Remby

My Feedback: (3)

Had a piston fail in flight. Was using 25% nitro fuel with 22% oil, The Cox Medallion was on it's second run. The engine shut down, but could hear a "Whrrrrr" as it got closer to landing. I was thinking the prop screw had come loose, but the piston had split apart on top (copper) from the ring (steel)sleeve. The rod had a little play, but nothing abnormal. The sleeve pushed out of the cylinder by hand, was not frozen.

First one I ever broke in many years of flying Cox engines. I tore down and replaced the cylinder/piston, and cleaned/checked the engine. It was able to be run and reflown with the replaced parts.

Is that good luck or what? Like I said, never happened before. Your thoughts are welcome, thanks guys!

Tee Bee

A friend of mine cracked a piston top a while back but his rod clearance was a little loose. I've never cracked a piston but have broken a conrod in flight. I guess these things just happen from time to time. Glad you were able to repair it.

icerinkdad

Racing Cox reed valve engines in control line I used to see this happen occasionally. If the ball joint connection is loose enough to feel then every time the engine turns over the end of the pushrod is pounding away at the top of the piston from the inside. A small diameter prop or unloading the engine in a long dive will accelerate the wear causing quick failure sometimes.
The piston reset tool from Cox International is a good investment.
Bob

PeterC

I've had a number of failures of Cox engines. In one the ball socket became so loose that it pounded a hole in the top of the piston. In another on a Tee Dee pylon racer a long screaming dive was terminated by the engine going silent and the prop windmilling in the slipstream. I had broken athe crank pin off the crank. I thought that was quite amazing since I rarely used any more than 20% nitro and usually used sport props.
The reset tool is a good investment for anyone planning on using Cox engines because they will all require resetting at some point in their life.
Peter

AndyW

My Feedback: (1)

The only time that happened to me was when I ran a brand of 25% nitro that had 18%, all synthetic. No castor.

icerinkdad

The Cox warenty in the 60s and 70s was only good if you ran fuel where all the oil was castor. That little ball and socket joint is hard to lubricate and gets hot... castor doesnt boil out if the engine is a bit lean.
Bob

Yuu

AndyW and Icerinkdad..... do you suppose an "ashless" two cycle oil, like chain saw two cycle oil, would work in the Cox Engines, say, 32:1 mix, as an oil addative? I suppose, if it's designed to burn, it wouldn't be too good at the ball socket at the top of the hot piston. But still, those chain saws can really develop the high rpm, and they're air cooled. I have read in the threads here that regular gas and oil mix will run, but not without a glow plug heater, as the alcohol required to keep the catalyitic reaction is not there. What do you think?

AndyW

My Feedback: (1)

The problem with our small engines as opposed to the "big", two strokes, chain saws, dirt bikes and the like is that they use needle bearings on the connecting rod. The engines are big enough to accommodate the mass and bulk of such a rod. Our engines are too small for that so we have plain bushed connecting rods. As such, those need far more lubrication to prevent failure.

Cox rods are even worse in that respect. Standard, pin type pistons have two holes in them allowing for lubrication to get into that area. "Shrouded" rods as you find with Cox, some Fox and all VA engines have the piston end of the rod nestled deep within the piston. Not a lot of circulation of fresh fuel/lube gets in there, I'm afraid.

And that points out the superior lubrication qualities of Castor. Castor works in such a severe environment, synthetics don't. At least not yet. Downsides with Castor is that the oil has a higher viscosity and presents more drag. Especially in the cold. Plus, at very high temps, Castor caramelizes. That's just a nuisance of course, but that's in fact a good feature as it will prevent galling under severe conditions of overheating and/or starvation of lube at overlean settings.

In my former day job, we sold an EP (extreme pressure) additive that enhanced an oil's capacity to resist failure. This was demonstrated with a device that had continuous lube delivered to a bar rubbing against a rotating flywheel. You would add weights to the bar to deliver more and more pressure against the rubbing parts. Any standard oil would have the parts squeal while the treated oil would just run happily away. It was amazing, you could add triple the weight and still not get a failure. Apparently, all oil gets this additive and adding more made it better. I never did try the additive, for some reason. Wish I knew now what it was. As I recall, it was quite expensive.

