jetpack

My Feedback: (1)

Can anyone share the dimensions of a Cox Thermal Hopper prop nut, and what its made of?

You would need a caliper and a set of radius gauges to measure one accurately enough for it to be machined. I know what one looks like through pictures, but none in-hand to measure.

Just the hex size, hex thickness, total length, base diameter, nose radius and blend radius I believe is all there is to it, besides the tap size and the material.

If anyone has a digital camera it would be great to have a perfectly straight side picture of one, or a detail shot of anything I've missed so it can be drawn out on computer for dimensions.

burtcs

JP: An associate has several of these engines he uses for comition free fligh. He is of the opinion that they are nothing more than a reed valve TD. Having seen these engines close up, the spinner and bolt do indeed look like the TD parts.

Could you be thinking of another engine? A Space Hopper maybe which I don't usually see to many of these. Or could your engine have a threaded stud screwed in? I which case unscrew it and replace with TD units.

HTH - Steve B.

jetpack

My Feedback: (1)

No, I wish they were a TD spinner and screw, these look more like an acorn nut. The Thermal Hopper motor is equipped with a stud that they screw onto.

The Space Hopper might be the engine your thinking of. They have what looks like the conventional TD spinner, but the spinner is smooth all the way across, no lip like you will find near the base of it.

The pic on the left is a Thermal Hopper and the one on the right is a Space Hopper.

EASYTIGER

My Feedback: (119)

Machining one would be a real headache! They are not that uncommon on the collector circuit, just ask around for a Thermal Hopper Prop Nut, it's not that rare....

gcb

The Thermal Hopper was the second engine produced by Cox in 1953. The Space Hopper, I think used some TH parts with a newer reed design and a beam mount. The TD was derived from these, deviating from the reed induction.

Here are some pics of the acorn nut, and some equivalent parts between the TD and the TH.

George

jessiej

And, in their day both engines ruled the 1/2 a world!

jess

BMatthews

So the prop stud is actually part of the crankshaft like with larger engines? The picture sure makes it look that way.

jetpack

My Feedback: (1)

Thanks for all the replies [sm=thumbup.gif]

George, those pictures are great, thank you...
I would have never guessed Cox was already using the hollow spinner back without them. It is amazing how far the designs were carried to save weight without sacrifice. My main reason I enjoy these engines.

Easytiger you are not joking...
The back groove and tapped hole on the prop side would need doing first...then a fixture machined (a studed faceplate) to tighten it down to be put back into the lathe. Then comes the radii with all the tool grinding unless your CNC, and then to the indexing head for the hex on the mill.
That would take me about five hours with no coffee and donut. I would like to skip those steps and get right to the polishing but I keep looking on ebay with no results as of yet, and most TH's I see are in bad need of one. I can work on break time at work, or stay after hours occasionally if I decided to machine one, so thats why I'm taking this route till somethings come up I guess.

Jess your right, but I can be more right by saying they still rule at this bench.

Tim Wiltse-RCU

jet,

I am sure Burnis Ray has them. I had a friend of mine off the internet make me some a couple of years ago too.

LAter,
Tim

EASYTIGER

My Feedback: (119)

I was about to say...Burnis Ray or Dan Sitter. Like I said...it's not that rare a part, nor that expensive. But an awful lot ot work to make one! Sure always thought it was cast and tapped, not machined? Could be wrong.

gcb

EASYTIGER,

You may be correct. Not being a machinist, I can't really tell the difference. Cox has some fancy tools so possibly it is machined, but they couldn't do this with such a labor-intensive process and sell them for a quarter (or whatever) even back then.

Tim,
How did your friend make them?

Actually I could use four (3-SB Jr's and one TH) and perhaps a couple of spares. I just assumed they were not affordable.

George

Edit: I pulled this part of my response about the acorn nut from the engine section since most responses seem to be here.

Hex size=3/8", hex thickness=5/64" at highest point, total length=3/8", base diameter=1/2" with 1/32" from back to base of hex, tapered ~45deg., thread size=5:40. Don't have tools for radius. There is a flat spot on front ~7/64" in diameter.

Back of nut is cut out from 3/16" around threads to 3/8", leaving 1/16" lip. Depth ~ 5/32".

Eyeball accuracy...until someone provides more accurate readings.

These "acorn" nuts were used on the Space Bug and Space Bug Junior also.

EASYTIGER

My Feedback: (119)

I'm not a machinst either, but it LOOKS to me(like it does to you) to be a cast part.
Cox later got away from cast parts entirely...that was part of their whole deal...if you look closely, after the thermal hopper and space bug, there are virtually NO cast parts on any Cox engines. Almost none.
GCB points out the reason WHY these are not that rare...the space bug used them too, and they were and are pretty common engines. Also, many of these acorn nuts were sold as spares.

