Don't think there's a way of "filling" the slits.
Just need to hit the local lawn and garden shop and look for old ryobi's and swap the cylinder.

I'm gonna try to swap cylinders with my "currently in use for the lawn" ryobi with my newer twin ring that has slits....

I had bought a reconditioned one with single ring...and found it had the slits too....I was bummed...single ring with the slits...sigh...

4 Ryobis, now scatterred around the house in various stages of conversion....two twin rings, three slitted cylinders (one unknown)...

Maybe some phone calls are in order to local lawn repair shop...

CrazyHerb

Stihl used to sell a weed-eater that used the Ryobi cylinder and block. It was called the FS-36. I just went down and checked 4 cylinders used in the Stihl and none had the slits. This might be an option for you guys. The bore diameter is 35mm. I don't know if they are 26cc or 31cc. I don't have a 31cc motor apart at this time so I can't do a swap. I hope this info is of some help.

The fact that the front lugs were not tapped only means that the engine was a "short block" The Torx head screws used in the weed whackers is a self tapping screw. There was never a shroud installed on that crankcase.

Enjoy,

Jim

Well I checked some of my Ryobi's and assorted clones. I have one 2 ringed Ryobi with slits. One single ring Ryan without slits. Also one of the Ryobi/Ryan motors that Cvan mentioned from a Stihl weedeater; 1 ring no slits.
Like Herb mentioned, It would be interesting to see how a 2 ringed piston would run in a cylinder with no slits.

I know a guy that has a lot of older Ryan weedeaters I may have to go visit him.
Adam

I've flown this combo a few more times with the 18x6W APC "fun fly" prop. I get 7750 RPMs, and I still haven't messed with the timing yet! Recorded thrust with a cheap digital fish scale was around 14 lbs.
I am hoping to try a 20x6 prop this weekend and tweak this thing some more...

Now what will it run with the Cylinder that has the slits in it??
Adam

I'm not going to risk ruining the fit between the rings and the non-slitted cylinder to do a "test run" with my twin ring piston in the slitted cylinder, sorry.

I think there's too much worry about the slits...

Just run what you got and go fly!

Slits or not....these are still just weedwackers we taught to fly. Most run great in any configuration....

I think the BEST quick mod to these is to open up the reed limiter....to allow more fuel/air mix into the case....

If you "happen" to stumble across a jug without them swap it....but for now...just get it on a plane and go have some fun!!!

I may try "pulling the gasket" trick to get a little more compression just for giggles.....but...I need a quick/cheap airframe first....actually two of them!!!

Best of luck with all your mods, and thanks to all who contributed, a pics worth a thousand words....are we there yet???

CrazyHerb

I have to agree with CrazyHerb, Quit stewing and go have fun. As far as quick and easy hop-ups go I gained about 700rpm by opening up the baffle in the muffler and enlarging the exhaust pipe. You will have to do some welding for this. Just use a grinder on the seam around the muffler to open it up. Make sure when your done that there is no way for oil to pool in the muffler.

Cvan

Hi everybody!
I've just faced the same problem with my RYOBY 31. I've been trying to get enough thrust from it for a month. It definitely has low compression (slits are there!) and seems to be overheated when in the plane. Still yesterday I dared to take my Junkers 13 in the air and it was a success though the plane is 9.5 kilos and static thrust only 4.
I know its wrong but the prop was... 22x8. Only this one gave the highest result at around 4000 rpm. Any comments?
My eng was used in grass trimmer (or whatever you call it), I use 1:20 fuel mix (still not sure its correct).
I wonder how you check the eng temp and what do you do to lower it? Seems in the trimmer it had a side ventilation, not front.

I would think that you are over propping it - that would explain the excessive heat. I recommend using an 18 x 6, or maybe an 18 x 8 prop. You should be taching over 7000 - maybe even closer to 8000. That gives the engine a chance to unload a bit. As for the mix ratio - my Ryobi instructions say 32:1. 20:1 would definately affect the operating temperature!