Andrew

Yuu -

Your point is well taken. The COX design differs from almost all other IC engines in that the rod/piston joint is located at one of the hottest points in the engine. Heat must conduct to the edge of the piston and down the skirt to be drawn off into the cylinder walls and dissapated into the fins. For the typical rod/wrist pin setup, heat comes from the cooler skirt wall and migrates along the wrist pin. A lower heat source coupled with being bathed in a cooler fuel/air mix allows the rod bearing to run much cooler. Add the fact Andy made about more area being exposed to lubricant vs the shrouded ball joint in COX engines and you have a recipe for early failure without good lube.

One thing I believe gets overlooked regularly, except by us old f**ts who flew the iron/steel setups of past years, is that castor carries a lot of heat out the exhaust. The synthetics, with a lower flash point, burn adding heat to combustion. Thinner synthetics likely don't help cool the ball joint. Castor can be really efficient in cooling the joint and maintaining good film strength.

I'm inclined to believe that film strength and a high flash point are the two properties that give castor the advantage with COX engines.

That's ma' story and Ah'm stickin' to it.

andrew

Yuu

Thanks, Guys, for the replies. I mixed a batch of 78% of 93 octane gas and 22% castor, and shook it up good. I walked out to the test stand area and fueled her up... and the castor was already re-coagulating in the bottom of the fuel bottle. The engine ran, with a battery connected to the glow head. That batch was gone, so I mixed another batch of 1 part model fuel, 1 part 93 octane, and added .2 part castor. The fuel ran without the battery connected, but ran much slower, at 5200 rpm max, w/ or w/o battery. Not enough power developed to fly my plane, I'm affraid. Our flying field is closed due to mud and thaw, but my test area here at home is 'open' for testing. Looks like another 'good idea' has been shot down. I'll end my testing before I bust a rod or crank pin.

ProBroJoe

Hey remby, lookie what I found on the Cox engine Wiki: http://en.wikipedia.org/wiki/Cox_model_engine

"The Venom was Cox's last attempt at making a really fast 049 mouse racing engine. Again taken from ideas learned from years of competition, this engine put all those ideas into an off the shelf product. It used the Killer Bee crank shaft loosely fitted into the crankcase, and a cylinder with porting very similar Tee Dee cylinder and tapered like the Tee Dee and with a lightened piston like the Tee Dee. The rest of it was like a Black Widow. The problem was that the production engine was not the same as the prototype. A mistake had been made in manufacturing and the piston was lightened too much. This made the engine fast but the piston weak and they would blow the top off the piston after a few runs at high speed."

Kinda sounds like what you experienced, huh? [X(] (Wouldn't surprise me with so many interchangeable parts lying around at the factory...)

Remby

My Feedback: (3)

ProBroJoe;

Yes, it kinda describes what happened. This was a stock Medallion, not reving up too much, but it still failed. I have others that are Tee Dee based without any, as yet, failures.

I would really like to see a full Castor based 25% fuel, I do not care for Klotz as I find it flashes off, the piston turns black from the flash burning, I doubt the failure would have happened using all Castor lube.

The Wilpeda stuff is cool, have ran across it myself. The lising for the RTF Cox models tells me how many of the C.L. planes I had and wrecked over the years. The Super Stunter BF-109 was a favorite flyer.

Andrew

Remby -

Do you recall which Klotz lubricant you were using. They make a number of different products ranging from pure synthetic to synthetic/castor blends to an all-castor lube. Klotz BeNol is pure degummed castor - pure in that it doesn't have synthetic lubes mixed in, but it does have some additives. Here is a [link=http://www.klotzlube.com/techsheet.asp?ID=2&submit2=View]BeNol tech sheet.[/link]

I use the BeNol to mix with other fuels to bring the castor percentage up and have not noticed any problems, other than oily planes and discoloration on the external parts of the engines.

andrew

Remby

My Feedback: (3)

Andrew;
The Klotz I speak of is pre-mixed, in the Sig 25% Champion fuel. I am unsure as to which type they use, but know using that brand causes the flashing with Cox engines.

