XJet

I've put together another 049 from the various Cox bits I have here and was wondering about compression.

The engine runs okay for a start but then it sags a lot and won't hold a needle-setting.

When hot, the compression leaks out from TDC within a couple of seconds -- but when it's cold the compression stays for about 5-6 seconds.

Is this too worn?

If it is, I might try nickel plating the piston or the bore and see what happens.

If it's not a worn piston/liner then I'm a bit stuffed as to what the problem is.

I've replaced the gasket between the tank and the crankcase and the gasket between the venturi and the backplate. I've also put some silicon tubing over the needle-valve so it cant' leak there and replaced the internal fuel-pickup tube with some small diameter silicon (kept the spring in it too).

One other question -- should the reed valve be fitted so that it's sprung open or sprung closed by default?

I tried both ways with little perceptable difference and I can't for the life of me remember which way they go. It's a metal reed for what that's worth.

Japanman

Hi Xjet,
Sounds like it is overheating- try adding a few(1 or 2) head gaskets.

Hey, I was just about to try to hit you up about plateing a clinder (how are you going on the conrod?) I have for just this reason- the pistons are pretty hard so it should make the engine last just about forever!
All the reed engines I have seen should have the reed floating with no positve or negative sealing- might be the cure of the over heating. you can cutout your own reeds from mylar- of yours is suspect.

J.M

BMatthews

I don't think it's leaking, just warm oil letting the compression leak by.

As long as it's EVEN wear and not due to a scored piston or cylinder I think you'll find that a fit like that really honks quite well. I have always found that my hottest Black Widow type Cox engines were the ones with a piston that would turn over through TDC with only a little torque on it in about 5 or 6 seconds when lubed with something light like 3in1. Your's sounds like just a perfect fit to me. If it's an older twin bypass port type with the open exhaust rather than the slits it should be a speed demon.

If you can tach it and compare it to some other C&P sets on the same bottom end I think you'll find it's the faster one unless they are all that loose.

XJet

I don't think it's overheating -- I'm using the same fuel (25% oil / half castor) and 20% nitro) as I run in my other engines and if it sags so much that it stops it's still free and easy to turn -- but there's just a lot of compression leakage between the piston and bore.

I'm still waiting for some time to make a rod. I'll probably just go ahead and plate this cylinder anyway -- I wonder if I should strip the black oxide from the fins and make the whole thing nice and shiny :-)

The crankshaft journal in this crankcase has ovaled too so I'm going to have to bore it out and sleeve it I think. I've got some bearing bronze here I can use for that. It's the least-bad crankcase I've got and surprisingly, I haven't seen an old cox engine in a garage sale around these parts for quite a few years so I'm stuck with fixing up the bits I've got.

All the reeds I've used have been metal (phosphor bronze by the look of them) and they have a definite preload built into them. If you fit them so that the bias is towards open the give the brief "squawk" I recall from my early days of working on Cox engines when you blow against them so as to force the valve shut -- the other way they make no noise because they're already closed.

Where do you get the mylar you're using to make your own valves?

Big Al-RCU

X Jet
I have always assumed that a new reed (such as comes in the old Cox overhaul kit) was dead flat and had no right or wrong direction and that any bow in a used one was a result of use. But (excuse me for stepping into your thread) I hope someone can comment on the basic theory of the Cox reed valve. I have wondered if the shape of the wire snap ring/keeper was critical. Is more free travel better (equivalent to longer valve duration)? Or is less travel better (minimize losses from the constant reversal of the reed mass)? Or is resonance a key factor? Suspect that there is more engineering involved here than meets the eye, but that some of our members can describe the ideal setup. I have an old PeeWee that always ran like crap, but wonder if it wasn’t something simple.
Al

gcb

Al,
All the springs I've used will hold the reed shut when installed backwards, and the engine won't run.

Others' experiences?

XJet,
Could it be something simple like the plug seal leaking when hot, in addition to the piston fit?
Also, would you plate the entire cylinder bore on a tapered cylinder or just the top? I have neither the skills nor tools for plating and honing, this is just a question.

George

DesignMan

How about varnish build-up on the cylinder walls? If they are dark brown, you have a problem. Clean them up with 0000 steel wool or a Davis diesel cleaning brush. Probably the crock-pot/antifreeze cooking will do the job too.

DICKEYBIRD

When you say you assembled this engine from bits, does this mean the piston & cyl. are no longer a from-the-factory matched pair? If so ck. the piston O.D. with a mike or caliper to make sure it's ROUND. I don't know if they wear out of round or the bad ones were ground from the factory that way. Mix 'n match parts can work well in a Cox but you have to be very selective.

Set your mike or caliper to size and lock it down. Carefully slide the piston in & out of the mike at 4 or 5 different radial positions, comparing the feeling of the friction at each position. You might be surprised what you find going through a handful of old pistons. I've discarded quite a few pistons that felt like a "good" dry fit but were in reality out of round. This is an artificial friction fit and don't seal worth a hoot. A somewhat loose but still round piston will run much better than a snug oval one.

Reedies that won't hold a needle when hot are usually over-compressed, over-propped, the crankshaft is too tight, (doesn't sound like yours is) the piston is too tight, or the piston is just plain worn out.

Good luck!

ajcoholic

My Feedback: (10)

Bruce,
I just mailed you three heads for your Norvel. If I would have seen this a few days ago, I would have sent a few crankcases for the cox reedvalve engines. I have a whole BAG of them..

