Firefly 46 glow part 3


Last month we left the workshop having completed the cylinder head, barrel, liner, crankcase, bearing, and backplate of this feisty home-brew .46. We’ve established that nothing so far has been that taxing in terms of manufacture, and I’m pleased to tell you that the same ease applies to what’s left. So, let’s crack on and get it finished. 



Enjoy more RCM&E Magazine reading every month.
Click here to subscribe & save.

Drawing number WF46-0008. This is a satisfying piece to produce, the most critical dimension being the diameter that mates with the crankcase bearing. Ensure that the material used is of the correct specification and take your time; the shaft must spin freely in the bore but without any slop. There’s no hard and fast rule here, but if you smear a little light oil on the shaft (I use 3 in 1), insert it into the bearing and turn it upside-down, the shaft should slowly slide out under its own weight. I recommend leaving this final sizing until all the other operations have been completed.

The crankpin can be turned as an integral part, but this operation will require a different setup. Check the position of the slot, decide where you’re going to start, and then check it again – the relationship between slot and crankpin is critical and can easily be misjudged. Having marked out both ends of the slot and with the crankshaft positioned using your chosen method of holding it, begin by machining the flat that the slot sits in (view on arrow B), which gives a very useful guide and allows you to gauge how far you need to travel. There’s not a great deal of material to remove to break into the 8.5mm bore, and having established a breakthrough the initial slot can be machined using a 6mm slot drill, finishing with an 8mm ditto.

The thread on the end of the crankshaft is quoted on the drawing as 1/4 UNF x 28 tpi; this suits commercially available proprietary prop nuts, however if you choose to go your own way (as I did), the choice is yours. 




Drawing number DRG WF46-0020. The simplest way of acquiring a prop driver is to purchase a stock item or use an equivalent one from an engine that you already own but aren’t currently using. If you’re going along the route of a flat milled section on the crankshaft then this is certainly your best option, since the shape of the hole in the centre is usually cast or broached and isn’t easy to reproduce. Take your time when machining (or filing) the flat on the crankshaft; the trick is to hold the ‘shaft in such a way that the driver can be tried for fit without removing it from the vice. This allows you to remove a little material at a time until the required fit is achieved. Speaking from personal experience, the difference between a tight fit and a very slack one is surprisingly small.



Your decision to produce a prop nut will be determined by the type of model that you’re planning to put your Firefly in (there’s not much point in making the engine if you’re not going to fly it!), and whether you intend to use a spinner of some kind. A standard nut and washer (1/4 UNF or similar) is perfectly satisfactory, but if you choose to make your own there are a number of shapes that can be used.

A standard prop nut washer will be required if using an ordinary hexagon nut. If you’re going to purchase a ready-made prop nut, make sure that its thread is the same as that which you’ve produced on your crankshaft before parting with any hard-earned. I produced the nut for my Firefly from aluminium and this has proven to be perfectly adequate, with the thread showing no sign of stripping.


Drawing number WF46-0009.

Note the offset on the big end before embarking on this piece. Start by machining a blank that’s 41.5mm x 9mm x 8.2mm, this size being much easier to hold whilst drilling and reaming the holes. Start by machining the relief in the centre of the con rod with a standard 10mm end mill, and finish off with a 10mm ball-nose cutter. (If you don’t possess such a cutter then careful application of a round needle file will do the job). At this stage both ends will still be 8.2mm thick, and the 6.04 still needs to be finished. By holding the central web of the con rod between two pieces of 3mm material, both sides of the small end can be milled in one setting. There’s no need to press a phosphor bush into the big end. 


Drawing number WF46-0010. Take a piece of good quality cast iron (Meehanite, if you can get it), and cut a length of around 40mm. Turn a 22mm diameter down an 18mm length at one end, reverse it in your chuck and turn the diameter to fit your liner. A good, sliding fit is what you’re after, so take your time here. When happy, machine the 12mm and 19mm bores. You can now use the 22mm diameter as a service piece to hold the piston whilst you complete the machining.

