Max bore on UL cylinders?

looking for opinions.
If UL cylinders can be safely bored out to STD ULH. Main problem is the ULH STD bore is 3.421875 and all after market STD ULH pistons are 3.4375. I am at the Max oversize for UL pistons so it's either go up to the ULH bore or sleeve the cylinders. I would have to bore about .030 to .035 larger to reach 3.4375. Using torque plates goes without saying.

Reason,
Well the rear piston in the UL (FlatBoy) decided to go south today. Not sure what caused it yet seeing as how there is just under 2000 miles on it since the rebuild. Only change to the motor over last year besides a rebuild is the flywheel scraper clearance has been opened up to .020. Both front and rear skirts on the rear piston are scored bad. Piston clearance was set at .0025 on both pistons. No smoke from the rear cylinder and no indication that something was going wrong. Just suddenly started making a lot of noise. Front piston looks great. Used the same skirt to wall clearance last year and had no problems and got 10,000 miles on it. I did replace the pistons over the winter with a set of Dixie pistons, last year I had Eastern Pistons. Had my choice between both of them this winter and chose the Dixie pistons. Both have the same makings inside but are definitely different in weight and machining. Dixie appeared to be machined better but now I am not sure. The Eastern pistons were lighter and the wrist pin bore and the counter bore for the lock rings were not concentric with each other. The expansion strut in one of the Eastern pistons was bent.

Figures just as the weather is starting to get good.
Jim

also check timing or overheating problem which might have been from overboring hot spots in the cyl
sounds like the rear cyl got too hot
either of those could have caused it
i wouldnt nessisarly blame it on the pistons but the tiawan china ones are very poorly made
i'd personaly sleeve them to std bore
sleeving isnt a problem all the rocket techs think it is as long as its done correctly

Jim!

I was told that Dixie and Eastern pistons were the same, with a logo inside that looks like crescent moons within a full moon. Were yours different?

(Taiwan and China are completely different countries, and so is South Korea, where those pistons reportedly come from. They are very good hardware.
Oriental rings, however, can be suspect!)

As you know, galled skirts are the end result, and not a cause; So we are looking for where the fuse was lit, not where it went off.
Heat comes from combustion and friction. If you feel that oiling is not an issue, and the borejob had been final-fitted with plates,...then then that only leaves combustion getting out of hand. That takes either a gross miss-timing..... or just a tiny bit more oxygen.

We know you well enough that timing wasn't the issue!

As far as maximum overbore, you can only make that decision by measuring what you have left in the cylinders, directly over each base nut hole.
I use an altered caliper, the old-fashioned kind that look like bug's pincers.
(Unfortunately, this forum does not support photos, and your email is not listed.)
.070" remaining wall is as far as I have pushed it on a Flatty, with no overheating or other problems. (I assume the machine is still in service in Florida climate, and the jugs were even powdercoated.)

Beyond that, you face the tedious and expensive process of sleeving properly.

Good luck, and let us know.


...Cotten

Cotten,
email is flatboy@quik.com

Both pistons have the I think it is JCC (not sure if that's it can't look right now) but the Dixie pistons also have another marking HDC I think. I am in work right now but will check the markings when I get home. The Dixie pistons are heavier, the wall thickness on the skirt is more than the Eastern pistons. It appears to be a heat problem but I have not located the source yet. The manifold was not leaking and the head gaskets look fine. Timing is correct, oil pressure and flow is good. Both pistons when removed appeared to have sufficient oil on the skirts, (well as best as I can tell on the rear). The wrist pins and inside the wrist pin boss of the pistons have plenty of oil.

The ring land clearance is correct, the bore was done correctly with torque plates and checked and re-checked before assembly.

The rings used were, cast top ring, Total Seal gapless second ring and 3 piece oil ring. The oil ring had 8.5 to 9 Lbs of drag.

Valve clearance was OK, checked them both hot and cold.

The baffles have been removed from the cases. There is no skirt oiler installed in the rear. The flywheel scraper clearance has been opened up a bit to see if it would allow more oil to be thrown up to the pistons. The case deck has been cut to lower the cylinders to help raise the compression, the cylinder head gasket surface was cut to make sure it was flat and to raise the compression, heads were also cut for the same reasons. Compression is 6.5 – 1.

There was no detonation before the piston died. I agree that some how the rear got hot just have to try and find out why.

I know, I'll blame it on the new crappy 10% Ethanol gas we have to use out here now.