Tim Wiltse-RCU

JetPack,

It has been over a year since I have heard from Dirk but give him an email about the Cox acorn prop nut [email protected] tell him Tim Wiltse up in NC sent you.

LAter,
Tim

injunnut1

Easytiger and all,
The Cox acorn nuts were most definitely machined, as were the Thermal Hopper cage mounts(sold for only 75 cents, was it?). If you will simply take a close look at the acorn nuts you will see tell-tale machine marks every which way but loose. Rumor has it that Leroy Cox was once so frustrated with the lack of sales on the T. Hopper cage mounts that he loaded up several bushel baskets full of spanking new ones and threw them in the foundation of one of his buildings just as the concrete was being poured! The one and only cage mount that I ever tried out on the test stand did not fare so well ..... upon peaking the T. Hopper engine out to full bore RPMs, the cage mount tore apart like Popeye shredding a Spinach can!

I met the Dirk guy a couple of years ago here in Florida and entertained him to a bunch of unique 1/2A engine stuff ...... he in turn gave me a few of his repro Cox spinners of the T. Hopper and Space Hopper varieties ...... he does do very NICE work and he was telling me of the rather involved procedures in machining the acorn type spinners in particular. On the other hand it was Dale Kirn, I think it was, who was once telling me that during a full production run the machines at Cox would spit out the Cox spinners like recycled corn kernels being splattered against porcelain![X(]

ajcoholic

My Feedback: (10)

They are most definitely machined from bar stock. I have made and sold many replicas (I machine mine from 1/2" diameter bar stock aluminum.). I first made a form tool, from a 3/8 lathe tool bit that cuts the front curved portion of the acorn nut. Then I part it off and turn it aroun in the 3 jaw chuck, drilland tap the back and if you want, with a very small boring bar ground from a 1/4" tool bit cut the relief. Then, I chuck the nut into my little 4" 3 jaw attached to my sHERLINE rotary table and mill off the hex in my vertical mill.

Yes, it is a lot of work and set up, however I had a number of cox thermal hoppers, space bugs and SB jr's that were missing the nuts so I tooled up and made a whack. I may have a few left, let me check.

I do know I have one or two originals but they are staying with me

If I have one of my repros I will send it your way... if not, sorry,

AJC

jetpack

My Feedback: (1)

Thanks everyone for your help, this is great.

Hats off to AJC, he was kind enough to find one and send it to me in the mail. Thanks very much for your effort, it really made my day!

When I bought the Thermal Hopper it wasn't in too bad of condition, just the normal wear and tear expected...everything was complete except for the propnut.

I have spending time with these Cox motors recently, taking them apart, cleaning and restoring them for a collection I have started.

I posted a couple pictures to share it. I removed the parting line of the crankcase casting, and gave each part a cleaning, deburring and a polish. I think I spend maybe a total of 25 hours on it. Its all oiled and assembled now and pops over real good. Its great to feel the compression and listen to the reeds click

I dont think the glow head is the right one, but I know the cylinder is correct...it came with the three-piece piston/rod and it still needs a new crankcase gasket.

This one I bought to put into a copy of a "vintage" type build-up of a Berkeley Models Douglas F4D-1 Skyray where it is called out on the drawings as one of the two motor options...the other is an OK Cub .049 which I already found.

gcb

For goodness sake don't use the aluminum impeller as provided with the kit. They have been known to fly apart with a much less powerful engine than the Thermal Hopper.

George

jetpack

My Feedback: (1)

Yep no worries well aware of that one from peoples post on the
Skyray, and from the looks of it as well. Its just a simple soft aluminum stamping that I would not trust behind a brick wall on anything other than a wind-up. This build-up is just for display.

If I build one to fly it will be all modern DF propulsion.

gcb

Excellent.

Perhaps the original should have been referred to as a DUCK! fan.

George

EASYTIGER

My Feedback: (119)

I had one fly apart, but that was because I used it on a Fantom 16 doing over 30k! Got a nice little scar on my arm from it. I would not be concerned about it flying apart with a space hopper, probably not going to happen. But St. Martin, a very experienced guy, said a conventional propeller(he used a wen mac) that fits into the ducting was much more efficient than the stamped impeller. He flew a Crusader back in the day.
Don't kid yourself...the Berkeley DF models were very carefully designed and tested and thought out, the impeller works.

I have the Crusader with Kamdax power, and recently built the T-33 for the same. Crusader flies well, the T-33 does not have enough wing and will be re-powered with electric. Next one on my list is the F-11F, will be EDF.