If you want some Cox parts, email me what you need...

AJC

XJet

Thanks AJ, let me know what I owe you.

I have checked for leakage around the plug but I haven't checked the piston for any ovalling.

Unfortunately this isn't an open-port cylinder although it does have double transfers.

If someone can tell me the thread used on the bottom of the cylinder and for the glow-head I could always make up a new piston/liner pair myself. Does anyone know?

ZAGNUT

My Feedback: (1)

the first thing i do when rebuilding a reedy is to toss that old star shaped bronze valve....the old crusty ones always cause problems.

some old floppy disks (this one is sony) have a nice little square of mylar inside of the same thickness as the cox reed. the stock reed is a 7mm wide rectangle but will still seal well at 6 mm wide as long as it is a good fit and can't move around.


dave

BMatthews

For the old metal reeds I found that all my problem engines had one thing in common. The reed would squawk with light air movement. Flexing or flipping the reed to avoid the saxophone sound always restored the engine to good running. The mylar reeds have never shown any musical symptoms and all have ran well. I'm a mylar fan.

A possible source may also be draughting or graphic arts supply outlets for the thinnest mylar sheet they have. And it's possible that you can use the actual floppy disc material itself. I'm just not sure if the oxide coating will come off or not. Might be worth playing with some solvents. But I also think that the disc material itself is probably TOO thin and flexible. I'd have to check it against the OEM reeds.

gcb, the spring is meant to limit travel and not to clamp the reed down. You tried it backwards... But it is possible that rebending the arm to control the reed flex may work for some aspects but I can't think of any. In any event the new plastic cap types don't let you play with this.

XJet

Okay, I've got myself a piece of mylar from inside an old floppy. I can't for the life of me remember the shape of the mylar reeds -- is it the same as the bronze ones?

Should I just use the old reed as a template for cutting the new one from this mylar?

ZAGNUT

My Feedback: (1)

simple rectangle with ends rounded off to fit the backplate recess. width is 7mm on the stock ones.

bmatthews: the reed mylar is .2mm thick while the disk is much thinner and would probably get sucked right out of the backplate[]


dave

edit: added high quality digital image of cox reed

Japanman

If everything you try seems to have no effect, try this:
Slap on a 5by3 prop- I toasted one of my very precious cylinders here when the carb came loose in flight and the engine went lean... It didn`t want to run with a 6 by 3 after that (doing just what your engine is doing) but ran fine with the 5by3 . Mind you, It was REALLY stuffed - about 1 second to bleed down compression when wet with fuel.

How about I make you a rod in exchange for plating one for me? it will have to be 2024 though. I am really interested to hear how the cylinder plating goes.

J.M

ajcoholic

My Feedback: (10)

.049 base threads (and glow head threads) are 40 TPI

AJC

(you dont owe me anything for the heads, they were ones I have had - used but good)

gcb

Thanks. That's what I was trying to tell Al. It doesn't run with the reed clamped down. I found this out the hard way, long ago.

George

XJet

I'll try to get time to plate a cylinder this weekend but don't worry about making a rod for *me* -- I don't need one (just) yet. However, once I've finished moving house (ugh!) again you can send me anything you want plated and I'll do it in my next batch.

It's quite interesting to note the difference when turning over a Norvel without a plug fitted and a Cox without its plug. Considering that neither should have any significant differential in expansion between the piston and liner, (being steel/steel and aluminum/aluminum) it's amazing that they both run so well.

Now that AJ kindly posted the thread pitch for the Cox cylinders, I might also have a go at making an aluminum/aluminum piston/cylinder set and see how that turns out.

Silvaire

My Feedback: (50)

Since this thread has discussed cox reeds a bit, I was curious about some teflon reeds I saw offered for sale. There are some currently offered on "that" online auction site as auction #5901115018. This seller also sells Davis Diesel items on a regular basis, and perhaps these teflon reeds are from them. So... has anyone had any experience with these teflon reeds?

gws1

My Feedback: (13)

The oxide coating does seem to wear off of the floppy disc material given enough running. I have had good success with them though. That is what I was using on my V-block reed assembly. I need to get back on the project again.........

Greg

Japanman

Thanks- Let me know when the move is over and you are ready to go. At the least I`ll send you my spare norvel rod- Then you can have something to get dimensions off should things go to custard with the rod in your engine.

J.M

XJet

Getting back onto the topic of Cox compression - I've noticed that since I've been swapping plugs around and trying different numbers of head gaskets, my Cox now blows bubbles between the plug and the cylinder (when cold or hot)

I've tried a brand new washer, annealing an old one, lapping the plug's mating surface and everything short of using some teflon tape on the head-thread -- but the damned thing still leaks.

The mating surface on the cylinder seems clean and smooth -- but still it leaks.

I've tried three different plugs -- and it still leaks

I'll go over it with a bright light and magnifier but, short of a crack, I'm stumped!

DICKEYBIRD

Dey do make 'em crooked sometimes! (The glowplug threads in the cyl. that is.)

BMatthews

It sure sounds like that would be the problem here.

Japanman

I had the same problem - I used loctite master gasket (as well as the copper gasket)- this fixed the problem. Gun silicon would also probably work. The advantange with master gasket being it will not glue itself to the aluminium head- though you might need a bit of heat to get the head off.

J.M