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Cox Per Wee or Tee Dee

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Benjamin Day28/10/2018 14:26:14
50 forum posts
This is probably the wrong place for this inquiry, if it is please do move it along to the correct place!
Anyway, has anyone any experience in making a replica Cox Pee Wee 0.020cc i.c engine? Or similar tiny design. Or know if any plans/drawings are available? It looks to have about 6 parts, and it looks tantalizingly do-able (to the eager novice eye) lol
If I cant find plans or any usefull reference, I'm just going to approximate what i can. Can anyone advise on what fit the piston should have? Or any other critical fitment issues with this kind of mini.
Thanks for any and all input. Random anecdotes welcome too....
Jeff Dayman28/10/2018 14:41:06
1787 forum posts
45 photos

I spent a lot of time playing with and working on engines from Cox when I was a kid. The bore to piston fit is extremely important and the factory lapped the bores and the pistons to very close fits, in the order of .0002" to .0004" clearance. Any wear from use in dusty conditions would ruin Cox engines fast. I believe with some lapping practice a careful home shop machinist could make cyls and pistons without much trouble.

Cox used a unique sphere ended piston rod which was ring - staked (permanently assembled) into a cup in the piston. Presumably they did this operation with a carefully stroke-controlled arbour press because I never saw one new that had more than .001" to .002" end play. This op would need some special tooling to be made to get similar fit if same construction was to be used. Very do-able though, with a simple fine screw thread stroke control on the press and some dummy piston rod and piston cup practice pieces to dial in the stake parameters and fit.

Benjamin Day28/10/2018 14:56:58
50 forum posts
Thanks for the reply! Closer than I was expecting, but im going to focus on the "very do-able" bit lol
I didn't realize they wore out quickly, I think some material selection could keep any ware on the piston which looks to be the easier part to swap out? Making the piston a consumable part. It's looking like a better and better project by the second!
Thanks again
John Haine28/10/2018 16:45:09
2995 forum posts
160 photos

You might consider starting with something a bit bigger where the fits are less challenging?

Neil Wyatt28/10/2018 17:54:13
17686 forum posts
697 photos
77 articles

I've got one of each. Despite the apparent simplicity they are examples of excellent engineering. I imagine they could be incredibly frustrating.

Bear in mind they also have special glo-heads as ordinary glow plugs are far to big.


herbert punter28/10/2018 18:37:52
89 forum posts

The ball-ended conrod design was used in several makes in that era. Cox sold (and the people who do them now still sell) a re-setting tool consisting of a metal base to put the piston in and a slotted tube to fit around the conrod. The end of the tube is then hit with a hammer until the required fit is obtained.


Edited By herbert punter on 28/10/2018 18:38:37

herbert punter28/10/2018 18:46:22
89 forum posts

There is a plan for a 0.3cc deisel MM137 available from Sarik Hobbies which might be a better choice.


Benjamin Day28/10/2018 19:12:05
50 forum posts
Starting with something a wee bit bigger might well be a good idea John, how much bigger would I need to go to get more forgiving tollerences d'ya think?
Neil, i was hoping suitably tint glow heads were available from the bay (sure I've seen them on there) but isn't it just a bit of platinum of the right resistance in a threaded holder? Kanthal could maybe substitute? (I think I'm forgetting something about a reaction between the nitro fuel and platinum....) I know you are super busy but if you could take some measurements of one, or both the Cox engines you have, i would be very grateful, but its a big ask for a busy guy so no worries if you don't have time. And agree its probably going to be devilishly frustrating and to be honest I would probably be disappointed in a way if it fired up first time.
Bert, for some reason i thought it was just a small ring? I must be getting parts/names confused, either way, thanks for the you have a link for the .3 plans? That might well be my best bet, thanks again.
Also, are these as much fun as they look? I want to send it screaming through the foamies at the local park lol
Old School28/10/2018 19:59:41
323 forum posts
26 photos

If I was starting again a 5 to 10cc glow ignition engine big enough not to be so critical on fits. Find one where there is a series on how to build it.

John Olsen28/10/2018 20:37:13
1028 forum posts
86 photos
1 articles

You should take a look at Ron Chernich's Model Engine News site. Sadly Ron himself is no longer with use, but the site is still up as I write, and there are plans on there for a number of small engines, plus any amount of advice on how to make them. **LINK**

There have been plans in some of the magazines, including Model Engineer, and castings are available for a few engines. Not the really tiny ones, but then I agree with those above who suggest making a slightly larger one first

It is all quite doable in a model engineers workshop. Most of the commercial engines were made at a level not too far above that anyway. Cox would actually be an exception, they had a really high class setup, and this meant that they were able to make all the parts interchangeable. You could dismantle a bunch of engines, randomly mix the parts, then put them back together and they would all run. With hand lapped pistons it is not so easy to achieve that sort of repeatability but that does not prevent you from making a working engine.


herbert punter28/10/2018 20:38:10
89 forum posts


Neil Wyatt28/10/2018 20:45:24
17686 forum posts
697 photos
77 articles

The Sparey Diesels are good, and you can get cast crankcases.


