Cox 020 PeeWee rebuild questions

Good morning (here) balogh,

Thank you for that info on how to isolate and leak check just the reed. I will do that if I get a chance today. Also, I woke up thinking that adding the castor oil-soaked thread to the big backplate to fuel tank joint could prevent the full seating of the backplate against the fuel tank. I don't want to overtighten the backplate screws, but if that is a possibility, it could prevent full compression of the tiny O-ring seal between the backplate and fuel tank. Has anyone ever 'doubled-up' on the little O-ring seal to insure sealing at that location. My little O-ring in the rebuild kit seemed pretty thin.

Also, I realized that my oil-soaked thread had a gap of about 1/16" between the ends. I think I need to correct that and get as close as possible to a "full circle" of thread.

Thanks,
Dave

Dave,

I am not really experienced with tanked COX engines, I fly them all with external tanks.

If you have a doubt whether the O ring will properly seal after the oily thread is tucked into the perimeter groove on the tank bowl, then remove the O ring and put a piece of thread into the groove under the O ring to compensate the added thickness of the thread in the tank bowl perimeter, then put the O ring back, and the original geometry will be reinstated.

Some folks here, I recall Roddie, are slicing thin rings from silicone or rubber tubes for a replacement O ring at least for 049 Bees, but I guess the same will work on a Pee Wee as well providing you have the silicone tube of proper diameter.

And yes, try to avoid voids, and overlaps between the ends of the thread tucked into the tank bowl perimeter groove, because that will compromise the sealing where there is a void or there is no overlap.

In any case, do not overtighten the tank backplate screws because that may result in a crack, i.e. a damage beyond simple repair.

Hi Dave, Here’s how I do mine: With the tank and backplate off the engine, And the O ring installed, I like to be able to feel a little squish between the two parts when pressed together with the fingers…. That tells me the O ring is contacting both parts well …. When I can’t get that with the factory O ring I make a ring from silicone fuel line….. I use a piece of brass tube in a drill press with the fuel line slipped on…. Drill press running, Cut the end of the line off with a razor knife to make the ring…... Caution if you try this…..Sometimes it takes a few tries to get one the just right thickness…..

After the O ring is installed, the oily  thread in the bowl will have just a little squish factor as well to help with seating….Yes, The two ends should touch but not overlap…. It may take a few tries to get the just right length  ….

balogh wrote:Some folks here, I recall Roddie, are slicing thin rings from silicone or rubber tubes for a replacement O ring at least for 049 Bees, but I guess the same will work on a Pee Wee as well providing you have the silicone tube of proper diameter.

Don't know about Roddie, but I posted some time back doing that with a Babe Bee, worked fine. Some questioned my trimming accuracy, but just like in all things mechanical, requires "feel", and cutting a slivver thin enough. A new very sharp scapel, Exacto knife blade, or single edge older style razor blade is required.

Since I had small silicon fuel tubing inner diameter of the venturi to fit the aluminum fuel bowl joint, silicon is very flexible, restored the venturi integrity.

It took a couple tries to get it "right", but with nothing to lose and all to gain and waste is minimal, for those who have good manual dexterity skills, I say, "Give it a try."

An added advantage is, if the gasket needs to be thicker than a new factory one because of one reason or another, resolves this issue nicely.

Sorry George, my bad, my memory did not serve me well..

Hi Dave, if you've renewed all gaskets supplied with an overhaul-kit... I wouldn't worry about the tank bowl to backplate sealing. Assure that the mating-surfaces are clean. When you "start" the screws that mount the tank assy. to the back of the crankcase; start each screw just "two turns/threads"... then alternately tighten each screw the same # of turns.. which should result in more "even-seating" of the two surfaces.. once the screws begin to tighten.

Obviously... you need to be sure that the reed is sealing; before you assemble the tank to the back of the engine.

Has the position of the fuel pick-up been discussed?

balogh wrote:Sorry George, my bad, my memory did not serve me well..

No problem, András, we all make minor mistakes here and there, but the good thing is, no one here takes it seriously. We just forget and move on.

I enjoy your explanations, builds and flight videos, you certainly put these Cox engines to good use, plus have good pointers on how to get the most out of these engines.