Jim

ethonol does wash the oil away as it passes ask any 2 stroke mechanic to show you a cyl that had it ran thru it
i'm sure he has a pile of them out back
i still lean towards timing or hot spots in the bore from being too large
although i dont like the import where ever the fuck they come from pistons i dont honestly think that was the cause
and yes a few degrees timing can cause a cyl to gall and the points gap does change the the timing
i think it was 1 degree tining change for every .002 point gap change

Scoot,

Timing is and was fine, bore OK straight and true, comment on the gas was just me being a smart ass. Still trying to find the actual cause for it. More than likely something caused a lean condition on the rear cylinder. Should have checked the manifold before I tore it down but didn't. There looks like there was enough oil getting to both pistons so I have ruled out lack of oil as the cause. I have found it a bit harder to read the plugs with the Ethanol blend. Or something was leaking just after they went to it out hear. I noticed that the plugs were burning more of a gray ashy color just after the switch. I though I might have developed a manifold leak at first. Checked it at that time and there was no leak. Checked the valves at the same time and they were fine. I also richened up the carb a bit to see if it made any difference in the plug color but didn't notice much.

Checked the cylinder deck last night and it's perfect. Haven't checked the head as yet. Can't find any leaks in the manifold inserts or the intake port of the cylinder. If I can't find any leak there using a Helium leak detector then I would say it's tight.

Jim

funny how we think we are being smart ass's and thing fall true
go to your local chainsaw shop and ask about what i said
then rethink your i had plenty of oil
str8 band true bore doesn't have shit to do with HOT SPOTS in the bore
read up on it
i never did but i listen and learn from guys that been doin this b4 a mechanic was called a tech or rocket Scientist
its all about common sense

hey man i'm just tryin to help cause if i asked i'd like some help too

No problem Scoot taking your suggestions and looking to see if I can spot them.

Still looking for the cause but still haven't found it. Don't think it's not enough oil but might be a combo of things. Possibly the rings scrapping to much off the walls causing lack of oil but the front looks fine and it gets less oil trown up to the piston or leaning out somehow still looking. Brought everything into work today and leak checked it all again and found no leaks in the manifold, the intake port, the cylinder around the intake port, the bolt holes, the inserts, the gaskets or the seals. Checked it cold and warm with the helium leak detector, if that didn't find anything then everything is tight.

Looking at the bore and the piston, the gauling is from top to bottom on the cylinder front and back and the same on the piston. There is no place that looks like it may have been tighter or hoter and caught the piston. It looks more like both front and back skirts on the piston expanded and started to seize. So it is leading me to heat caused by a lean condition. If the bore were to tight or there was not enough oil getting to the skirts from the get go, I don't think I would have gotten close to 2,000 miles on it before it died seeing as how flatties run hot.

Also concidering opening up the bore a little bigger on the rear when I get the pistons. Maybe, not sure yet.

Jim

was the weather any hotter then its use to?
did you get your gas anywhere new?

No and no. The weather has been relativly cool for this time of year.

Got another question,

At the bottom of the stroke when the piston is at it's lowest point, does any portion or amount of the skirt protrude below the spigot of the cylinder or should the entire piston stay completely up inside the bore? Still have to look and measure a few things. Cases have been decked quite a bit, but not sure if the spigot of the cylinder was shortened by the same amount or even if it matters on a flattie.

Jim

you would have to deck a lot for it to matter

its when the pistons going up that the oil hits the cyl

Jim!

I know this is a dirty subject, but can we exclude the possibility of particulates from below?

Can you examine the galling with a strong magnifier to determine if there is anything other than aluminum there?

No sparkles I hope.

You might even scrape a bit of the galling into a small glass dish and wash it with a little clear solvent. While its under solvent, take a magnet beneath the dish and hope your hairs don't stand on end.

After its dried to where you can get it to cake upon a piece of stainless wire, place it in a gentle propane flame and watch for color. Green would be the worst.

If either of these test are positive in your mind,...then you will probably want to swab the oiltank for more clues.

....Cotten

Cotten,

The test you are suggesting may not give me an answer. The reason being that the there was enough damage to the piston to gall the cylinder and the rings. So there would more than likely be iron and steel from the cylinder and rings in the oil tank and embedded in the piston. I have not pulled the lower end to split the cases yet but will probably do so this weekend. There was and probably still is metal in the oil tank. I haven't removed the oil tank to give it a good cleaning. At this point I think it would be foolish not to tear the motor down and have a look at everything and to make sure all the debris is out of the lower end.

As far as the cases being decked, there was approximately .030 to .050 removed. Not sure if that is enough to make a difference.

I can see no other cause so far for what happened other than the rear piston getting real hot front piston looks good. Just haven't found the cause.

Jim

Maybe this is stupid.
What if the ex valve does not seal as it should, either from small valve to stem clearance, carbon on seat etc. Could this give you a leaner mixture?
Normally there should not be much oxygen in the ex pipe, but if the pipe is not sealed to the cylinder it could get some air in that way.
GuS