DC Wasp Rebuild

[William ChithamParticipant @williamchitham75949]

I have a DC Wasp .049 glow plug engine which I would like to recommission but it is missing a couple of key components, to whit the rear crankcase cover and the needle valve. As far as I can remember the rear cap screwed in to leave fairly minimal clearance to the crankpin, I think I can reverse engineer that. The thing that has me stumped is the needle valve. I have the tube it screws into (the spraybar?) but the threaded end is broken so I will have to remake that. I have probed the through hole with various drill bits and established that there are two steps. From the threaded end the first few mm is around 1.5mm diameter which I imagine is just clearance. Then it narrows to approx 1.4mm which extends to near the fuel pipe end where it steps down again to approx 1.1mm. I suppose this second step is the “valve seat” where the needle regulates the fuel flow. What I’d like to know is what sort of clearance there should be around the parallel shaft of the needle to allow the passage of the metered fuel to the spray hole and what sort of angle should the point of the needle be? Also a material recommendation for the needle would be much appreciated, as far as I remember the original was a just a steel needle with the end bent around 45° to turn it and an aluminium shroud with the internal thread for adjustment.

[William ChithamParticipant @williamchitham75949]

Might need a new prop as well…

[bernard towersParticipant @bernardtowers37738]

try adrians model engines or motor boys for some inspiration on you valve and needle. it is fairly basic and the finer the needle taper the finer the adjustment.

[Ramon WilsonParticipant @ramonwilson3]

I’ve made quite a few spraybars/needle valves over time. It’s something that can be made fairly easily. Firstly the taper is not that important as long as it is concentric and reasonably long. Using piano or music wire of 18-16swg I grind them by hand on the side of an off hand grinder wheel to give an approximate taper then finish them off using a needle file by spinning in the lathe with the very point supported on a piece of wood held in the tool post – crude, simple, but effective. Usually if the taper is long enough then there is sufficient clearance for the fuel flow. I would think that you would be fine with 1/16″ music or 16swg piano wire. Once the needle and housing are made push the needle in the spray bar until it seats then screw the thimble on fully home then back off one turn. Solder the needle to the thimble with it all assembled – this will ensure that the needle closes when fully home – surprising how that will affect starting/running if it doesn’t! Centre the outer end of the thimble to create a reservoir for the solder to give a good join.

The back plate obviously needs clearance but again is not important save that the deeper it goes the less crankcase volume. If you made it with say 5 thou clearance and fit the backplate with a (copy) paper gasket that should be fine.

Finally very small glow motors such as this require a fuel with a reasonable nitromethane content – I would say a minimum of ten percent but preferably 15.

Hope you get it running okay

 

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Allternativly, take the easy way out and purchase a standard NV from PAW Engines, 01625423891. Cost about £10 inc postage. You might have to make a small bush to fit it in the intake.

Keith.

[Andrew TinsleyParticipant @andrewtinsley63637]

Keith beat me to it! Support PAW in these days of electric flight!

Andrew.

[William ChithamParticipant @williamchitham75949]

“We do these things not beacause they are easy but because they are fun” to misquote JFK. Love the PAW site though thanks for the steer.

I have had a few goes at the backplate and have one now that screws in but I’m not convinced I have the right thread form. I’m at 5/8″ x 36tpi but I’m using a 60° tool, maybe should be 55°?

Thanks Ramon and Bernard, I have seen those sites but yet to find a drawing that clarifies how the needle valve actually works. I have made a drawing from the remains of the original and attached a section. What I don’t understand is whether the needle meters the fuel by it’s position in the reduced diameter “A” or by obscuring the jet as at “B”. Ramon’s suggestion “…push it in till it seats..” makes me think “A”.

 

 

[bernard towersParticipant @bernardtowers37738]

Definitely “A”

[Ramon WilsonParticipant @ramonwilson3]

William,

The needle always meters the fuel at it’s point of contact at the far end ‘A’ – the hole (or holes) is just to let it out! if the needle is very close to the same diameter as the bore then the taper has to be long enough to allow fuel to reach the hole or the needle slightly reduced in diameter to do the same.

Just in case you are not certain – If the spray bar has a single hole then it (the hole) should point downwards into the venturi – if two holes then it should be set so these are at 90 degrees to the venturi

[William ChithamParticipant @williamchitham75949]

Ok great. The original needle must have been smaller than the bore, otherwise the point would have protruded out of the end. Seems to me better to ensure there is always a parallel section at the jet, other wise the taper would influence the fuel flow twice. Presumably the annular space around the needle needs to be at least equal to the area of the jet.

I’m expecting to have to experiment with the orientation of the jet hole, I’d read somewhere that 90° might be best but not that that applied to a two hole set up.

Thanks for the help, just need to make a sensitive drilling tailstock attachment now!