Jens Eirik Skogstad28/10/2018 20:58:35
394 forum posts
22 photos

The Cox engines had cylinder and piston who was cyanid case hardened while the parts of piston who had layer of copper was not hardened. Not possible in a model engineer shop since cyanid case hardening is poison.. frown

Edited By Jens Eirik Skogstad on 28/10/2018 21:05:33

Jeff Dayman29/10/2018 12:02:14
1787 forum posts
45 photos

Agreed about cyanide hardening not being a good idea in the home shop Jens Eirick.face 21

However if you want to do case hardening in the home workshop on selected surfaces you can use case hardening powders carefully applied. I have used chalk wash and typing correction fluid "Liquid Paper" (I think called "Tippex" in UK) as a mask for areas to not be hardened using Quick Hard powders. The powders are used by heating the part to red heat, dipping the part in the powders or (sprinkling certain areas with powders from a spoon) and reheating to red heat/quench in water. The process is repeated until required hardness and depth of case is achieved. Note that many free machining steels will not case harden with this method due to lead content. Most low alloy and plain low carbon steels will harden.

Should you ever want to coat steel with copper plate, you can dissolve copper sulfate in sulfuric acid, and brush it on steel. It will leave a copper plating on the surface. Many older toolmakers used this rather than marking out fluid or dyes for laying out cut lines on steel in shops I worked in years ago in industry.

Regarding hand hammering Cox piston rods into the piston cup with the repair tool set mentioned- I think this would require extreme skill and some luck to get a good fit. It's likely to go from perfect to scrap in less than one hammer hit....but you might be lucky.

Andrew Tinsley29/10/2018 12:39:10
1044 forum posts

Unless you have plenty of experience with very fine fits, I would strongly recommend that you start with a much larger capacity engine. Making stuff down to the fine limits needed for very small engines in the .1 to .2 cc range is NOT simple. Doable, yes, but certainly NOT for a first IC project.

Over tightening the ball joint on the smallest TD (I gave too heavy a blow on the resetting tool) turned a clapped out engine into a good runner. That demonstrates the fine limits required. Now it was only a tap with a 2 oz hammer that caused this change. I most certainly didn't whack it, but more force than I usually use. This expanded the piston very slightly and caused the transformation!

Do yourself a favour and build something from 1 cc upwards as a first project. If you have never built an IC engine before, trying to replicate the smallest Coxs is a mugs game. You are most unlikely to get a good runner. You would have to use a Cox glowhead too. Small diesel engines need fits every bit as good as Cox engines. A 5 cc Sparey diesel is a good starting point for getting into IC engines.


Jens Eirik Skogstad29/10/2018 21:55:41
394 forum posts
22 photos
Posted by Andrew Tinsley on 29/10/2018 12:39:10:

Do yourself a favour and build something from 1 cc upwards as a first project. If you have never built an IC engine before, trying to replicate the smallest Coxs is a mugs game. You are most unlikely to get a good runner. You would have to use a Cox glowhead too. Small diesel engines need fits every bit as good as Cox engines. A 5 cc Sparey diesel is a good starting point for getting into IC engines.


As usual the model engine with long stoke and large of piston length is not difficult to lap and fit piston/cylinder due there is less leakage than a short stoke engine with short of piston length who can be difficult for a new beginner who are creating the first model engine.

Danny M2Z29/10/2018 22:51:04
830 forum posts
282 photos

The Motor Boys Plans Books have been released into the public domain here **LINK**

There you should be able to find something that is capable of being made in a home workshop.

* Danny M *

Marcus Bowman29/10/2018 22:56:30
162 forum posts

I agree with the comments re: choosing a first engine carefully.

Ron Chernich's site used to sell a book of plans, which I have, replicating some of the engines from the past. Sadly, the book is no longer available. Plans for the MATE were printed in Model Engineer, some long time ago, and that was designed for beginners. It depended on an aluminium extrusion, no longer available, but I am sure there was an article about making it from a block of aluminium.

Neil mentioned the Sparey engines, and they are not a bad place to begin.

There are also plans and build instructions for a series of splendid looking engines in the Super Tigre series, available free, here: (its the third DVD you want). OK; not quite truly free at the moment, but archived at low cost. This is not a beginner's project, though - not at all. Gorgeous though.

Roy Vaughn30/10/2018 00:42:05
30 forum posts
3 photos

Cox engines were made on very advanced machinery (for the time) and would be very difficult to reproduce at home. Not just the fits and finishes but the plastic intake housing. A much more certain bet would be one of Chris Boll's designs. My first motor was his ML Midge of 0.8cc. The BollAero at 1.8cc would be even better. The most critical factor in getting these small diesels to run is the piston-liner fit. Both components must be separately lapped to the correct fit. Expect to need several goes, i.e. pistons, the first time you do it! The site is the best source for techniques.

Roy Vaughn30/10/2018 06:10:29
30 forum posts
3 photos

Correction: The ML Midge is a Mark Lubbock design.

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