Hello All,

I performed the reed leak seal test as described in the attached by balogh. I had the mylar reed in place and it locked up solid. So, I put the copper reed back in to try it. I got intermittent lock-up but mostly a nice high pitch whine. I put the mylar reed back in. I used the old retainer clip wire. The mylar reed rotates freely. When I first put in the mylar reed, I used the new retainer clip. As mentioned earlier, they can be a bit thicker than the original. With the new clip, the mylar reed would only partially rotate around before catching somewhere, I assume on the thicker clip. There doesn't appear to be anything damaged on the old clip, so I reused it.

I visually looked at the tiny seal sideway, with the seal in place and I could see the upper edge of the seal above the seal groove. So there should be some "squish". I remove the new fuel pick-up tube as it wants to stick straight out. I tired rdw777's "squish" test but it must be my inexperience. I did not feel any squish. I inspected the sealing surfaces of both sides. That is when I spotted a large pit or pore in the backplate casting right where the sealing surface is. See attached photo. Maybe filling pit/pore with a little JB weld epoxy and finishing flush could cure that. Or is the backplate a lost cause? What is the consensus?

There has been no discussion about fuel pick-up tube placement. But I tried to position it the same as the old one, to the side of the venturi tube with the angled-cut end at the bottom of the backplate so it would hopefully pick up all or most of the fuel.

Again, thanks again for all your help,
Dave

To all (continued),

I will attempt the oiled thread under the O-ring seal as well as try to locate some silicone tubing of the correct size. That tiny 020 is, well, tiny!

Dave

Dave

Did the mylar with the thinner circlip finally pass the suck'n'blow test? That small pore would not disturb me as long as it is in the middle of the annulus area and is not open towards the air inlet channel on the backplate. The O ring will sit on the pore thus prevent leakage of fuel into the air intake.

That hole in the casting may just be a void that popped open during the cleaning process… If it doesn’t go all the way thru you’re probably OK…. Although it is right where the O ring seats….. Worth to try and use IMO….

If you want to look into the O ring seal a little more…. Lightly damp the part of the backplate that mates with the ring with castor oil using a Q tip…Then “paint “ it with a Sharpie marker….. While the ink is still wet push the parts together firmly and twist just a little… ….Then pull apart and examine if you got a witness mark from the O ring…. The ink washes off with alcohol easily….

LooseSpinner99 wrote:
There has been no discussion about fuel pick-up tube placement.  But I tried to position it the same as the old one, to the side of the venturi tube with the angled-cut end at the bottom of the backplate so it would hopefully pick up all or most of the fuel.

Hi Dave! That position we'll call the "6 o-clock" position.. and would be the best position for the pick-up.. if you were to install the engine in an RC model airplane.. or surface-type of model.. (car, boat etc.)

If you decided to install the engine in a control-line model airplane; you'd want to rotate the pick-up to the "9 o-clock" position.. because of the outward centrifugal-force of the flight forcing fuel to "that side" of the tank; assuming a conventional set-up where the model fly's counter-clockwise in it's normal attitude. The same would apply to running a tether-car.

Here's a look at two of my Bee (.049) backplates with their pick-ups in the 9 o-clock (L/H) and 6 o-clock (R/H) positions.

If you pressurize the tank once assembled and it doesn't leak, you don't need a gasket in the v-groove of the tank. Closing your needle valve down ( Not forcefully) and pressurizing your tank with a proper syringe will tell you where all your leaks are. If you do this and hear air hissing during pressurization, it leaks in only a few areas. Air leaks escaping from the rear is a indicator of screw heads leaking, or venturi o-ring. If air is leaking directly from the screen it's a o-ring leak. Air can escape from the tank to backplate connection, crankcase to tank connection, needle valve seat, needle valve itself or screws retaining the backplate. The threaded portion which retains the needle valve is a press fit into the backplate, this can also leak air around it. While this seems miniscule, all of these issues can leak making problems.

Hi All,
Another progress report.
To balogh: Yes, the mylar reed sealed just fine! Infact I've been out to the garage workbench several times this morning and every time I pick up the fuel tank and perform a suck/blow test on the reed. It passed every time. Also, I think you make a good point that leakage around the fuel tank to backplate without any oil-soaked string would be minor and not affect the operation or the reed/venturi. I'll bring this back up again when I show the results from the "witness mark" test I ran as recommended by rdw777.

