Anyone have some advice for freeing a Cox piston that is gummed or varnished in the cylinder? I am trying to resuscitate my 1953 Space Bug Jr which has not run in 50 years. Little run time on it as I recall, and the liner looks good, no apparent rust. Hope to free the piston (stuck about halfway between TDC and BDC) without damaging it, as this early version cylinder is unlike the ones starting in 1954. This is my first model engine, bought by my dad.

Also need to know how much play is acceptable in the ball joint where the rod is held in the piston. Any way to measure reliably?

BTW I have the still quite nice Firebaby it was mounted on, wood fuselage, nylon stocking material doped to wing underside. This dates from maybe 1957.

David gives good advice! My favourite method is to remove anything plastic and place in an oven set about 200 degrees Centigrade. Leave it to cook for 10 minutes or so and then remove with oven gloves and rotate prop driver, should move after heating , if not turn up the oven a little and leave for about 10 minutes. This has never failed for me!
The other question was what play is acceptable in the little end joint. I suppose the correct answer is none! Hold the piston down on a flat surface, grab the con rod and try pulling it up and down. Even small amounts of play can easily be detected by doing this. If there is obvious play then you need a reset tool from Cox International or Davies Diesel Developments. Follow the instruction until none or virtually no play remains. The ball joint should rotate freely, but have virtually no up and down movement.

Regards,

Andrew.d

Once I get things freed up I like to completely disassemble and do the antifreeze in a crock-pot thing.  It cleans things up a lot.  It will leave some gummed-up castor behind, but transmogrified into loosely-stuck slightly brittle stuff that'll come off easily with a stiff brush.

Some complain that the antifreeze can discolor aluminum.  It can.  I haven't had it do so to Cox anodized aluminum, but the Space Bug is cast, so if you care about your case coming out permanently darkened, think twice about using the method.  Me, I'm a slob who just cares about how it runs in the end, so I just chuck it in the pot.

Thanks guys! Looks like the oven is the best bet. The crank spins freely, no problem there at all. Inside the motor looks new. Reed valve is probably good too but haven't checked (not sure HOW to "check").

A million years ago I had one of these rod-play tools when I ran TeeDee 049s in free flight (which I still do) but can't locate it for some reason. Actually it is probably 65 reasons ganging up on me. Is Cox still in business in some form, as indicated by the advert banner I have seen here?

Two things.  I think you will find that your SpaceBug Jr. has a three part piston: an outer shell, a plate holding the rod in place, and a snap ring holding the plate in place.  Not much you can do about piston slop.

I have wanted someone to try this, and it might work on your engine as a last resort.  Take the head off, fill the cylinder above the piston with water.  Replace the head.  Put it in the freezer.  As the Ice expands, something will give.  

Success!

Piston now freed (penetrating oil plus rotating with prop mounted). Problem is, while piston travels freely in bottom end of stroke, it binds as soon as piston crown rises above exhaust ports. It will turn over, but something is clearly wrong. The cylinder looks good, no visible problems. Any ideas? This is the 1953-54 cylinder with a flange at the bottom. Can this piston/rod be substituted with a later model, maybe?

How badly does it bind?

Does it seem to bind with the piston clocked in one direction to the cylinder but not another?

Is there any visible rust, or a ring of pitting or discoloration in the cylinder anywhere above the exhaust ports?

Does it bind "hard" (i.e., with a nasty squeaky fit), or does it have a gummy feel to it?

The engine ran, once -- do you remember why you took it out of service?

A bit of a tighter fit above the exhaust ports is a good thing (to my knowledge TD cylinders were tapered), but a really-o truly-o bind isn't.

I would just go here and get a new assy.  Should screw right on, I think.

http://coxengines.ca/cox-.049-sub-induction-cylinder-piston-spi.html

         Hello Mike, I was quite surprised when I opened up a Babe Bee I had with this same set up. Initially I thought it was some kind of Cox clone. The exhaust cutout is considerably smaller than a normal Cox .049 in height and width. Mine was also equipped with the W element in the glow plug. The backplate appears to be typical Cox with the exception of no casting indicating the company logo or even made in the USA. The hole was quite small in the backplate and the plating was exceptionally poor in the respect it just didn't resemble the quality of later versions. I cleaned it up also noticing the play in the circlip and retention cup holding the conrod. I also noticed that the rod is aluminum. The engine runs, just not to my liking. The cylinders are extremely thin and it sounds like this bind may have been due to the removal of the cylinder using the tool that protrudes through the exhaust slots. I've taken a piece of K&S tubing which will fit into the bottom of the skirt of the piston. If the fit is loose wrap tape around the tubing until it becomes a press fit. The rod will be in the center of the tubing. Insert the piston upside down in the cylinder from the bottom up. Drop a few drops of Brasso onto the piston and rotate in a up and down fashion. This will remove that burr making the cylinder useable again. I just wouldn't expect any high performance numbers from that piston design. I feel that play really is hurting the performance rpm's. Ken

Mike, when I said to rotate the cylinder with respect to the piston, I meant independent of the crank -- out of the engine, if possible.

I'm assuming that this engine still has a ball and socket joint instead of a wrist pin -- right?  I have both a TD 09 and a Medallion 15 where both the cylinder and the piston are out of round.  This works fine when the ovals match up, but when the piston's big way is aligned with the cylinder's small way, they jam.  Since the piston tends to rotate in a Cox, the engines won't run for any length of time.

If yours has this problem, then you're probably out of luck unless you can find a new piston/cylinder.  If it has burrs, as Kennith suggests, then his solution may fix things.

I have a Space Hopper with the three part piston.  I replaced it with a modern piston and it runs very well.   

  once you get everything apart soak everything in Dawn Power Dissalver for a few minates and  then scrub with P/D and a green scotchbrite pad it will remove all of the hard grease and carbon . tear of a piece small enough to go in the cyl again with P/D, a small Brass bottle brush along with the P/D works good for this also  you may still have some carbon or caster in the sleeve keeping the piston from going up, the power dissolver wont hurt the plastic so you can clean everything . be sure to clean the surface that the reed sits on, and if it still has the copper reed ,might want to replace it with a mylar reed.

as far as for the cyl make sure there is no bur left by the tool taking the cyl loose from the case. if you get the rod too tight take some Brasso or dupont rubbing compound and oil and fill the piston up and work the rod all around until it smooths out ,it may take a few moments then you will have a perfect rod fit. . use Dawn dish washing soap and HOT water to clean everything spending extra time with the rod to get all the compound out ,then oil everything the crank should spin freely in the case ,and the piston should go to at least the top of the cyl  Dont forget to replace the O-Ring between the tank and the backplate.