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What model is this 70s OS 61?

Started by British Victory, October 14, 2018, 14:09:37 pm

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British Victory

As the title says, just bought this engine on spec attached to an epoxy glass fuselaged wot 4 mk2 from the early to mid eighties in the last century.
The carb is an os 7M and the silencer an os 744 there are a couple of pix nearby showing the cast in lettering. It looks like it's abc.
there's only one f in RCMF,
               John

PDR

It's an OS61 FSR-ABC from about 1988 to the mid 90s. They are very nice engines (I still have one in my engine collection), although they are bigger and heavier than many of the other 60/61 engines of the time. Rock solid runner though - powerful, smooth and excellent throttling. They went especially well on a mild pipe an ED no3 or Irvine 60 pipe if you can't find the original OS blacked steel one. Best run on an 11-8 or 12-6 (they were well matched to the old Graupner 12.5-5 and 12.5-6 wide-blade props) on the silencer, or an 11-7.5 on the pipe. 5% nitro or straight fuel, preferably castor-base.

I would carefully strip it without turning it over and make sure none of that crud has found its way inside. Remove, clean and re-seat the bearings and then when it's all clean put it back together light oiling to confirm the compression's OK. It's really sad to see such a fine engine in such a sorry state of neglect!

There's an article in the "masterclass" area of the articles & downloads section on preparing two-strokes. As this is a used rather than new engine you can ignore a lot of it, but it has useful advice on dismantling, cleaning, seating bearings and reassembly which would apply to this engine.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

British Victory

Many thanks for your help and advice. It runs, helluva finger biter though. It came with a wot4 mk2 withe the epoxy glass fuselage and that was in a sorrier state than the engine, all assembled now with new servos and ready for service.
I've got at least one of those graupner 12.5x6 somewhere, failing that I have a couple of bolly 12.5x6. Only have 15% nitro now, maybe that's why it's a bit nippy.
Whilst it looks shite on the outside the internals are oily as Candy's whatsit and it's quite smooth to turn. It's been hanging up in a barn for the last 30 years.
TBH I only bought it for the plane, I have several similar 2 strokes including a 61FX but whilst the mounting bolt pattern is the same the rear needle precludes fitting it as it fouls the mount so it'll get used. Carb is a swiss watch in disguise!
there's only one f in RCMF,
               John

PDR

October 14, 2018, 17:40:24 pm #3 Last Edit: October 14, 2018, 17:55:00 pm by PDR
Yes. that eccentric-head screwslot shaft to give fine control over the mid-range mixture is watchmaker stuff! OSs of that era were all superbly made - far better than that crude Enya rubbish people seem to praise these days.

If you're running on 15% nitro then you should probably decompress it a bit - one or two 5 thou head shims would be an idea to stop it pitting the piston crown. It should also get rid of those finger-biting habits.

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

w8racer

Pete, how does the OS 61 FSR compare with other engines of the same era - Webra Speed/Champion 61 etc?
Robert Welford

PDR

The OS and the Webra were definitely the preferred engines of the period. The OS was better made and a bit more powerful that the Webra, but it was bigger and heavier. It was also significantly more expensive, but that was a feature of the OS importer until Irvine took it on.

The Webra was quite nice (the only difference between the Speed and the Champion was the latter's slide carb), but they needed new piston/ring/liner sets about every 18 months [of 20 flights per weekend use] whereas the only time I can remember doing that on the OS was when it dropped a plug element and dug a small trench down the side of the liner.

When used in the original manner (max 12" props) the bearings lasted indefinitely, but if used in the later "quiet mode" using low revs on high-pitch props both would consume rear bearings every couple of dozen runs.

Of the other engines of the time there was the HP, but that was a shadow of it's original self - gutless and poorly made. The Merco was still around, but it was non-schneurle and well down on power like the Enya G7 (and the Enya G8 was barely any better). There was a better Enya (the XF) but it wasn't as good as the Webra and no one imported it into the UK (I still have one in a schluter helistar).

Other than that there were a few less popular ones like the HB/Veco. In standard form it was like the Merco, but with slightly better throttling if you could set up the Perry carb (fiddley). It had a "PDP" version which had "Perry directional porting" which was similar to schneurle porting, but with less under-piston ventilation so when used at very high power they had a tendency to pinking (detonation from the overheated piston crown). Then there were the various SuperTigre 61s which were good for the money, but not really in the same league.

I had several of the Webra Champions which came in pre-cherished aeroplanes I picked up at auction, and a few Mercos - two Mk3s which I bought 2nd-hand as my first 60-size engines, and a Mk4 which I bought new and was very disappointed with because the Mk3s were better. But all the 60s I actually bought after that were OSs. A couple of FSRs, a couple of VRs  and a couple of (very nice) VFs. I think the 61VF was probably the nicest engine they ever made, but most modellers seemed to have difficulty installing rear-exhaust motors so they never really caught on.