I used to purchase the Hobby People fuel (NFX brand), and found it was a full castor blend @ the 25% formula. That fuel left NO deposits, other than the varnish residue that was normal for anything I had ever seen from years of running the 1/2a engines.

But the Klotz in the Sig Champion 25% fuel must be the source of the black deposits. Nothing else I see in that formula should be the cause of such deposits. If it is flashing of, it means half of the oil is not lubing, just making smoke.

Sig just lists the synth oil as Klotz.

Andrew

Remby -

I believe the Klotz lubricant in the Champion fuel is the original Techniplate, which is all synthetic. They list their Champion fuel as 50/50 Klotz/Castor. Since they carry Techniplate as one of their oil additives, likely it's used as their synthetic additive. The flash points for Techniplate and BeNol (castor) are 460 and 555 degrees respectively, which could certainly explain your piston coloration.

I've been buying high nitro/low oil car fuels, then adding BeNol to bring the total oil up to about 21% (18% castor) - with the higher nitro content, I can add oil and still retain a nitro percentage around 22.5%.

andrew

Remby

My Feedback: (3)

Andrew;

Thanks for the data. It looks like the Klotz would be the issue then with that fuel. I think it a good fuel, but for the 1/2a Cox engines a full castor is the ticket. I am going to find a shop that can get the raw parts and mix a 25% all Castor fuel.

The fuel may not have caused this failure, it may have been a week part or whatever, Am still going to look into that full castor based fuel blend.

Thanks to all the input guys!

dcbdbis

I was one of the mgrs of the engine line at Cox when Roy still owned it, and until Leisure Dynamics "flew" the company into the ground. I worked closely with R&D and QC.

I know, I just dated myself.

In the test cells, there was a lot of alternative oils tested, and alternative fuels. The reset tool was essential. If the ball socket comes loose, it's a matter of time until failure. The pistons are a copper alloy, plated with chrome. They are soft and malleable. Ensure the piston reset tool is used on a known flat surface, or you may find that the piston itself has been distorted. And the more times you reset the ball-socket, the softer the socket becomes, and the more often it needs reset. It's certainly a catch 22.

Synthetic oils have been used successfully, but with premature wear on the crankcase/crankshaft bushing, and on the top end of the connecting rod. I have personally turned the engines in the test cells > 35,000 rpm (you read me right). On castor, the engines suffered no damage. On synthetic, the engine would heat-seize mid-run with the result that parts went all over the place.

Push them into 35 - 50k rpm range, and the glow plug packing material fails.

We did test some engine mods that worked. This only works in a non-throttled setting. If you want to "spook up" the .049 on a reed valve engine, remove the reed and carefully drill out the air intake to 0.062". Fabricate a replacement reed from 0.010" mylar.
This mod will take an engine turning a 6x3 @ 16k to about 18.5k. This mod will NOT allow the engine to be throttled down with the cylinder sleeves. It can't maintain enough low pressure to keep fuel flowing when throttled back.

The other item we used to do is to position the cylinder so that the transfer ports of the cylinder were on the sides of the engine and not located fore/aft. This allows the crankshaft to provide a little centrifical force to the fuel/air charge and you'll gain another 500 rpm. CAUTION: do NOT shave more off the cylinder to accomplish this than 1/4 of a turn of the cylinder. If you do, you'll adversely affect the piston's location in the cylinder, overly raise the compression ratio, and in worse case cause piston/cylinder interference. This mod works on the TeeDee engines as well as the reeded engines.

To cut the crankcase down to accomplish moving the ports to the sides....use a known flat surface (glass) and lapping compound. Go slow.....Once you are done, it's likely that the top of the C'Case is NOT perfectly flat anymore, thus compromising the cylinder seal. Use Loctite Blue (medium strength) to both seal and hold the cylinder into place when you reassemble.


