extra300crazy

I have the chance to get an O.S. 120 II Gemini NIB and I'm looking for advice as to it's running characteristics. I've been using O.S. 4-strokes exclusively for nearly 30 years. I currently have the 56a, 72a, 81a and 110a that I use in planes. I've used their older ones with air bleed carbs and prefer the two needle type.
I would appreciate any and all information that the members here could give me regarding tank position, tuning quirks, operating it successfully without onboard glow, prop preference and the like. My hope is to use it in a 60-90 size sport plane instead of the usual Cub.
Any help would be greatly appreciated.
Karl

daveopam

My Feedback: (9)

I've got some time on one. It was on a Cub however. It is a strong reliable unit. Love the sound of it. mine liked a 15x8 with 10-15% fuel. With the onboard glow it would tick over all day long and take throttle at any point. The only drawback to using it in a sport plane might be how you fly it. This is not a pumped engine and there is no muffler pressure for the tank. If you fly around inverted or do a lot of steep pulls you could starve it out.


David

jaka

Hi!
I ran one 30 or so years ago and it was as sweet running as any OS engine is.
Ran perfectly on 5% nitro and 20% castor oil (rest methanol). Suitable prop is a 16x6 or 15x8, but i recommend 16x6 bef´fore a 15x8 if you have a larger 1/4 scale CUB in mind.

extra300crazy

I'm grateful for any tips you can offer. I would hesitate to use any sort of muffler pressure on it as I think it was meant to be run with the stock headers. I've heard that using flexible exhaust extentions will caise it to overheat.
Karl

daveopam

My Feedback: (9)

One other thing to mention. Installing it can be a bear. Keep in mind you can't get a one piece cowl over a twin unless it is a big round one. On the cub the cowl and engine were put on as a unit. It took a few extra holes in the cowl for a long ball driver but was not terrible once you did it a few times. I also fed the throttle linkage through the firewall as the engine/cowl went on.
Never ran anything but the little chrome exhaust stacks so I don't know about the flex pipe.

David

extra300crazy

Dave,
I've seen split cowls and know how that's done. On a couple of tight fitting single cylinder installations, I had to feed the throttle cable in as you suggested. I know that the engine would lend itself well to an RV-4 style cowl or possibly a revolver 60 cowl with some creativity. Does the engine run well on suction alone for the tank? I think if I get the engine broken in properly and set the air bleed correctly, I should be alright.
Karl

daveopam

My Feedback: (9)

It's a fantastic running engine. Never had a dead stick. Would idle for long periods without issue. The air bleed carb is not ideal but with remote glow on under 1/3 throttle it's a peach. I would not be afraid to recommend it to anyone.

David

extra300crazy

Dave,
Did you have any luck with it without the glow driver? Having to use one really wouldn't be a deal breaker for me. Was your fuel draw and delivery reliable? After all these years with the single cylinder O.S. 4-strokes, I'm lusting for this one because .....well life is too short to fly mediocre engines!
Karl

daveopam

My Feedback: (9)

Sorry, but the glow driver was with the engine when I got it. I ran it on a test stand with out it but never flew it that way. It could be started with one glow driver and then while it's running fire the other cylinder. ALWAYS used a OD "F" plug. Never had an issue with fuel draw.
You can make a home made glow driver with a little work.

David

extra300crazy

Dave,
Thanks for the reply. I wound up getting the engine at the Toledo show yesterday and am very happy with it. I did notice that although the crankshaft is 5/16-24, the main lock nut securing the prop and going through the center of the prop measures 12 mm. Is that the diameter that you ream your props for?
Karl

daveopam

My Feedback: (9)

Mine used four screws and I'm sorry I don't remember the size of the center ream.

You are going to love that engine.


