Correct threadsize for a Cox Babybee

[Timothy Payne]

Does anybody know what the threadsize is for the four long studs that secure the tank to the crankcase on my Cox .049. - I know they're one inch in length, but need to cut them down as the 8cc plastic tank I bought for it leaks badly.

Upon contacting xenalook they suggested the bolts (supplied with the tank) might be too long for the older Cox crankcase.

Are the bolts too long or are these plastic tanks not that good!???


Any suggestions,


Tim

[Tim Wescott]

2-56.  The metal tanks, at least, use special screw heads (or maybe just plain old pan heads) to fit in the recesses in the tank -- socket head screws work.

I've never used a plastic tank, so I can't say much about them.

[Timothy Payne]

Thought so!

Cheers Tim

[Tim Wescott]

As to whether your particular screws are too long: if the screws are snug (I don't want to use "tight" when we're talking about a plastic tank!) and the tank still squirms around when you twist it on the case -- your screws are too long and need to be shortened a bit.

I assume you know the right method to shorten a screw so it'll still work.

[Dane Martin]

i have had nothing but good luck with the plastic tanks. the screws are slightly too long, that's true but easy enough to shorten. the tanks are wonderful for speed, racing or free flight, but are not stunt tanks. they hold up great, though.
a little brass tubing and some JB weld will fix the stunt-ability for sure 

[Phil Krankowski]

The screws are "fillister" head.  It is a standard type of head, but not a common type.  Better supply houses like Fastenal and Mcmaster-carr should have, or be able to get easily.

A bottoming tap can be used to make the threads deeper instead of cutting, some cases benefit from a few extra threads anyways.

Using some aluminum safe sealant may also have similar results to cutting the screws, but cutting might still be necessary.

Phil

[RknRusty]

Phil makes a very good point. Cox bored all four holes in the crankcase deep enough, but frequently only tapped them for a few turns. You can almost always use a 2-56 bottoming tap and add 5 or 8 more turns to the screws. Then the ones that seemed too long are not too long any more, and the crankcase will mate more securely with less chance of developing air leaks. Some of the bottoming taps have an extension on the tip that may need to be ground off. I found one on e-bay for a couple of bucks and free shipping a few years ago. While you're at it, get yourself a 4-40 tap too.

This might be bad manners, but check out coxengineforum.com. It's a small but amazingly helpful forum. You can click a button to hide the nagging log-in pop-up.
Rusty

[Timothy Payne]

Cheers guys,

Cox International actually acknowledge the problem of using plastic tanks with older engines, stating that the studholes aren't as deep.

They suggest adding an extra gasket where the tank meets the crankcase, or indeed using shorter bolts. - I'll have to get in touch with Modelfixings for these as I use them for replacement bolts for Fox and McCoy engines.

Here's one for yer!

Are the steel backplates still available cos they are far better than the 'plastic' remoulds. (Funny how some stuff was of far greater quality when it was first manufactured!!!).



Tim (2-stroke man)

[RknRusty]

Cheers guys,

Cox International actually acknowledge the problem of using plastic tanks with older engines, stating that the studholes aren't as deep...

...Are the steel backplates still available cos they are far better than the 'plastic' remoulds. (Funny how some stuff was of far greater quality when it was first manufactured!!!).

Yeah, Bernie at Cox International and Matt at Exmodelengines are both top notch customer service guys. They both bought up all of Estes old stock at the same time and work together to find us what we need. Bernie is a long time airplane modeler too.

Neither of them have any metal backplates except when they pick up some that might come from a one-off sale somewhere. They're always on the lookout for lots of old parts. Matt refurbishes a lot of stuff too, and either one will put you on a waiting list. It doesn't hurt to ask in case they have some that aren't listed, but don't count on it. Everybody wants them. Go easy on the torque and the plastic ones last fairly well though.

You can also ask over at coxengineforum.com. Someone might sell, trade or give you one. We swap bits and pieces all the time over there.

Good luck,
Rusty

[Timothy Payne]

Will do that Rusty. - Thanks for the advice (I never thought about deepening the thread holes!).