[petetwissellParticipant @petetwissell]

I remember the DC wasp – it was the first engine I had when I started out in model flying.

As I remember, the engine was almost worn out by the time it was run in!

[William ChithamParticipant @williamchitham75949]

Progress on the Wasp refit. I finally managed to make a rear crankcase cover that fits. It took about five attempts. I ended up recutting the thread in the crankcase. I have also made the spraybar, broke the first one in the lathe but the second attempt looks alright. The original was threaded 1/8″ x 48 tpi which is BSF I think though the charts all seem to start at 3/16″. Needless to say I haven’t got a taps and a die that size so I setled for M3. Waiting for a delivery of piano wire now to make the needle. Jet is drilled 0.9mm as near as I can match to the original.

[bernard towersParticipant @bernardtowers37738]

Well done William you are getting there, I did a needle over the weekend by spinning the needle in the lathe (Peatol) very fast and using a Dremel with a fine sanding disc attached (not a stone) and I have to say the result was really quite good. Amazing what you can do without too much kit!!!

[Ramon WilsonParticipant @ramonwilson3]

Yes, nice result on both those parts so not too long before you can try it. If I recall correctly the original had a ‘Quickstart’ coiled wire starter spring that hooked behind the prop blade to give a rapid turn over rather than the usual hand flick start. Did your engine have one fitted as well as a working glo plug – I’m not certain but think it likely that it was probably fitted with a short reach plug originally so a standard plug may sit too deep for it. Also a reminder that these small engines start much easier and run much better with a higher nitro-methane content 10-15% at least. Unless you know of someone using such you may, possibly, have difficulty in sourcing small quantities of fuel of that content. Whatever best of luck in getting to run soon

[Neil LickfoldParticipant @neillickfold44316]

Some spray bars do the metering like the B example shown. This is normally for the single sided NVA design where there is no fuel inlet on the other end. Some competition F2A engine use the side hole and bore diameter for the fuel regulation, but most will use the A arrangement and have the step quite close to the cross hole.

The more you make, the easier they will become to make, and easier to figure out what is important.

It’s always fun making little parts and very satisfying when it finally runs again too.

If you can’t get a short reach plug, either add an extra plug washer under the plug, or make a washer of the thickness so that he plug when in the head, the bottom of the plug is very slightly, 0.25mm to 0.5mm max into the head. They run much better this way, than having the bottom of the plug recessed by 0.5mm.

20% nitromethane content fuel used to be a common fuel for the small 049 sized engines. They will work on 10% nitro fuel, and even No nitro fuel. But the no nitro fuel is very difficult to get a consistent run from the engine each time. 10% nitro makes the same power as no nitro, but allows for a much easier needle setting and a more consistent engine run. 20% was like the sweet spot of running small engines.  Run some No nitro fuel through after the days flying or use and the use some AFT automatic transmission fluid as an after run fluid to preserve the engine from the rust that can form from using Nitromethane containing fuels.

There used to be commercial available 15% Nitro fuel used in RC pylon Racing of the F3T class.  It is still a flown class so maybe available as an order from your local model shop. It has 18-20 % oil content. You may want to add some more oil to the fuel as it is not a chrome cylinder engine. I like to run about 25% to 30% oil in my small engines.  Every 100mls of commercial fuel, I add 10 mls of castor oil making 110mls at a time.

[William ChithamParticipant @williamchitham75949]

The engine did have the Quickstart device, I think I removed it when I got the hang of starting it without. I still have the hooked plate but the spring is lost. I have got a glow plug and probably a connector clip thing for it, I recall running it on a 2v lead acid battery. Leeds Model Shop sell a 15% nitro fuel by the litre:

https://www.modelshopleeds.co.uk/catalog/product_info.php?fcm_pricerange_high=750&fcm_pricerange_low=1&keywords=nitro&sort=1a&page=2&products_id=104575

I’ll find some Castor oil to add, I used to buy it at the chemist along with ether to mix up fuel for the diesels, can’t remember where the methanol came from.

The crankshaft bearing and the piston fit feel very good but there is a slightly worrying amount of play in the little end. This is a ball and socket arrangement which clicks noticably as it goes over tdc but I suppose when running it is almost always under compression so perhaps that doesn’t matter.

William.

[Ramon WilsonParticipant @ramonwilson3]

Sounds like you are well on the way now William. I’ve just looked at the Southern Model Craft site https://www.southernmodelcraft.co.uk/online-store.php and they do a litre of 15% – ALL castor based – fuel for £8.50. So many others are synthetic oil based which is not a good idea to use in an engine previously run on all castor though by the sound of it you are well aware of that fact.  Though now under new ownership I have used SMC fuels for years without any issues so can highly recommend them.

Hope it continues to go well for you👍

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