David

extra300crazy

Dave,
I'm open to any other hints or tips that haven't been posted.
Karl

daveopam

My Feedback: (9)

I'll give that a thought Karl, but it's been a while :-)

extra300crazy

Thanks dave,
Karl

jaka

Hi!
No pressure needed and No glow driver needed! It idles just perfect without one, and that on just 5% nitro.

extra300crazy

Thanks for the input jaka, I normally run 15% in my O.S. singles so that should make the idle perfect. I've located an old fox metric prop reamer that goes 10mm-12mm so I'm covered on that one.
Karl

THERCAV8R

My Feedback: (283)

I have one on a Sig space walker 1/4 scale. Run it on jet cat 20/20 ys fuel. never had a problem with it no mufflers and no pressure to fuel tank. The cowl is split where the top half comes off. 16/8 zinger wood prop. great engine.

extra300crazy

THERCAV8R,
What does your Spacewalker weigh? Does it have enough power to do basic aerobatics such as loops and cuban eights? I'm impressed that it flies the 1/4 scale Spacewalker well, as most people prefer the 160 instead of the 120. It thought that the 120 would be well suited for a 60-90msizes sport model. The now discontinued GP RV-4 comes to mind. I thought that a Xoar 15/7- 16/6 would work well. I just saw the new SIG 1/4 scale T-CLIPS high wing hot rod at the Toledo show at around 9-10 lbs. that just might be the ticket. Just saying..............
Thanks for your input,
Karl

THERCAV8R

My Feedback: (283)

Well you can't hover it but it will do loops and point rolls with ease Cuban 8's are no problem either. it doesn't struggle but you can't do 400 foot loops. it's not a speed demon but a space walker isn't suppose to be either. normal take off roll distance 100' to get airborne. never really weighed it but over 10 pounds for sure. The model is covered with iron on fabric and painted. remember that the spacewalker is like a cub, get it moving and it will fly. I have seen many spacewalkers with the 120 twin on it. They all do well. The T-craft would be a great choice. The 160 does have more power and remember the twin 120"s do not have the same power that the singles do but they sound a lot cooler. Also depends on what you have to pay for it, is it a value? parts are scarce for these engines. once broken in and adjusted you never have to touch the setting again as long as you keep the same fuel and prop. Your prop choices are sound. I have found the 16X8 best for my type of flying on 120 four cycles. also I chose the single nut and jam nut instead of the 4 bolts to hold the prop on. the 4 bolt arrangement comes loose.

extra300crazy

THERCAV8R,
I've been flying the O.S. 4-stroke singles for years, I'm just new to the twins. I picked it up yesterday at the Toledo show NIB for a good price and am very happy with it so far. Although I love my aerobatic Extras, I only do IMAC and precision manuvers not 3D. My plan is to put it in a mildly aerobatic sport plane in the 9-11 lbs. range. I'm impressed to hear of the performance of the engine in the mid weight 1/4 scale planes. Please keep the advice coming, I welcome all of it!
Karl

MX240

My Feedback: (4)

The OS 120 Twin is no longer made and has been replace with the OS 160 twin.
No problem through.....still a darn nice running twin. I have had one 120 twin and at the present two OS 160 twins. All run great. Been using them for twenty years. Sure not a 3-D type engine. The first ones would break a connecting rod on high RPM dives.
I also run glow drivers below a forth throttle just for safety of my scale airplanes. If the carb is set right they will run without it.
I also use a vibrating style Perry pump on all mine. Make a metal bracket under.... and use the forward engine mounting bolts. Yes, the twin runs smooth .....but the Perry pump still works and will keep the fuel flowing on uplines.
Jim

MX240

My Feedback: (4)

I might also add, do NOT make any hard nose first landings and break off the low mounted carburetor.
It will take the case with it. Big $$.

This is a advantage to a Satio twin carburetor that is mounted in the back of the engine.

extra300crazy

MX240,
Thanks for the tip on the ocillating Perry pump. I got the 120 instead of the 160 because of the price and the fact that I'm really not interested in anything bigger than a 60-90 size model, in spite of the fact that they are both the same size and weight. A lot of plane designs need the extra nose weight and I think I have that covered. Thankfully I've got my landings down pat by now, but your point is a good reminder. I did some home work on the engine's history, and was aware of the first generation's short comings.
No 3D intended, just some average aerobatic sport flying. I've got my Extras for the IMAC stuff anyway. However, that engine on something like the 70 size GP Revolver or similar......................
Karl

MX240

My Feedback: (4)

Thanks, I couldn't think of the word "ocilliating" Perry pump.
You look in "My Gallery" and you will find a picture of my Hanger 9 C-182. Wingspan is 96" & 16#.
Wonderful size for a OS 160 twin that just shoe horns inside the scale cowl.
Has the power to do three loops or three rolls...if I want? On the 160 I use a Zore white prop 17-8.
That would be too big for your 120.

extra300crazy

MX240,
What did you have your 120 twin on? Weight and prop size would be helpful.
Karl