I have to agree with Kraut. Cut back on the oil and your prop size. I use a 40:1 gas oil mix in all my engines and an 18X6 prop on my Ryobi's. An 18X8 will also work but you will probably loose around 600-rpm and some thrust.
Good luck

Hi guys:
so many facts to learn.
New double ringed piston, new jug cilinder with the slits for decompression, wich one should be the best for R/C airplane?? new piston and old cilinder??
thanks for your support.
tato

Unkov:
The extra oil in your engine probably saved it from cooking. As Kraut says run 18-8 and 32-1 mix. Your engine will like you for it. May get more power with a slightly bigger carb too.

Tato:
New piston in old cyl would be better if the old cyl is in good shape. No scratch marks in the cyl.

Results of changing to non-slotted head, and rings and other changes.
Ryobi motor #2 was old lower unit, new carb (no return spring), reed valve limiter opened to .125", definned and lapped flywheel, NGK plug .022", new coil coil/flywheel .010" (business card)
new Zama carb; Stock muffler with .5625 ID pipe insert
Time on motor under for 4 hours.
Starting easy with Tower H-D starter
Prop DynaThrust 18-8
Best RPM range 2700-5940 (carb would not shut off enough air to kill engine)

The changes:
Replaced head with used clean older non-slotted version (lightly sanded with 500 grit);
Replaced rings (2) with Bowman replacements,
Piston & plug decarboned;
Silver sodered hole in butterfly closed;
Reattached return spring (There was evidence on bore and butterfly that there was end play in shaft. may have been part of not closing enough);
Timing clearance .006 @ center contact and over .010" on outer contact (whatever it is called)
Time on motor : 32 oz of fuel
Observations:
The older head has a vertical post in the exhaust port that looks to give a slightly smaller exhaust area.
The started will not turn engine over without loosening the plug slightly for compression release. Tightened while running.
RPM range: consistent and smooth from 1750 to 6200

Now I need some help on the ign. clearance(s). What should the clearance be? (Range) If they do not touch can they be too close?
Do they need to be the same? ( Had everthing Locktite coated and discovered that due to differences in parts I could not even get close to matching gap clearances.)

Getting this engine ready for a new SPAD (cough, cough! )

Ever since I can remember I have used the business card method for setting the coil to flywheel spacing. This has always worked well for me and I haven't noticed any gain in RPM by closing up the gap.
Good luck and have fun!

Coil to flywheel is not to critical. As long as it doesn't touch when it's running to .015 would work . .010 is what the book calls for. The closer, the stronger the magnetic field, the more spark voltage, but even at .015 it is more than enough voltage required to fire the plug.

I have spent many hours trying to improve the engine performance by adjusting the gap and found that it makes no difference if the gap is slightly more or less then the recommended. Business card method is the BEST.

I could try to tig-weld it at work but then you'll have to find a machinist to round the jug afterwards.
PM me if u wanna try.

Hi converts! My friend and I have done the "jug swap" from single ring to a twin ring engine, the jug without "slits" came from an old weed trimmer that we picked at a swapmeet mine is from a leaf blower , we where lucky enough to find that one in an almost new condition and only payed $10.00 for it, we are getting 8300 rpm with a 18x6W APC prop using Mobil's MX2T oil at a 50:1 ratio but we also fly at sea level, the real change that we noticed besides the increase of rpm's was the "torque" it seems to have improved abuot 15 to 20 %, my friends ryobi has a CH electronic ignition and can easy hand start his engine, mine I need to start it with an electric starter. so I recomend to start shopping at the garage sales or swapmeets and look for some good "old version" of weedies or blowers, that's what we are doing.
Bill Pomplun.

or you can get one here

http://www.outdoordistributors.com/

and get these (Jug no slits and twin ring piston assy)

MTD-753-04323 $32.66 1 CYL:31CC:EPA:EF2S
MTD-753-04367 $22.25 2 PSTN ASSEMBLY:W/ROD/RINGS