OS reduction gear unit
 

I have an OS gear reduction unit, model GS A-2, that is NIB. I need some info on this unit, such as when it was introduced, retail cost and what the ratio is. It was made for the .61 FSR, so what size props will it spin? Any help will be greatly appreciated!
Thanks,
Pat

OS reduction gear unit
 

All I can tell you is that the gear reduction ratio is 1.895 to 1. This info is off an old OS brochure I still have but no info was given for prop size. The age of the unit is around 20 years or more now. The cost then and present day value now??

OS reduction gear unit
 

Pat,

Shemp is correct about the age of the engine. It came out in the mid-1970's. There were two ratios, but we don't have any information on those engines. They were discontinued long before we became the O.S. Max service centers. We only have information on when they were sold, and nothing else.

As far as we can recall, the gear units were never sold as separate items, but only in conjuction with the .61 FSR engine.

We're sorry, but we'd not know what props would be used. Probably something around 15"-16" in diameter.

You can check the ratio by counting the gear teeth. This would then allow you to calculate prop RPM. The .61 FSR could easily turn 14,000 RPM, The 1.895 ratio stated would give you a prop RPM of about 7,400. Any prop that would turn this RPM on the ground and fly your model is a good one.

Please, though, check the exact ratio of your prop unit, and then calculate the estimated prop RPM before you use the unit. If you accidentally under-prop the engine, you'll over-rev it, and parts for the .61 FSR engines are very scarce.

OS reduction gear unit
 

HI FOLKS!
I am glad I found this thread!
About 10 years ago a .61 O.S.VF ABC GS rear exhaust - geared ( 2 gear boxes: A1 - 1:1.4 and A2 - 1:1.89 ratio) was laying on a hobby shop and I bought it second hand without getting any information about the engine performance or the prior owner.
I maneged to gather some information on it during the last years, and now I am trying to use that engine on a 1:6 scale Tucano- 72"wingspan, 11.6 lb weight.( engine completely cowled, home made muffler, nose gear on the right location!).
Accorging to the leaflet on that engine, the 1:1.4 gear ratio (A-1) was meant for stunt, ( recomended props : 13-14dia. x 9-7pitch) and the 1:1.89 one (A-2) for scale( 18dia. x 6-8 pitch props).
As for price, one add I found on a 1980 magazine cover, indicated 150.00 for one gear unit ( I can check for that info later)
I have actually test flown the tucano with the 1:1.89 unit running a 16x10 master scimitar wood prop at 7,300rpm - engine at 13,800rpm and burning a 20% nitro fuel.
The plane flew right, but it is taking me a new glow plug per each flight, and I wonder what I can do fix that.
Too much compression? There is a .30mm metal shim under the cylinder head.
Poor ventilation for the cowled engine?
I wood like to hear from you guys what could be done to cure it.
If there is interest, I can e-mail or attach sketches, pictures and test bench results with different gear ratios / props.
How can I check if the home made muffler is delivering enough pressure? Pressure nipple is located right after the engine"s exhaust outlet.
Hope you can help me!
Geared.

OS reduction gear unit
 

On ANY model engine, blowing a plug per flight usually indicates a too-lean condition, too much heat, or an over-rev. A good-quality plug should easily take the RPM, so I doubt it's that.

Most likely, the engine's running a bit lean, and you're getting some detonation. Examine the glow plug element. If it's being beat-up and deformed, detonation is the most likely culprit. If the element's gone, it could also be too much heat.

An engine will detonate if it's too lean, if the compression's too high for the nitro, or the tuned pipe is too short for the RPM. To much detonation, and you can burn a hole in the top of the piston.

Pull the head and look at the piston. If the center of the piston is starting to look like it's sand-blasted, and you can see a ring on the piston top that corresponds to the squish band, then you've confirmed that detonation is happening.

The cure is to run richer, add head gaskets, lower the nitro content of the fuel, or lengthen the tuned pipe assembly...or any combination of these.

The geared engine combo turning near 14K is really working hard. The engine would tend to run warm because the heat of the gear train would be transmitted back to the main case of the engine, which would be hard-pressed to keep cool. You need to make sure you get very good aiflow through the fins of the cylinder and head. A lot of airflow over the gearbox helps a lot. Using fuel with 20% oil or more will also help. The extra oil gets thrown overboard, taking a lot of heat with it.

Hope these ideas help out.

OS reduction gear unit
 

Thanks Bax!
I will try less nitro, reduce the compression, improve the air flow near the cylinder head. The only item I can not change is the length of the muffler: it goes inside the the tank compartment. I am not using a resonance pipe. Thats why I need to know how to be sure that the costomized muffler is delivering enough pressure to the fuel tank!
Is there any way to check that?
Thank you.
Geared

OS reduction gear unit
 

If you can richen the needle enough to make the engine drop into "4-cycle" type running, you likely don't have a fuel pressure problem.