cloning a venom jr

Use parts with good fits to each other and it should be good.
If it goes well runtime would probably be just shy of a minute.

Are you talking about a Texaco Jr.? A Venom Jr would basically be a killer bee crank with zero drag driveplate, #4 cylinder with lightened piston and an enlarged venturi in the 5cc tank. Then after that to make it a true Venom Jr. you would need to add cross vent tubes to the 5cc tank and use a vent-less backplate to make a 5cc stunt tank.

A Texaco Jr is mostly a sure start cylinder on a black widow crankcase with a 5cc red tank and a 5 fin head.

Here's the tank you probably want,
from the Mercedes tether car.
I think I've seen the tank come up
on e-bay once, a long while ago.
Sounds like a cool project though.
Bob

photo from a Lousyflyer post. tnx.

Just what is the difference between a normal and a "lightened" piston?

Anyone? Anyone?

The way I see it, The inside of the piston wall near the skirt is ground down so it is thinner. Some are shortened/lightened by grinding off some of the skirt.

lousyflyer wrote:Just what is the difference between a normal and a "lightened" piston?

The "lightened" ones blow up....

lousyflyer wrote:Just what is the difference between a normal and a "lightened" piston?

About one gram. Difference in RPM depends on engine.

ahrma_581 wrote:

lousyflyer wrote:Just what is the difference between a normal and a "lightened" piston?

The "lightened" ones blow up....

Because, if I recall correctly, of the way it was lightened. I think the whole skirt had been thinned from the inside. For a flyer, I'd go with the short SPI piston like the Killer Bee type. It may be a little heavier but it's strong.

I rooted through some of my stuff and found a piston that has been lightened. I forgot that I had it (no memeory problems here, right?). The skirt has been thinned and shortened.

The normal pistons that I have weigh in at an average of 3.2grams. The modified piston weighs 2.83grams.

The normal pistons are averaging .405" tall, whereas the shortened one measures .361"

If you get a killer bee crankshaft, is it specifically machined to run with in conjunction with a lightened piston?

lousyflyer wrote:If you get a killer bee crankshaft, is it specifically machined to run with in conjunction with a lightened piston?

Hmmm, good question. The light piston would need a proportionally lighter counterbalance wouldn't it. If I knew Cox production history, I could say something like, "yes, it originally appeared with the Killer Bee and its lightened piston, so the KB crank is smoother running with that light piston." But I'm not an expert on that, so that's my best guess.

Now that raises other questions. You put a KB crank with its (I'm told) lighter counterweight into a 1995 BW which has a tall heavy piston. Does that introduce more vibration and wear?
Should you mount the black Cox prop, which ALWAYS has a heavy sided hub, with the heavy side opposing the piston throw, thus adding to the counterweight? I think so. Should you add hub weight to a perfectly balanced hub for this purpose? That rubs me the wrong way, but I suppose you should.

My observation of Cox black rubber duckies is the heavy side of the hub is always the side with an injection molding divot. So if you only have the capability to balance blades, but not hubs, use this divot as a reference mark.

There has to be information on this somewhere. I would prefer knowing that I am not going to waste a crankcase by running out of balance assemblies together before I start trying to use a lightened piston.

PM Paul Gibeault and ask him. He developed high performance reedies for Cox and is a champion mouse racer. He's a member here.

Done. I would like to hear what he has to say.

Did you link him to this thread?

Thanks, Bernie.

Sorry to be late in replying...haven't been here for awhile.
I do have a thought or two.
First off, no part swapping is ever a waste of time with a Cox! The odd swap will produce a better running engine for no apparent reason. (well, the reason is the fits & tolerances are better "matched" but, it certainly is not visibly evident).

Cox engines will never be balanced. Some are just better balanced than others is all. i.e the Venom/Killer Bee crank engines with the heavier crank throw.
The factory "lightened piston" was ground a few thousandths thinner on the inside wall. The exterior dimensions were the same. Some seems to work OK, but when run fast (20,000+ rpm, especially with the Venom) the piston often failed in the crown area. They may work OK for lower rpm & general sport use. They are unreliable in a racing engine & therefore not used by me really for anything.
Interestingly, it isn't the thinning of the piston but rather the sharp edge left at the inside top of the piston (by the non -radiused cutting tool) that causes the failure...

Bottom line...GO AHEAD & PLAY!! You're not going to hurt anything.

Best of luck, Paul G.

lousyflyer wrote:There has to be information on this somewhere. I would prefer knowing that I am not going to waste a crankcase by running out of balance assemblies together before I start trying to use a lightened piston.