To rdw777: I performed the witness mark test as you described and recommended. I planned to do it with 1) the stock tank and backplate, 2) thread in the tank to backplate joint, and 3) thread under the O-ring and tank to backplate joint. Unfortunately, I could simply not get the thread to stay in position at the very tiny radius of the venturi seal. I tried polyester and cotton string. Neither would work. This might be possible with the 049 but I couldn't do it on this motor.

This first witness mark picture is with the stock tank and backplate, no string anywhere. It looks like it would seal ok



This second witness mark picture is with the oil-soaked string 'packing" in the tank to backplate joint. No sealing at all!



Given this result and the fact that I cannot get the string to lay in the venturi seal groove, I'm thinking of assembling the fuel tank to backplate without the string. Balogh made a good point that if leakage occurs, it should be minor if there are not large defects and the joint is clean. A leak at this location would not affect venturi/reed valve operation.

Cotton fibers wrapped around the backplate screws are another story. Because the upper backplate screws break through into the crankcase, sealing at least the upper two backplate screws is, I believe important to do.

But I have a question, has anybody used the cotton fibers on the 020 sized engine? Maybe wrapping oil-soaked cotton thread would be just as effective. Please let me know what you think of that. That would be easier than trying to manage the loose fibers!

To roddie: Thanks for the input on the tube placement. I can see the type of engine use will dictate where the tube pick-up opening should be!

To Ken Cook: Thank you for your input too! I think I'm headed down the same path you are describing!

To all: I made measurements of the fuel tank venturi seal sealing surface inside and outside diameters. It is an odd size. The ID is about .104 inch (2.64mm) and the OD is about .156 inch (approx. 4 mm). That means I would need a tube with a approx. 1/32-inch (0.7mm) wall thickness. I checked Amazon and McMaster-Carr. A quick look for silicon tubing in this size range found nothing. That is why I am thinking hard about just reassembling the motor without the string in the tank to backplate joint and re-try leak testing. It would be every odd size tubing. On the other hand, they do make the stock seal somehow. The stock seal looks like it has a square cross-section, like a flat washer instead of an O-ring which would have a circular cross-section.

Thanks to all of you in this adventure!
Looking forward to your additional comments!
Dave

Dave

If the 2 top screws break into the crankcase - if I understood it right- then the crankcase will breath into the tank through the threads and no matter how well the reed seals, the crankcase operation is impaired. This cannot be repaired by winding threads around any or all of the screws at their head, outside the tank. The reed is then bypassed through the screw threads. The breakthrough should somehow be plugged, or the entire crankcase replaced, in my opinion.

Dental floss?

balogh wrote:Dave

If the 2 top screws break into the crankcase - if I understood it right-  then the crankcase will breath into the tank through the threads and no matter how well the reed seals, the crankcase operation is impaired. This cannot be repaired by winding threads around any or all  of the screws at their head, outside the tank. The reed is then bypassed through the screw threads. The breakthrough should somehow be plugged, or the entire crankcase replaced, in my opinion.

Andras, The photo below is Dave's from the 1st page.. and shows the thread breakthrough.



I think it was mentioned that this was typical of the PeeWee case's upper 2 holes for the 2-56 screws when being drilled/tapped in production. The screws themselves should have a .500" length of thread.



I have to wonder though..  if there were users who tried to substitute the longer (.687") screws from a .049 Babe Bee engine, which if over-tightened.. could have caused the breakthrough?

Thanks, Roddie,  for me the breakthroughs are a concern, because the screw  threads will not seal tight and the crankcase is thus open towards the fuel tank no matter how well the reed seals. I am not sure what nitro-resistant material could be plugged into the breakthrough holes from the crankcase side, but I would suggest this by-pass route, leaking  the periodically pressurized and depressed crankcase space,  must be eliminated for an efficient engine operation.

I checked one of my PeeWees and indeed the breakthroughs are there, too. If this is typical with all the otherwise good runner PeeWees, then maybe is less of a concern than what I thought.

Hi All,

Great discussion above! My initial thoughts when I found the breakthrough was that the cylinder threads, once the cylinder is in place, further blocks crankcase to fuel tank communication path. I know, thread is not a sealing feature. I just checked my assembly (fuel tank off, piston and cylinder assembled). I can still see part of the breakthrough hole on the crankcase. So, the cylinder threads only partially cover about half of the breakthrough holes. Sorry a picture seemed impossible. But I'll try if anyone what to see what I'm seeing.

