Farmertec 272xp P&C, or What's Wrong with this Picture?

I've been trying to get the piston into the cylinder on the 272xp upgrade of my Husqvarna 61. No matter what I tried, I just couldn't get it in. I tried a homemade compressor, I bought a Stens ring compressor set. Nothing worked.
Last night, about 3:30 am, as I lay there wishing I could sleep, something occurred to me. This morning, I took the ring off the piston and put it in the cylinder.



The red arrow points to the "gap". It measures .006 in.!

 

I sparked a little off each end, now it goes together with no problem! I went ahead and ordered a Caber ring anyway. When I tried to set the cylinder, I noticed the surface wasn't flat. The gasket was still there, so I removed it. The surface still wasn't flat, there was a SECOND gasket under the first! The idiot who replaced the piston hadn't removed the original gasket. Let's see, who WAS that idiot?

Tightened down the cylinder, measured .018 squish. Should I leave it? If so, anybody used Permatex #2 as a sealant?

 

I've had to file off rings before and learned what you just learned. Check in jug first. I use Prematex Motoseal if base gasket delete. .018 is pretty good, tho.

 

.006'' is probably in the ball park for that saw.

 

The pin is .060

 

The pin had better not be affecting the ring gap in the cylinder. The rings are shaped to fit around the pin, not run into it.

 

Yes, and yes. Should be fine...and 0.006" is fine too...whatever was causing the piston to not go in, that wasn't it.

 

Funny, as soon as I widened the gap it went in with no problem. To be clear, the pin is 10x wider than the gap was.

 

Yup, and the ring is made to wrap right around the pin. It has no bearing on the ring end gap at all...at least it shouldn't. If you had to open the gap up to .060" (+) to get it in, then your piston/ring(s) are not a matching set...in other words, one of them is wrong.
Either that or the ring was on top of the pin...but if you struggled with it that much, I doubt that would be the case each and every time you attempted to install. At least you didn't force it...I've seen stuff brought in for low/no compression that were "just rebuilt" turns out the rings were either upside down, or just sitting on top of the locating pin...even seen pistons installed backwards.
A wide end gap will contribute to blowby. Hopefully the Caber ring will work for you right outta the box.

 

All the above are true, .006 is where I'd want the gap on that saw. Like stated above you've got some other issue going on. If you look at the pin in the ring groove, it is typically not dead center in the width of the groove. The rings are typically not flat on the ends either, so the gap between the bottom of the pin and the ring groove is where the long ends of the ring goes. I'm wondering if you possibly were trying to put it together with the ring upside down.

 

I see that brenndatomu and I were typing at the same time...

 

Next time you measure the piston end gap push it in with the piston just above the exhaust port. Then measure it there.
The .006" sounds ok to me too. The .018" of squish will work but is starting to get a little tight. The bigger the piston the more expansion so on saws with a little more CC's I try to give them around .020" squish or more. Just my 2 cents.

 

Yes, the pin is offset towards the top of the groove, but the ends of the ring were flat so they were binding on the pin, preventing the ring from compressing adequately. It will all be resolved when the Caber ring arrives.

 

Ahh, yes so either the wrong ring or it was made improperly. The Chinese parts don't have the best reputation for consistency...you get one of something and it is fine, then the next one is made wrong (or at least poorly)
There is a rule of thumb on ring gap...I don't remember exactly, but its something like 0.002-0.003" per inch of bore diameter.
That may vary some by size and type of engine too...IDK...it'll get ya in the ballpark though

 

If it was me i would open up that gap a touch, ive always heard .004-005 per inch of bore for general 2-stroke applications. A little more for "modded" engines. being a 52 mm bore that would be a bare minimum of .008.

 

Wiseco says you generally need .004-.005'' per inch of bore on a 2 cycle.

 

Well there ya go.

 

Yes?