Hope this helps,


Dave Babb
Former Cox employee.

combatpigg

My Feedback: (3)

Thanks for this excellent information! You've tested this engine beyond what most would ever dream of and your findings are worth their weight in gold.
I ran TD 049s at 23-24,000 for years with no predictable failures.
At much more than that the ball sockets would crumble soon enough to make you want to go out and buy a Cyclon.

dckrsn

Dave-

Excellent post. Your work at Cox sounds like a book I'd like to read.
35k rpms! Now there's a song I'd like to hear.
Thanks for mods. I have coppied and printed them.
Bob

dcbdbis

On turning an .049/51 @ 35k+ RPM. It's not with a prop you would fly with. Just enough to cool the engine. I was doing a study on high-speed cataclysmic failure points.

FYI


Dave........

Andrew

Dave

Thanks for the information - knowledge coming out of the original COX factory is always most welcome here. Please don't hesitate to bring more to the forum.

andrew

dcbdbis

On the mylar reed. Some folks when we would race, would swear by the 0.10" mylar reed. Others 0.012" in the same material.

Personally, I've observed some deformation on the .010" reed after a few runs. Enough to not seal at low speeds and make starting a genuine P.I.T.A. The 0.12" version while costing a couple of hundred RPM's didn't seem to deform. Can't remember why the 0.012" mylar reeds didn't make it to production.

Personally, and off company time, I fabricated reeds out of teflon. Same basic performance, same basic dimensions. Now days, making a reed would mean careful work with an exacto. We had a hand operated die. Very similar to a modern day whole punch, only shaped like the reed. Some material, a cutting board, and a quick tap of the hammer would net you one reed.

The plastic/teflon reeds will not have the naturally sealing arc of their metal cousins. All this affects is starting if your not vigorous enough.

Here is another tip. The reed sides have curved cutaway points. The closer to the actual seal you make these cutaways, the faster your engine. But the sooner the reed gets tired. You'll have to experiment...Suggestion, go with a three point contact on the reeds for retention, instead of the factory four. Going with two, and you'll suck the reed every time. Been there, done that.

And on the reed retaining spring, re-arc it and give the reed another 0.015" of room to move in toward the crankcase. Start with 0.010 of extra room and move up from there. If you suck a reed, you've gone too far.

To increase crankcase pressure (slightly), reduce the c'case volume by eliminating the thin gasket between the tank and the c'case. Seal with a high quality, high temp automotive silicone. Anything else and the fuel will attack it. You can also use the newer sealant called "The Right Stuff" available at automotive stores. affectionately called the "black death". It's called that because if you get it on your clothes, nothing will take it out, following by your wife killing you for ruined clothes....:-)

In either case of the silicone or the black death....Allow it to cure overnight at room temperatures, or the sealant will be dislodged in the first run. remember, there is not a lot of seating area..... And don't slobber a lot on the insides, or it'll squeeze out onto the reed rendering it useless. Better to have a tad excess on the outside than on the inside.....

Another trick when we wanted longer run times, was to drill a hole in the tank, and run fuel line from the internal nipple, to an externally mounted tank. Distance is critical. Keep it close.

If you go with the plastic/teflon reeds, the urge will be to use an electric starter. I can't tell you how many engines came back to us "in warranty" with the front of the c'case ground off from an electric starter. Aluminum upon aluminum does not make for a good bushing material.

To convert for electric start: Measure the front prop hub to c'case clearance. If it's not 0.010" or greater, you're going to have to grind on the c'case to create clearance. Get some hobby sheet brass in about 0.010" thickness, drill a hole the size of the crankcshaft and fabricate your external radius. PLace this "shim" between the c'case and prop hub and re-press. Seat the prop hub completely. If it does not spin freely, you need to remove more material from the front of the c'case. Once proper clearance is achieved, (just enough to spin freely, or about 0.0015").....then just add fuel to this "bushing" before each start for lubrication. You'll not eat up the prop hub nor the c'case.


Dave............

dcbdbis

Clarification: Drilling out the air intake without damaging the seal.