A tiny drop of JB Weld could be used to plug the holes. It is an epoxy-based adhesive that is supposed to be pretty tough. As long as the surfaces are clean, it should adhere well. And it would be partially held in place by the cylinder once it is installed. Another option could be marine grade RTV silicone sealant. Again, the tiny drop of sealant, applied with a toothpick, would also be partially captured by the cylinder threads once the cylinder is in place. It seems unlikely that it would go anywhere. And the RTV silicone would handle the vibration maybe better than the JB weld.

The concern I heard the first time I suggested using something like this was that tiny pieces could break off and go anyway, causing problems. At the time I was suggesting using the type of Loctite that forms a gasket like seal on the surface beneath the backplate screw heads instead of the cotton fibers. I got some pushback on that so I will stay with the cotton fibers.

This is all great stuff!

Dave

Dave

Maybe a small piece of white teflon ribbon that plumbers use, tucked down to the Bottom of the threaded screw hole on the crankcase with a pin or needle, could be a solution. It bottoms out when the screw is  in place and when the screw is tightened, will fill the cavity at the bottom, forming a plug gas tight enough to retain the compression and depression in the crankcase. The breakthrough looks smaller than the screw diameter so the teflon "ball" will stay in place especially if the cylinder is also in place.

This is the teflon tape I am talking about, you should find something similar in plumbers' store:


It is called P.T.F.E  thread seal tape..

Hi balogh,

I thought I posted a reply to your latest comment! But I don't see it here. I am familiar with the PTFE tape for plumbing connections and I have some. i tried some cotton thread wrapped around the screw beneath the screw head, just as you would wrap Teflon tape. That did not work, lots of leakage out of the screw head areas. Instead of trying the cotton fibers, I went right to the Teflon tape. The below pic shows what one upper screw and one lower screw looked like. The upper screw has additional teflon wrapped at the beginning of the threads as well as around the screw head.



I assembled the engine and leak checked it. Amazingly the screw heads did not leak! Also no leakage at the crankcase/tank and tank/backplate joints! The only other fix I could think to do is use a gasket making Locktite under the screw heads. But I got a lot of pushbacks on that for fear of pieces coming loose. I still might try it.

However, I found a new problem. The next source of leakage is at the needle valve insert on the back plate. How does one go about repairing that. I looked up CA glue and it is fuel proof. I could apply thin CA glue to the joint between the threaded insert and the backplate.

I disassembled the engine after to see what the condition of the Teflon tape was. See the below pics. It looks like I might have used a little too much. I maybe limit the wraps to 1-1/2 to 2 wraps around. Also, I think the Teflon tape would be a good use for the upper screw thread tips where it goes into the crankcase.






Question about the leak check "locking up". I'm using a plastic syringe to perform the leak checks. Should I expect plunger to really stop moving or will there be some small leakage that is acceptable as long as I don't detect any leakage at the critical joints and interfaces?

Thanks,
Dave

Thanks Dave for experimenting on the idea..I would not necessarily wrap the screw tips in the tape, just drop or push a piece of teflon fodder into the screw hole in the crankcase where the breakthroughs are i.e. a piece of tape swirled into a ball and let it be flattened and jammed into the breakthrough by the screw when tightened, that should suffice. With this leakage potential eliminated, and the reed having passed the leak test, I would not worry too much about the remaining leakage found with the syringe..

If the leakage is outside the NV insert, then it may cause fuel leakage..if, however, the leakage is between the NV and the insert along their threads, that will cause fuel metering problems and can be eliminated by pulling a silicon fuel tube on the NV and the insert end/nipple to seal the threads.

Hi balogh,

The way you described the use of the Teflon tape - swirled up into a ball, I would gusss to about the diameter of the screw hole, sounds great and I will do that!

I cannot tell if the NV leakage is coming from the NV threads or between the backplate insert and the backplate. I'll try a little thin CA (and hopefully not get it down the NV hole! And I will, as you suggested use a piece of silicone fuel tubbing on the NV in case it is actually the NV threads leaking. Belt and suspenders as some folks say. I had the NV at what I believe to be "snug" tight, but not over tightened.

And also, thanks for qualifying the "lock-up" definition.

Dave