bryris

I picked this thing up on e-bay for $25 bucks or so. It was advertised as never used, but I believe it has been used. It is lightly used either way. However, based on my research, this is a 25 year old motor - maybe older. Its got the old strap on type muffler with the band that wraps about the jug. It turns over nicely, compression seems nice, etc. The only potential problem with the motor - which may not even be a problem - is that if I grab the prop shaft, I can push it in and out about 1mm or so. When I hold pressure on it, I can tell that the crank is rubbing the back plate a bit. If I remove the pressure, there is no rubbing, its quiet, and all seems well.

The only other motor I have available to compare is my relatively new OS .46AX, but it has no shaft play at all. Is this something that needs to be concerned over? Perhaps the older design of this motor just had this inherent characteristic in the design. Any ideas?

Also, I'd like to remove the back plate to look at the back of the shaft. Is it as simple as just removing the screws, looking, and reinstalling? Is there anything strange that is going to happen if I do this? I've never removed a back plate before.

Thanks!

vpresley

My Feedback: (162)

Hi

A picture would help. if its a bushing motor a little play would be normal for it. If its a bearing motor then that would not be good.


Vince

fizzwater2

My Feedback: (61)

sounds like a bushing engine, not a ball bearing engine. End play like this is normal. When the engine is running, the prop will be pulling forward on the shaft, so the pin won't be rubbing on the backplate. Probably not the first choice for a pusher engine, though.

However, using an electric starter on this may very well cause the pin to rub when you're starting. You would be best off to hand start this one, and if it were mine, I'd use a good dose of castor oil in the fuel, too. I had an OS 25 wear quite prematurely because I didn't have enough oil in the fuel. This was a control line plane that my kids flew (me, too) and the all synthetic fuel apparently didn't offer enough protection against lean runs on hot summer days.

luvtoav8

Most likely the result of an electric starter being used on this vintage, bushing bearing engine. The optimum end-play on these engine is about .003-.005 inches. Modern bushing engines have a steel thrust washer between the prop driver and the engine case to endure electric starter use. I have an OS 35-S with similar end play and it runs fine, just clatters a bit sometimes at partial throttle settings. If one wished, they could fabricate a washer to go between the prop driver and case to take up the excess clearance. The modern ball bearing engine you are comparing it to will have minimal end play as the outer bearing races do not rotate in the case (unless there is a major problem) thus the end play is limited to the sum of the close tolerances in the two ball bearings.

mike109

G'day As the other blokes have said, it sounds like a bushed (plain bearing) engine and possibly even a loop scavenged one. The fore and aft movement in the shaft is normal if it is.

It is also probably a steel liner with an iron piston and if it is, it will need quite a bit of running in before use. Lots of heat soak cycles with it allowed to cool between rich runs gradually becoming leaner but never lean for an hour or more. Short runs of a couple of minutes initially.

It sounds like one that I had that was a complete pain to start initially and which took quite a chunk out of my finger when it died and then started up again as I reached over it.

With its bushed bottom end and its steel/iron construction, it will need quite a bit of castor in its fuel, as much as 25% initially and 20 eventually.

A Photo would help.

MJD

My Feedback: (1)

> Is this something that needs to be concerned over? Perhaps the older design of this motor just had this inherent characteristic in the design. Any ideas?

The others covered it, this is normal and desireable for bushed engines. But don't use it in a pusher. 1mm is fine.

> Also, I'd like to remove the back plate to look at the back of the shaft. Is it as simple as just removing the screws, looking, and reinstalling?

If it has a paper gasket, you will want to gently twist/pull the backplate to see if the gasket releases easily. If it is gummed/baked on, you'll probably tear it if you remove the backplate. Beyond that, it is indeed as simple as removing the four screws and looking in. Don't reef the screws when you reinstall them, just evenly X-wise bring them up so they stop under load, then snug them up to a level appropriate for their diameter, no grunting.

