Re: piston/ring question

Fri May 10, 2013 6:50 pm

Don't you have to pull the pistons to measure that? Haven't pulled the pistons and really don't want to until I get ready to do a rebuild. Want to make sure the tractor is worth rebuilding first.

Re: piston/ring question

Fri May 10, 2013 7:51 pm

dbboss wrote:Don't you have to pull the pistons to measure that? Haven't pulled the pistons and really don't want to until I get ready to do a rebuild. Want to make sure the tractor is worth rebuilding first.
Yup. Gotta pull the pistons. Tractor rebuilding. Good question. I have more in my Cubs than I could sell them for. Other hand is that I use my Cubs as work tractors, mowing, trailer work, occasional garden tilling, etc.. Can't purchase another tractor to do the same work for my invested money.

Other hand, with an engine rebuild, it should be operational for another 40 or 50 years with a bit of maintenance.

I purchased a 154 Cub Low-Boy a year ago, thinking parts tractor. Engine has 105 psi, even, per cylinder, smokes a bit. Currently thinking I will do an engine tractor over haul this coming winter. It's a great mower tractor. But I can't come close to buying a new mower to do the same job for the money I have invested.

Just guessing, if you are thinking about getting your invested money back on a resale, won't happen.

Re: piston/ring question

Fri May 10, 2013 8:10 pm

dbboss wrote:Does seem looser at the very top than if the piston is 1/2 way down the cylinder. Also, what should be an acceptable gap between the piston and cylinder wall?


As you noted, the cylinder wear is less at mid-stroke. Most cylinder wear happens at the top and bottom of the bore where the piston changes direction. The cylinder wear is always less mid-stroke. Keep that in mind when measuring ring end gap if fitting new rings in worn bores. If you set the minimum end gap on a new set of rings near the top of the bore, it's highly likely that mid stroke the gap will disappear and can cause damage to the piston, rings or bores.

My manual says the maximum piston to cylinder wall clearance before rebuilding is .005".

Re: piston/ring question

Fri May 10, 2013 8:27 pm

I am panning on using it as a work tractor so I don't have to swap implements as often, plus it'll be a backup as I work on #1. So far I've got to replace a final housing (old one brazed years ago but busted... good this time), head was cracked and brazed but looks like it'll work, right ear has been cracked and brazed. Repair looks good and it isn't leaking any oil. I plan on building a brace for it if everything else works out & it will run. Hydraulic lines have been repaired at the pump flange (more braze). All of this has been found before I have even tried to crank it! Lol. You can see why I want to hear it run, check the transmission and the hydraulics before I invest in a rebuild on the engine. I know a lot of guys would part it out and not even try to rebuild it, but money is real tight, I got it for almost nothing, plus I hate giving up on a challenge, especially if I can keep plugging at it and it doesn't hit my pocket too bad. I got my money's worth and then some just out if the plows and implements that I got with it. Funny thing is the basketcase #2 looks worlds better than my cub that is running. Lol. #1 looks like #2 feels.

Re: piston/ring question

Fri May 10, 2013 8:53 pm

Most here will not invest in a rebuild of a motor that has a repaired ear. Not that can't be done and work for another 50 years but most will replace the block without ear damage. At least that seems to be the consciences most times it is brought up.

Re: piston/ring question

Fri May 10, 2013 9:33 pm

If the ear has been cracked and brazed, before I would use the Cub at all I would make sure that I got some ear supports installed. That may help the brazed ear last a lot longer. Good reward for a reasonable investment. Send Berlin a PM.

Rick is right, most won't do a rebuild on an engine that has a cracked ear. However, Ellie's mill has 10 over pistons and the ear is cracked. I am installing a set of ear supports now -- well trying -- and that should solve my problem.

Re: piston/ring question

Fri May 10, 2013 10:36 pm

The brace will be my first order of business if I get it going. I have a friend that can fabricate anything as long as I have a drawing or can explain it well enough for him to understand what I need. That won't be a problem. That's where I got my crank handle, middle buster mount, rear toolbar lift and valve spring compressor. Lol. If it can be built and we have the material, it gets made, not bought. Usually ends up stronger and works better that way anyhow. He would be dangerous if he had more than an angle grinder, welder and a drill press. He could build the whole tractor with a lathe and milling machine at his disposal.

Re: piston/ring question

Fri May 10, 2013 11:41 pm

I am no cub expert like some of the guys here and it may be a little different than a cub but I used to rebuild stationary gas compressor engines for the gas company. Some had been in continuous service since the 30’s so needles to say they were flatheads, some things I learned.

1.) Measure the cyl diameter. It will be more at the top than the bottom because firing pressure is higher at the top. Firing pressure goes around the piston to liner clearance and enters the ring groove at the gap in the piston ring. It then flares the piston ring out against the cyl wall. Therefore a higher pressure near the top of the cyl causes the ring to push outward harder at the top wearing the top of the cyl wall away. There should be a spec for maximum allowable cyl taper. I usually measure the top and the bottom and sort of average the 2 out. When deciding to hone/bore/reline.

2.) On your piston rings they expand when they come up to temp. You measure end gap for instance on the cold disassembled engine by placing the rings into the bore and using feeler gauges. When it is operating and the rings expand from heat that gap will close up. So if a man pushes the new rings into the TOP of the cyl and then measures the gap with his feeler gauges and sets it at book spec minimum. It is possible that at the BOTTOM of the cyl it will be lower than minimum end clearance. The hot rings will expand till they touch and tear up the cylinder.

3.) The firing pressure pushes down on the piston rings causing them to seal against the side of the ring groove in the piston that is farther away from the pressure. That’s why you always install rings with the indications stamped into the ring facing the pressure. You measure ring side clearance with feeler gauges to see if the ring groove wears out. It has been my experience that cast iron pistons last for decades and aluminum pistons last only a few years.

4.) Most important thing. It’s an old flathead engine not a spaceship. Out of the 100’s of engines we rebuilt very few were ever at book specs. It was always something like “not enough time to change that cyl during this outage” or the “we will watch those web deflections and see if they get any worse before we change the crank” and most generally every thing worked fine. You should be able to measure your piston to liner with a feeler gauge and see if it is close to the book spec. If it wasn’t burning a lot of oil I wouldn’t worry a bit. :D

Re: piston/ring question

Sat May 11, 2013 12:15 am

Thanks, gusbratz! Lot of good info there!

Re: piston/ring question

Sat May 11, 2013 5:30 am

My 53 Cub has a welded ear and I would have no problem putting rings and bearings in it. If I already had the head off I would drop the oil pan and push the pistons out and check rings and bearings. If the block needs to be bored, I would find a good block. Like Eugene said, I also have more money in my tractors than they are worth. A labor of love I guess.

Re: piston/ring question

Sat May 11, 2013 9:48 am

Good description Gusbratz, probably the best I have ever read. I had a neighbor that asked me about an old farm truck he had, he said that after it ran for an hour or two the engine would load down and finally quit. After setting for an hour or two it would start and run again for a while and then do it again. I asked him about temperature, oil pressure,and if he had tried a new coil etc. and then he finally told me that he and his son had put new oversize rings in it due to the amount of blow by it had. When I asked what he had gapped the rings at he replied "gapped? we just put them in". So I explained how and why rings needed the gap set. I don't think he ever did though, because I would frequently see the truck setting in the field a few hours, then it would be gone again. After 2 or 3 years of use like this it would run pretty much normally, though it finally wore out to the point it is ready for the scrap pile now.