jeffie8696

I remember a discussion regarding iron pistons in older OS FP series engines. Here we have proof. This is an OS35FP and the magnet isnt strong enough to be holding on to the wrist pin.

fiery

My Feedback: (2)

I have a .35 FP. It's a classic.

A friend has the iron/steel .40 FP. First R/C engine he ever bought. Purchased in the mid-80's and still powering a SIG Kadet LT-40 today.

Bill Adair

My Feedback: (1)

I have a .20 & .40, but have not run them.

Would like to find an instruction sheet for the iron piston versions, my but .20 came without one, and the .40 has an ABN instruction sheet.

Bill

jeffie8696

The 35 is unfortunately not mine, it is store inventory at Star Collectibles. I just happen to be the service department and grabbed it off the shelf for a "used engine maintenance inspection" after all it has been there for a while and I needed to be sure it was in good shape internally.

spaceworm

I built a Goldberg Eagle (original, not the II) for a cousin and helped him learn to fly. The iron piston OS 40FP he put in the plane vibrated so much the Monokote drummed like a rock band. I changed the prop to a carefully balanced one and it did the same. He gave up flying and i lost touch with him. But I wold never consider buying an OS FP engine after that. When did they stop putting iron pistons in the FP series?

jeffie8696

My FP engines are all very smooth, I dont understand what would cause a severe vibration from one iron piston or no.

airraptor

My Feedback: (66)

Try a 14x6 or a 15x6 prop and you will be surprised by the results

Bax

My Feedback: (11)

With the older FP-series engines that have the iron piston in the steel sleeve, you want to use a fuel with about 20%-22% oil content, with the majority of that being castor oil.

When brand-new, never-run, the engines will need to be run at full throttle and very, very rich..."slobbering rich". It can take 45-60 minutes of this very rich running before the engine will even allow the needle to be leaned. Once it's run-in enough to allow leaning, you can gradually work the needle until the engine will sustain running at full throttle when leaned just rich of peak RPM. The engine is ready to fly when it can run out a tank of fuel (8 ounces for a .35 or .40) without changing the needle setting.

The propeller to use would be a 10 x 6. 14" or 15" propellers are much, too much for that size glow engine. The engine should be running in the 11,000's at full throttle. You can prop it so it's running in the 12's, if your plane will fly with the smaller props ( 9 x 6, 7 or so).

airraptor

My Feedback: (66)

BAX for most applications the larger props are but they will turn those props. THEY ARE NOT too much for them in the right aircraft.

pilotpete2

The largest prop I ran on a 40 FP was a 12-4. That was on a Goldberg Anniversary Cub. It flew very nice and scale like with the 12-4 Zinger, turning a little oner 10K as I recall.
Pete

aspeed

I have one with an iron piston too. They will vibrate a bit more than an ABC or aluminum ringed one if revved high. That is just the nature of the beast. The old Fox .35s were nasty for vibrating, I had to put cheek cowls on so the plane would not shake. Later I changed to the LA .25s and vibration was not an issue at all. I think it is just the counterbalance of the crank being too small in the Fox's case. A friends baffle piston OS .25 made my hand tingle from launching it with a smallish 8-6 prop. My FP .40 is in a control line stunt plane and it had an 11-6 or 11-7 when I bought it. It did not vibrate excessively, but flew nicer for stunting at least, with a 12-4. Could be that the old motor your cousin had had a bent shaft from a crash or something maybe too, if it was not new.

Lou Crane

Aspeed,

I agree.

The Fox 35 crankdisk is barely 3/32" thick, except for the "counterweight" crescent. Modern engines have massive crankdisk thickness - sort of a built-in flywheel that reduces vibration.

An 8-6 on a MAX .25? OMG!

As with your 40FP stunter, higher RPM on a lower pitch prop works well. The engines of recent years were designed around sport RC use, and not to lug high pitch /low RPM conditions.

The high pitch props were part of the 4/2 cycle run, in my opinion. Iron engines with limited RPM capability could turn 10-6 props easily up to near their max on such props. When loafing along rich, firing could settle into the 4-cycle sounding run. Increasing load on the prop by maneuvering - with the resulting wing drag - did NOT result in a large (if any) RPM increase, but sounded like it did because combustions occurred more nearly at the RPM frequency. No skipped rotations.