If you can find SIG Champion fuel at the least I would run it on that, since it is 20% total oil and 50/50 syn/cas so there's 10% castor to start with. Adding a bit more will make it happy if it is low time as mike109 syuggested already.

I just nabbed a NIB Max .25 myself on auction, just wanted one as I never had a .25.

MJD

bryris

Thanks for the replies. I am 99% confident this was an airplane motor. I seriously doubt it had a starter on it. I'll pull the plate and see what I find.

jeffie8696

Sig sells all castor fuel with 25% oil content that would be ideal for this engine. I use it in mine.


Er, Ah , no you will hate this engine , it will never run well, you will have to buy special fuel for it that is really messy and it wont make good power,,,,,,what else can I think of,,,,, You better just ship it to me.

I have a .20 just like yours in the nose of this.

Here is a little video of mine. http://www.rcuvideos.com/video/OS-20-RC-Baffle-Ported

MJD

My Feedback: (1)

You should find someting like this.

Without the connecting rod in place, gravity would pull the crankshaft out. Forget about your end play. You can see here very light rub marks on the backplate, that's just from fiddling with the engine and flipping the prop, which tends to put a bit of backwards force on the crank as you pull it through. Imagine what happens when you jam a starter in to it!

MJD

p.s. yes it's a paper gasket. And make sure you use the right Phillips driver for the screws, they are prone to stripping the heads if you're not careful.

bryris

I see what was meant earlier by electric starter.

MJD - that is the exact motor I have. Thanks for posting those pics. I plan to put it into a .25 sized cub and fly the snot out of it. I'll just run it until I can't and either go electric with the cub or get another more modern .25.

Thanks again!

MJD

My Feedback: (1)

Those markings on mine are inconsequential btw, little more than surface burnishing and merely cosmetic. Quick starter blips on a primed and bouncy/oily engine are fine, but don't do what I see people do at the flying field as in jam the starter into the cone and spin away, then eat a sandwich while deciding whether they should prime it by choking the carb or popping the end of the muffler with their finger. Oh yeah, oops forgot the glow heat too, let's try again.. brrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr rp. You know the drill. I swear I can hear telepathic cries of fear and pain from the engine when that goes on.

MJD

jeffie8696

MJD Isee the same thing at my field. Most guys have spinners that are so blackened by the starter being jammed on the engine its crazy.

landeck

My Feedback: (1)

I still have the engine (OS Max 25 R/C). I flew it in the 70's on planes like the New Era III. I also have the .20 version. It does have a steel thrust washer between the prop driver and engine bushing. I used an electric starter on it without any problems. The engine is still in great shape and performed very well for me for years.

Bruce

MJD

My Feedback: (1)

Sensible starter use is pretty much always okay.

MJD

jeffie8696

Ieven go so far as to lube the thrust washer before use. Usually on a newly cleaned engine. Normal engine operation should force enough lube to it afterwards.

bryris

I was able to experience a few things with my first attempt to run this motor. It does run well, but I have a few questions. And I will post a photo or two tomorrow.

Stand by for the questions, as its past midnight right now. I'll post them tomorrow. Thanks in advance.

bryris

I brought the airplane out to the field last weekend and was going to maiden it, but it was too windy. But, I ran the engine a good bit and got it to a point where it was happy. It did require a new A3 glow plug to run well - not unexpected considering the age of the motor. But, once we got it running, it ran well.

A few things I noticed about it.

First of all, I totally understand the electric start issue you guys were talking about now. At first we tried to start it with an electric starter, but I just didn't like the sound it was making. It was either the rear prop washer rubbing against the output housing or the back of the crankshaft rubbing on the backing plate. Can you tell from the photo whether this motor includes the required prop spacer to allow electric starts without scratching the inside of the backing plate? After a little bit, I decided I didn't like the electric starting technique, so we switched to a chicken stick and it started fine with that method. If it is better for this motor to chicken stick it, then I will certainly do that - no problem.