The back plate is made from zinc. It's soft. the tank is aluminum, soft, but not quite as bad. The trick is to drill accurately to retain the original factory "O" ring seal between the tank and the backplate. We used to use an old c'case, bolted the backplate/tank assembly to this c'case for accuracy, jigged up the old c'case in a drill press, then drilled out the air-intake. You'll not have an alignment problem if you do this, nor will you damage the seal.

One other item: We had an engineer upstairs. His name was Dave (not me). I have since forgotten his last name. His goal was to make a reeded engine with c'case pressure to the external fuel tank. He drilled out the air intake to 0.080". Used a 0.012" mylar reed, carefully drilled and tapped the side of the c'case right on the circular run where the connecting rod bottom end runs, and carefully dremeled it off to prevent interference. The hole was tapped, but there is not a lot of material there. The small fuel nipple was then Loctite RED into place.

He only got it to run one time that I recall. But oh man, did it scream! I recall about 21k Rpm on a 6x3 glass....but was unreliable. Your tank must be small, and all the piping absolutely air-tight. The reeded .049/51 just didn't have the c'case pressure/volume to make it work. If you had a full tank, and allowed the engine to start with a full tank it would work. But if you tried to start it with less than a full tank, the little engine just didn't have the c'case pressure volume to catch up. TeeDee's were the exception. Key here is to use a small, I mean small fuel nipple and small line.

I would NOT suggest this as a mod. Too tweaky and too unpredicatble. With the "carb" opening this wide, maintaining a fuel flow without pressure is not possible. I've only seen it work with a 1 Oz tank, and then full to the gills before initial start. I don't remember if this was retested with a smaller than 0.080" carb opening or not. I would bet not. Too much unorthodoxy and too unreliable.

Now, engines are rare. Then we could go grab a handful of the engines and butcher away with impunity.

One other item of note: This pressure mod was only attempted on the plastic backed engines installed in product, like the plastic P-40's and such. That much drilling on an aluminum tanked engine would destroy the "O" ring seal.


FYI


Dave............

dcbdbis

Another item for clarification: The reeded .020....

We never attempted the hackery and butchery described above on this engine. I have no idea about different reeds, carb opening dimensions, etc.... Aligning the intake ports side to side yes......but none of the tank/air intake mods.


One other item on hopping up .049/051's.... after we found a cylinder (pre-honing room) and c'case fit that aligned up the side transfer ports like we wanted, we'd take the cylinder into the honing room and start gently honing it to shape. We wanted the last 0.020" of piston travel, right at the top of the piston's travel to be a slightly friction fit. Slight. You could feel it with your fingers, but not enough to really stick it hard. Another way to measure this was the last 15-20 deg of crankcase movement was a slightly friction fit. This yielded the best compression retention post-breakin. It took longer to break in, but once you took the time.....You would have good compression long after the "stock" piston/cylinders went the way of the dodo. Too tight, and you would fail the ball socket on the piston's bottom.

The shape of the cylinder would be cylindrical until the very top of the cylinder. Then it would become conical and narrowing. The reason? heat distortion. When running, the cylinder would become a cylinder with the application of heat distortion, instead of the stock cylinder which would be slightly "Y" shaped under run conditions. The purpose was compression retention.

This type of cylinder never went into production. The average user would lean their engine out at the start and run their buggies, PT-19's and P-40's right out of the box. We HAD to set dimensions so the engines would not seize out of the box, or our warranty returns would skyrocket, and the nasty spirited customer support phone calls became gruesome. Remember, we were at the entry level for 99% of our customers.

Now in reality.....we're talking dimensions of +/- 0.0005". I've not checked, but I don't think parts are readily available anymore to do this type of minutia. But I present it here for historical purposes.


Now for some comedic but historically accurate information. We had one phone call come into the support line, from someone (sounding like an adult) who was slightly inebriated. They asked Kyle if we were a porno service......You see......e answered the phone with: "Cox action hot line, may I help you?" Keyword that got fouled up I'm sure...."action"......


Hoping some of this old information helps.......



Dave

Jim Thomerson

I had understood that Cox ball and socket pistons were copper plated, then ground on the outside running surface, and then nitrided to make them hard where the copper was ground off. I take it this is not correct.