Others may have different opinions of course, but they are wrong (also of course!).

:)

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

paulinfrance

I had a preference in my guided missiles of the Webra on a tuned pipe and a silly little prop ( 11 x 7 ? )
The OS engines were really good in Helicopters, ( it was OS or Rossi ) the carburettor arm was longer on a bearing ( I my memory is ok )and it had an extended crankshaft for Hiem and Vario  mechanics for the clutch.
Mode 2 THE only way to fly

paulinfrance

Quote from: paulinfrance on October 16, 2018, 07:33:21 am
I had a preference in my guided missiles of the Webra on a tuned pipe and a silly little prop ( 11 x 7 ? )
The OS engines were really good in Helicopters, ( it was OS or Rossi ) the carburettor arm was longer on a bearing ( I my memory is ok )and it had an extended crankshaft for Hiem and Vario  mechanics for the clutch.

I still have an aero one, on loan to a friend still running well.
Mode 2 THE only way to fly

paulinfrance

Mode 2 THE only way to fly

British Victory

PDR, thanks for that info re increasing combustion space(!) Shims of the correct type are like rocking horse poo so I got my trusty calculator out and worked out how thick should a plug sealing washer be to achieve the same increase in volume as a 5 thou shim, it worked out at 1.5 mm ish.
An off cut of 8mm od copper pipe, a 6mm dowel and a bit of practice with a micro tube cutter and I managed to cut one off at about 0.057" (I only have imperial mikes).
Annealed it and formed it round a 6mm drill to get it as nearly round as possible and on it went.
I was a little bit concerned that the plug element would be shrouded but it seems to run and throttle ok on 15% nitro, of necessity short runs as it's v. noisy so I'll try it this afternoon at the field, if it's good I'll fly it as that's the only way to tell.
Don't have a battery for my tacho but it doesn't get too hot with a bolly clubman 12.5x6 on the front.
TBH I think it massively overpowers the wot 4 as it's the early narrow chord version and it weighs 2.6Kg which is .6 of a Kg too much imho, previous owner said it had unlimited vertical with a graupner grey 11x7 so we'll see.
The paint has dissolved off the fuselage, wings and tail feathers in way of the exhaust stream so it's possible that he was using high nitro anyway, didn't think to ask when I called in to pick it up.
there's only one f in RCMF,
               John

PDR

My original (plan-built) Wot4 Mk1 had a Merco 61mk3, weighed 4lbs2oz and had trouble landing because the motor idel was too fast and gave enough residual thrust to maintain height(!).

My second was a kit-built Wot4 Mk2 from the first batch of kits, with a Webra Champion (slower tickover) and weighed 4lb4oz. This would hand-launch vertically by just releasing it.

My 3rd one also had a Webra Champion, but it weighed 5lbs1oz because it had a set of retracts in the top of the wing for inverted landings.

My 4th Wot4 was also a kit-built Mk2, but lightened a little and powered by a 1st-gen (non-surpass) OS 40 4-stroke. It weighed ~3-1/2lbs and while it lacked full vertical capability it was a really delightful machine to fly.

People talked about getting their Wot4s "down to 5lbs" but I just couldn't understand how they built them that heavy!

PDR
There are no shortcuts on the long, hard road to success. But if your dad's rich there could a limo service...

British Victory

my last w4 I got down to under 4lbs, 3lb 10oz I think with an irvine 39 absolutely fantastic performance (the 39 replaced an irvine 72Q and several ounces of tail lead) I thought, if anything , it had better performance than the 72 particularly as the 72 had a 12 oz tank and the 39 got by with a slec red which is 6 ounces iirc.
Any how the os61 performed flawlessly, unlike me as I had a brain fart, didn't realise the engine cut switch on the futaba 14 sg stops the throttle servo from working,(I was using it as an idle down) so too high to land opened up and no engine, cracked the fuselage, sorted now but my red face will haunt me for a long time.
Quite pleased that I kept the os in the plane but I now have plans to swap it for a TT 42 GP as I have one going spare and the OS can go to a new home as hot rod wot4s aren't my bag anymore.
there's only one f in RCMF,
               John

Skyleader

The O.S 40 fsr was a popular weapon in my local club in the 1970s.
By the way PDR, I ran gallons and gallons through Enyas. Bullet proof and powerful. The air bleed carb wasnt efficient though. But still loved them. 😉
'Dont just stand there; get one up!!'