Secondly, the exhaust appears to be a "strap on" type of system. Does anyone know off hand (a long shot I know) the exact specs of the screw that holds this muffler on? The head on mine is fairly stripped. Its in tact enough to manage, so its still flyable and all - but I'd like to replace it soon.

Thirdly, I noticed that some glow fuel was spitting out of the exhaust flange while we were running it. If you look in the photo, you can see where I am talking about. Did this motor originally come with a gasket that fit inside that flange? Should I put a thin application of rtv sealant in there?

Lastly, the throttle arm busted on me at the field. It appears to be a unique type of throttle arm. If you look at the photo with the carb in clear view, you can see the throttle arm jams up against the copper plate and the screw holds it down. The LHS had no idea what to do with this. I do have most of the arm still and might be able epoxy a servo arm piece onto what remains of the throttle arm. What would you recommend? Neither of the photos of this are great, but hopefully you can get the idea.

Thanks!

Mr Cox

For the thrust washer, simply look at the gap between the case and thrust washer, and turn the engine over while pulling and pushing along the prop axis. It should turn smooth in both positions if it is not touching the backplate.

The throttle arm is a very common one, shouldn't be any problem finding a replacement. In the mean time you can use the inner hole, just make a corresponding change on the servo.

MJD

My Feedback: (1)

Why would the throttle arm just break like that in the field I wonder?

What is the vibration level of the engine/mount setup like? What I see in the pictures is that the grain direction is backwards on the side supports for the beam mount - vertical instead of horizontal. I don't think they're doing much for you like that.

I measured to be M2.5 x 0.45. Yes, those Phillips heads are prone to stripping unless you have the exact driver, and even then they still are.

MJD

p.s. I've extracted some more life out of scews like that by Dremelling a slot across the face for a standard driver, using a thin abrasive wheel. Rube Goldberg fix for sure, but it can make something works that wasn't before. Esp when they are stripped and torqued in place.

jeffie8696

You can probably make a throttle arm from a servo horn.
Iwill suggest an idle bar plug, worked wonders on mine. The way baffle ported engines are designed they tend to spray the fuel at the glow plug at low rpm and douse the flames so to speak.
Are you certain the thrust washer is installed under the prop driver? Ioften use a starting tecnique where I spin the starter then just quickly and lightly hit the spinner with it , works best when the prop is turned clockwise to the edge of compression so it can build a little inertia.
These engines like to start pretty wet too. There is a hole in the muffler for priming. Just make sure you have the piston up against compression when you prime it so you dont flood it or hydro lock it.
Dont give up, when you figure them out they run great.

bryris

Ok gents - its amazing how long I've been in RC, yet how little I really know. Turns out the thrust washer wasn't even installed. So, I installed it and it makes a HUGE difference. Hopefully, I didn't hurt anything before. I heard some rubbing sounds while we attempted an electric start before said washer was installed and it started up and we let it run for 10 minutes or so, but since the prop was pulling on the shaft, I don't think any damage was done.

jeffie8696

Iwould take off the backplate and hose out any particles just in case.

flyinrog

My Feedback: (1)

Good advice and what is that cool plane back in post #8?...Rog

jeffie8696

House O Balsa P-51 - .20 size. Nice plane but the kit was a serious build. "Box OSticks " Luckily my buddy Brad was out of work and wanted to try it so Ilet him build it. Iam going to try my hand at a Herr Rally XP , nice kit, lots of lazer cut parts.

bryris

I pulled the back plate and everything looked good to me. One of the screws did strip out the head despite my best efforts. But I vice gripped it off. I went ahead and reinstalled said screws again and just used the grips to tighten it down. I doubt I'll ever pull the plate off again, so its fine.

Do I need to torque the screws to anything in particular? I just tightened 'em up is all. Do I need threadlocker on these? If I wanted to go so far as to get a new screw to replace the stripped one, how would I go about it? I have the damnest time finding the screws I need at LHS or hardware stores.

Also, as to the muffler flange leak I have, how can I fix this? Can I put some RTV inside the flange or is there a better way?