Fox 35s were designed so long ago that the concept that a one cylinder engine could not be balanced to run without vibration ruled supreme. If impossible, why waste effort trying to balance it? Engines of the era were very light; today's engines are not - another way to reduce apparent vibration. We did have a few ringed, aluminum piston engines back then - they vibrated, too.

Light alloy pistons and massive flywheel crankdisks do help a lot, but it is still well to make sure props are in balance and mounting is straight and secure.

Sport_Pilot

Pretty sure it would be hard to find a glow plug cold enough to prevent 40 sized engine from detonation with a 15" prop, no matter the material of the piston and sleeve.

1QwkSport2.5r

Variable compression would do the trick to a point. Omit the glow plug and use a model diesel fuel and detonation would be a non issue. To a point, a model diesel runs on a controlled detonation, but being able to vary the compression would allow the use of a huge range of loads (props). But I think this is outside of the scope of the thread.

airraptor

My Feedback: (66)

Sport pilot sport pilot sport pilot. Why do you think it will detonate..... This engine will turn a 14 or 15 inch prop around 6000-7000 rpm 6,500 on mine and I used 10% fuel and a get this an old OS number 3 plug...... maybe my engine is just a special one.
Most assume you must turn the engine at a rpm that is normal. FP type engines have very low timing and small carbs so think about that for a second. I have seen OS 26 four strokes turn 15 inch props also at low rpm and run for 15 minutes on two ounces of fuel. Now with the OS 26 you couldn't open the carb all the way but a hair over half was good and it flew the plane it was designed for (old timer) very well.

1QwkSport2.5r

Diameter is only part of the equation - are we talking 14/15 X 3 or 4 pitch? That sounds more believable at 6-7k. Not that I don't believe you personally, just think it's a bit sketchy to get a .40 to turn 6,000rpm on a 14" prop with a moderate amount of pitch.

I have an ST .51 I've run as a glow and a diesel - it turned a 13x4W at 10,800rpm on 5% glow fuel and 10,500rpm using a 13x6 as a diesel. APC props of course. For that particular engine, I think a 13x4 would be the biggest prop I'd want to use with glow ignition personally. On diesel, I have no doubt it would turn a 15" prop, but I wouldn't try that with glow ignition.

Just my observations.

Sport_Pilot

I tried a 15" prop on a .91 four stroke and had detonation problems, so I would assume so with the FP. Maybe the compression ratio is lower than I figured.

aspeed

If it was not so darned cold, I would go out and try a monster prop. right now. I know a lot of people say not to plug up exhaust and over prop, but as long as they don't overheat or detonate, it is worth a try. It may require a few shims in the head. I read an article on a Fox .35 that welded a cylinder onto the opposite side of the case, and plugged the exhaust, and carb to get it slow enough for the plane, and it worked good enough, I think the guy in the article said it lasted well and putt putted away.

jeffie8696

Why not plug up the exhaust? I have seen exhaust throttles on many 2 stroke glow engines

aspeed

Yep. I don't see a problem with it. Years ago the mufflers were pretty bad, and they would overheat likely because of undersized holes in the muffler. I know they sucked power, but don't have first hand info on the overheating, just old wive's tales. I open up a lot of muffler exit holes even now. The .15 size F2D combat motors had a 8mm outlet and then they changed the rule to 6mm. Many of the LA .15 and similar motors are only around 4mm, and even larger motors like a .40 are not 6mm. A bit of back pressure is good for throttle/tank pressure though.

jeffie8696

https://www.youtube.com/watch?v=-h76s_N8WPs#t=22

aspeed

Looks like something to try on a bigger motor. The Norvels have a kind of wonky seal on the exhaust. There would be less suction in the carb, as the hole stays the same diameter, but then I guess there would be more pressure pushing the fuel through from the muffler? Quick trial, just use your finger.

jeffie8696

We have a very rare K&B3.5 outboard with exhaust throttle at the store

spaceworm

These exhaust throttles were often water cooled to stand the high heat.

aspeed

I had/have one for an inboard 3.5 K&B. I think I used a water cooled one on a Picco a long time ago. Boats are kind of an on off button. I should run one again, it's been almost 30 years.