To ensure that the elongation in the piston is square to the gudgeon pin hole, finish the hole first and then set the piston upright (holding on the service piece). Use a length of 5mm silver steel to set the hole at right angles to the machine table axis. Surprisingly, using a standard length of steel, you’ll be near enough by just eyeing it up.

Drawing number WF46-0011. You may find that the reamer for the gudgeon pin hole in the piston has cut slightly large; if this is the case you can always machine a pin from 6mm silver steel, leaving it just oversize enough to polish down to a size that’s snug enough for both piston and con rod.

Once you’ve made the plug for the end of the pin and pressed it in, adjust the overall length by machining a little off the solid end until the plug is just below the piston’s diameter. 


Drawing number WF46-0014 / 15.

This is a bit more of a challenge, indeed getting the hole in the barrel in line with the bore in the carb’ body will be a source of either great satisfaction or frustration, depending on your degree of success. This carb’ does work well, but you may feel that using a proprietary unit is a better option. I picked one up at the Nats swapmeet, and after ensuring that the carburettor as drawn does work, I replaced it with my swapmeet purchase. If you decide to use a standard carburettor, make sure to check that the mounting stub corresponds to the hole you’ve bored in the crankcase.


Most long-standing modellers will have an old .40-size silencer lying about, and having dusted this off, drill the holes in your barrel exhaust flange to suit. I used an O.S. silencer that easily fitted the flange.


Drawing number WF46-0018. Just a couple of things to look out for here. First, the thread for the needle valve; this is the fine version of a 4mm thread (M4 x 0.5) and, as in my case, may require a special purchase. The second point is more obvious in that the 3.2mm holes for the feed and outlet pipes need to be such that the copper pipe can be lightly pressed into them. These fits need to be fuel-tight and will leak if they’re even slightly loose. Accordingly, I soldered mine, but my brother took the other option and bought a metering unit complete with needle. If you do make your own unit, do ensure that the 5.75mm bore for the ‘O’ ring has a good finish and isn’t oversize.


Drawing number WF46-0019. This is a self-explanatory, personal preference option that requires minor modification to the needle valve.


That wraps up my observations concerning the manufacture of the Firefly’s individual components; please understand that they’re by no means definitive! Anyway, beyond what I’ve written above, here are a few of the ground rules I work by:


  • Read the drawings until you’re sick of them; you’ll be amazed by the amount you miss when you scan them first time around.
  • Don’t set a time limit. You’re supposed to be enjoying it, and once you start rushing things mistakes are almost inevitable.
  • Try and break each component into a number of operations. 
  • If you’re unsure of any aspect of materials, machining methods or design, don’t be afraid to ask. And remember, there’s a veritable wealth of information to be found on the internet.
  • Remove all sharp edges as early as possible, and don’t remove any workpiece from chuck or vice until you’re absolutely sure that there’s nothing left to do on it.

There are a small number mating surfaces that are crucial to the performance of the Firefly, but fit is more important than dimensional accuracy. It’s usually easier to turn a diameter to fit a bore than the other way round, so plan the order of your components accordingly. To paraphrase an old capstan lathe setter, “If they give you a limit, use it!” Don’t spend time and effort machining to an exact size on something that doesn’t need it. And if you’re so inclined, there’s ample material on this engine for some embellishment that will make your example unique.


Okay, so you can buy a perfectly serviceable, factory-built 40-size engine for not a great deal of money, and it will doubtless serve you extremely well. However, nothing compares to the satisfaction you get from making your own engine and seeing it power your latest airframe across the sky. Heck, you even could go the whole hog and build the airframe, too! Go on… give the Firefly a try and tell us how you get on.


Detail drawings for the Whittaker Firefly 46 appeared in the February and March 2012 issues of RCM&E. You can purcahse the plan at

Article Tags:

About the Author