Working with the OS 140 or 160 Engine 
(or Verne gets help, I needed it also (Editor))

All this discussion was originally from the discussion group on the NSRCA web site
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Rear Bearing Part Numbers

 Removing Connecting Rod

Removing Rear Bearing

 Installing a New Bearing

 Pressure for the 160 Tank  Mike Harrison's OS140 Setup


You can count on less than 50 flights on the original bearings if the experience of all I have talked to is any indication.  Fortunately, the bearing listed below are relatively cheap.  Most people seem to think running the engine dry after each flight day and using an after run oil of some type, Mobil 1 etc. seems to help.  Don't be surprised if the original bearings fail early in the engine's life.  New, higher quality bearings make a big difference.

Rear Bearing Part Numbers:

We've bought bearings from Purvis on 2 occasions. the first time we took in an OS bearing and asked for a better quality one. They gave us a sealed bearing from MRC. Part # 1904SZZ  the second time we took the MRC box to them and asked for more. This time they gave us an "equivalent" from the company Bearings Limited. Part # 61904-2RS. 

Both are good bearings. Mike Harrison believes that the Bearings Limited version was better than the MRC. I installed a Bearings Limited last September and they are still working. This is awesome performance, but I have no direct comparison data. 

Either will be fine, but if there's a choice, get the Bearings Limited brand. You should get 2 for yourself. Buying this way is less than half price from buying OS bearings, and the quality is better.

Lance Van Nostrand

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Removing Connecting Rod:

Anyone know how to get the piston pin out of an OS 1.4RX? (Ed: first remove the connecting rod) Working through the exhaust port, I got the circlip out but the pin seems to be stuck pretty good. I thought I read where the id of the pin was threaded but it doesn't SEEM like it is (Ed: it isn't threaded like the Webra). Can't see in there real well but the id feels pretty smooth. I want to replace the rear bearing before the Nats to avoid any possible problems. 

Thanks, Verne 

The OS 1.40 requires you to slide the conrod on the wrist-pin. There is enough travel to then allow you to remove the big-end from the main crank pin. (Ed: you have to remove the sleeve first)

The problem is that the wrist-pin gets dirty either side of the conrod. This prevents the conrod moving easily. I have had to reach in there with a scraper before now. Most times a strip of cloth with Mineral Spirits can be treaded around the wrist pin and then pulled back and forth to polish the wrist pin. Sometimes I have had to use Acetone as well. 

To remove the rod I hook my pinky around the rod and pull it towards the rear of the engine. It should move about 3/16". It will then come off with the crank near TDC. With the piston assembly removed I always polish the writ pin for the next time. Ed: now remove the crank before you end up with the next problem (see below)

If you disassemble the piston assembly make sure you orient the rod the same way that it came off. There is a chamfer at the front of the phosphor bronze bearing. 

(Hope this did not sound too much like a lecture?) 

Eric Henderson 

Eric, Not at all like a lecture. Last night about 10:00, I would've paid good money for it so I could fly today. Between the various responses, I was able to realize what you just explained and got it off. Next comes the rear bearing itself which is locked solidly onto the crank. Came out of the case okay with a little help from my heat gun, but coming off the crank will be another matter. I've been flying YS for so long now that I've forgotten how to get through these little chores. 

Thanks, Verne 

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Removing Rear Bearing:

Here's what happens if you heat the case with the crank still in place.  The crank and bearing may come out together.  That's what happened to Verne and here's the response from the list.

Now you did it! you shoulda tapped the crank out first, then heated the case to get the bearing out. Since the bearing and the crank are
both steel, they will expand fairly evenly with heat, so you may have to fabricate a gear puller to get the bearing off the crank.  Jeff Hughes

If you can stand one more take on the OS piston I'll share my experience. I have been flying OS 140's for 3 years which means I have been changing bearings at least twice a year. Everything that has been written on the list regarding the wrist pin is correct. The method I use for the bearing is as follows: Perheat your oven to 500 degrees (Ed: I use 400). Clean as much fuel off the case as you can. Put the case in the oven for 30 minutes. When you open the oven door, the bearing will be out of the case. If you are not this lucky, it will only take a slight rap to finish the job. Put the new bearing (stainless) in the freezer for the same 30 minutes. I remove any shields from the bearing. They only will restrict lubrication. Put the bearing onto the crankshaft and insert into the case. I know you'll know what to do next. 

