Is there a gasket on the back plate of a Cox black widow engine , if not cab n it be lapped in to seal, and what is best to use for lapping compound ??  I am thinking Tooth Paste..  Suggestions/ideas   ...

Hmm. You need a good seal on the tank if you want to pinch the over-n-under tubes to shut down the engine. (Mouse racing warmup.) Otherwise an air leak isn't a killer but may make it harder to needle.

A good venturi gasket is important. I have tried making these at times when I didn't have a real one. Never could slice off a uniform and appropriately sized piece. If someone has a trick for that, I'd like to know how....  You can definitely still buy these and that works for me.

I have used RTV on the v-groove joint before. Kind of a pain and you must not get any into the tank or it will float around in there and cause more grief a the worst possible moment.

If you have damage to the joint/seal area from being dinged or something like that I might try a sharp implement, but any additional gouges or scars just increase the badness. Parts that have been dumped in a bucket often have this problem. And, there are lots of buckets of dumped Cox parts out there.... (See disclaimer below.)

Toothpaste is not aggressive enough to do much good, unless you are alone on the ark and have 40 days and 40 nights with nothing else to do....except brush the hippos teeth, or work on your backplate.

The Divot


Disclaimer:  Very few Cox engines were abused during the SoCal Coffee Can Races. Entrants were covered by unique indemnification for the duration of the race. While the buckets could be shaken before the race, this practice was discouraged when it was pointed out that the assignment of cans was by drawn lots, and any entrant might be assigned the most shaken can.

              If the tank to backplate is leaking ( V-groove in the tank) not the venturi to backplate, use dental floss in the groove. I put it in my mouth making it wet enough to stick and lay it into the groove. Only provide 1/4" overlap. For screws that leak, I first replace them with new screws which sometimes works, if it still doesn't seal, I take a Q-tip and pull off a piece of cotton. I twist it tight and wrap it just under the screw head just as you would apply Teflon tape so that it tightens on the threads and not the opposite. This make a serviceable seal which unlike silicones and gasket materials need to be reapplied.

Most all Cox Reed Crank cases can be lightly lapped on a flat surface and oiled 400~800 grit to get the surface flat and remove the raised area around each of the 2-56 Screw holes.

Additionally a 2-56 Bottoming* Tap should be run in and add an addition 1.5 to 2 extra threads

The factory drilled holes are deeper than the cut threads. The extra threads help prevent the screws from bottoming out and giving false sense of tight back plate

* If Ace or Hobby shop does not have the $3.89 Bottoming tap..Buy the start Tap and grind the pointed tip back some to make it like a bottoming tap

Remember to flush the chips from each hole

I use Tamaya very small sized pointy Q tips.  I use a tube of PermaTex Anaerobic Gasket maker...A barley perceptible smear of the gasket maker around the V grove and back of each Filester Screw head. This is a non hardening and aluminum/plastic safe product

https://www.amazon.com/Tamiya-Cotton-Triangle-Xsmall-TAM87105/dp/B0026IBC38/ref=pd_bxgy_2/139-5551949-1491004?_encoding=UTF8&pd_rd_i=B0026IBC38&pd_rd_r=acbeb46c-8334-499f-814b-8508715d6197&pd_rd_w=8Mbt9&pd_rd_wg=zxGAK&pf_rd_p=f325d01c-4658-4593-be83-3e12ca663f0e&pf_rd_r=5E8CYA8K77DRZ2A0JP5Z&psc=1&refRID=5E8CYA8K77DRZ2A0JP5Z

  Yeah, we need to know if metal or plastic back plate. Even when new, you need to be careful torqueing these things.
  Type at you later,
   Dan McEntee

Gentlemen,

One way of cutting venturi tube O-Rings from silicone tubing: Use a piece of appropriate size music wire & run it through your ~ 1" piece of silicone tubing. This music wire will now become your mandrel with which to hold the tubing with.
Place this assembly into the freezer until frozen. Remove from freezer and chuck music wire into the lathe. Now, with a sharp Exacto mounted to your tool post holder, you can now
feed the blade onto the silicone. If you lock the tool post, then you can precisely cut off ( in thousandths) whatever thickness of tubing you like. Cut off different thickness's & try to see what works best for you.

I'm now trying O-Rings (made for darts)  bought at any shop that sells darts. A bit thicker than the Cox originals but worth trying I think, & can be locally bought cheap.

As far a sealing the backplate, since the tiniest piece of silicone glue coming adrift can totally mess up your tank, I don't do that. I have seen some metal backplates warp a tiny bit & just won't seal no matter how tight your screws are. Short of replacing parts my "good enough" solution is to dab (RTV) silicone glue on the entire outside (well cleaned) joint periphery & let dry.
(while I'm at it I do the tank to crankcase joint as well).  Another "almost good enough" solution is to wrap the (well cleaned) joint on the outside with Scotch Decorate & repair tape (CLEAR blends with all colors). This is a wonderful product that really sticks to many surfaces really well, plus can be done right on the field if need be.

These are just some suggestions that have worked for me & are proven on the mouse racing circle to have worked well enough. Experience has proven that the black nylon backplates tend to leak less (at the backplate tank joint) than do the metal ones. (Although I have seen both types leak!)

I hope the above has been some help.  Cheers,

Paul Gibeault (aka "Mr. Mouse")

A good venturi gasket is important. I have tried making these at times when I didn't have a real one. Never could slice off a uniform and appropriately sized piece. If someone has a trick for that, I'd like to know how....  You can definitely still buy these and that works for me.

I have used RTV on the v-groove joint before. Kind of a pain and you must not get any into the tank or it will float around in there and cause more grief a the worst possible moment.

The Divot

HI Dave,

Indeed, things are well frozen outside over here!  The lump of stuff on my motor is RTV sealant. Despite my best efforts sometimes joints leak. In this case (after cleaning of course) I added RTV silicone sealant to the crankcase joint as an extra sealing barrier. Not so pretty, but it doesn't hurt! ( I ran out of clear sealant which is less obvious...)
Cheers,

Paul

Now there's another good idea. I think I'll need a better fitting dowel though....

Thanks Ken!