I suggest you use only stainless bearings and expect to get about 100 runs out of it. The early warning sign of wear will be a slight rocking of the prop when the engine is hot. Once that starts you have about 10 flights left. I use after run oil religiously. 
Steve Miller

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Installing a New Bearing

Never use a torch. 

I remove everything from the crankcase and put it in the oven at 350 for about 1/2 hour. Then, insert the front bearing by tapping lightly with a maple block being careful not to cock it. Next, insert the shaft with the bearing already pressed on and put on the prop driver and a sawed off wood prop. Tighten the prop nut pretty tight and check for freedom of rotation. If ok, I put back in the oven for another 1/2 hour to 'normalize' the case, shaft and bearings. This REALLY helps to get all the rotating parts exactly lined up like it wants to be. Take out and set rear down and let it cool on its own. Check for free rotation after cooling and that's the best it's gonna get! 

To fit bearing on the crank.... 

I use a couple of blocks of maple under the overhanging edges of the bearing so that the shaft counterweight is suspended off the drill press table, then I close the chuck of the drill press and press the tip of the threaded end of the shaft and the bearing comes right off. When I put the new bearing on the shaft I ensure that it's all the way on by putting the threaded end of the shaft through the drill press table center hole and us a length of music wire up the rear of the shaft or a maple block on the counterweight and press the bearing on. Or, if I am lazy, just tap the shaft into the bearing with a small hammer and maple block. I prefer to press it though. 

Dave Harmon 
Torrance, Ca. 


Having been a Millwright many years ago one of the first things you learn is to never pound a bearing into place. Always press, never pound. Pounding causes microscopic flat spots on the balls, which leads to early failure. For the size bearings we are dealing with the drill press is a good choice. Another is to make a sleeve to go over the crankshaft and use your vise to press it into place. Always press on the inner race, never on the outer race as pressing on the outer will also cause flat spots when the bearing hits home and you put that little extra force on it to make sure it is all the way on. We need to take every precaution we can to make sure our bearings last as no one likes to change them any sooner than necessary. Hope this helps. 

Dennis Cone

Before you go to much trouble looking for that "sleeve" to use for pressing, sort through your socket set! 

John Ferrell 

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Pressure for the 160 Tank:

For those searching for devices to make non-pumped motors work more reliably....Iron Bay Models has just the ticket.

I have used this "Iron Bay" regulator for a couple days, notwithstanding the bearing issue, and it has been foolproof. Pressurizes the tank like a YS regulator, too! WHen you pull the vent, you get that SHWOOOOSSSSSHHH of pressure....very comforting. It is ABSOLUTELY a "demand only" regulator, meaning that ONLY the vacumn from the carb causes fuel to flow. And I tested the snot out of that thing using an insulin syringe to simulate vacuum in VERY small draws. It provides pressure, just by bumping the prop back and forth, and has a check valve from the crankcase tap so it gets to max, and stays there. Needles ended up just like the manual for the engine says (for normal pipe/muffler pressure), so that tells me it's not providing TOO MUCH fuel (like the CLines that I tried did - everytime), no hydrolocking, no siphoning when off, even leaving the tank pressurized (I forgot between flight attempts, and it didn't seep a drop) I'm sold on it and will get another for my PW Extra that that I'm converting to an OS 1.60 also. Here's the link: 

It is FAR superior to my unsatisfactory experience with the Cline, and may just make the OS 1.60 really a serious option for folks. Time will tell.

Bob Pastorello, 
Oklahoma NSRCA 199, 
IMAC 1320, 
AMA 46373

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                                                           MIKE HARRISON’S O.S. 140RX  SETUP

  1. O.S. F plug
  2. Cool Power 20% Pro Blend
  3. 18 x 10pn APC
  4. ES 2 stroke carbon fiber pipe
  5. Mac’s header-full length
  6. Mintor 1.40 dual plug head (OS F plug in front hole, OS 8 in rear)
  7. 2 inch between header and pipe
  8. Set the idle slightly to the rich side
  9. Set hi needle strong but rich, do not “burn the exhaust”  it should be a light caramel color.
  10. I tapped the header close to the coupler and ran a fuel line to the tank to give positive tank pressure during high rpm
  11. Take off the carburetor and put a high temp silicon bead around the bottom portion where the o-ring seals the carb to the engine and seat.  I suspect potential for air leakage there.

The ES pipe to use is the standard 2 stroke pipe that is the same size as the OS pipe.  I would set this all up this way then experiment from there.  I found some of this by accident, trial and error, and input from others.  Give it a try and see what happens.  I don’t know which items are how critical but doing all of this works great.  It is simply the best running engine I have had bar none.


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