A.S.P. 'Series 3' .15 (Stunt)

By Mick Lewis (Model Flyer June 2001)

When asked if I would like to cover Just Engines Stunt Conversion of the ASP 15, (plus an A.R.T.F. U-Key 15 stunt kit), I readily agreed.

So, read on and let’s see what we can come up with.

The Crankcase Back Plate Assembly

Well cast from alloy with no blow holes or flash. Just a good clean finish with the maker’s name, ASP, cast into the transfer port side of the case and its size stamped on the vertical web just below the exhaust stub. Round to the rear of the case, which has been drilled and tapped for 4 x M2.5 chemically blackened backplate retention allen bolts. Upon looking inside you will notice there are no cast or machined in transfer passages (all will be revealed later!). The case has been machined internally for big end throw and bearing fitment. Two good quality high speed bearings are fitted to the case, the rear measuring 10mm ID with an O.D. of 19mm and the front bearing has an ID of 5mm with an O.D. of 13mm. Looking down the crankshaft passage there is a small groove machined from the carb intake going forward and running into a full circle groove just behind the front bearing to prevent fuel mixture exiting from the front bearing housing.

Back to the outside, the casing has a very large exhaust manifold and is drilled for two M2.5 silencer mounting bolts which pass through it into the silencer threads. Moving forward, the crankcase has three full length webs running axially between the main case and the front bearing housing, with a further web between the main casing and the large carb stub which has been drilled M2.5mm for the crescent cut carb pinch bolt. Looking at the case from above, the carb stub is bored to 9mm for the Venturi and counter bored for the rubber O ring seal, leading down to the double flat ‘D; sectioned timed crankshaft induction port. Finally, the top of the cylinder has been drilled and tapped for the four M2.5mm head mounting allen capped bolts.

The back plate is once again die cast from alloy with a flat machined internally at the top for gas flow to the rear boost port and liner piston clearance. It sits very deep in the crankcase, sealing is done with a plastic gasket.

Liner and Piston Assembly

Schneurle ported liner made from brass with a hard chrome bore, finished by fine grinding. The piston is cast from high content silicon alloy and finished by lapping.

Right, here’s where the missing transfer ports from the crankcase are. The three induction ports have been cut full length to the bottom of the liner using the piston skirt and crankcase wall as gas passages. The lack of metal at the bottom of the liner also helps to reduce friction, which increases power. Looking at the liner, there is a single angled transfer port at the rear opposite the exhaust, with two boost ports situated one each side of the single unbridged exhaust port. Both boost ports once again angled for good gas flow. The liner has a taper of 0.03mm from top to bottom.

Returning to the piston which has been lightened internally by machining and fitted with a solid gudgeon pin which is held in place by a step at the front and a wire circlip at the rear when assembled. The con rod is cast from alloy and bushed at the big end, with two oil holes, the small end being unbushed with a single oil hole. Just Engines have informed me that later batches will have a C.N.C. machined alloy con rod

Crankshaft Assembly

Turned from a single billet of chrome steel, with a counter balanced flywheel of 3.5mm thickness. The flywheel has been fitted with a hardened pressed in big end crank pin, with a bearing length of 4mm and a diameter of 3.90mm. The main shaft has a gas passage 6.50mm diameter with an intake port of 8.20mm sq. giving an induction timing of 137o . Moving forward, the main shaft diameter is 10mm, stepping down to 5mm for the front bearing and prop driver assembly.

The prop driver is turned from alloy barstock with a dirt shield at the rear end and has been helically knurled at the front face for good prop grip. It is located on the crankshaft by a split tapered alloy collet. The prop washer is from alloy barstock with a good thickness of 2.50mm. Finally, the prop nut is steel with a M4 thread.

Cylinder Head

Once again from alloy barstock with deeply milled cooling fins on the top. The combustion chamber is a shallow 1.3mm in depth, modified hemi type, with a squish band of 2.10mm diameter. The head is fitted with a soft alloy head gasket and is held down with 4 x M2.5 allen capped bolts and has been drilled and tapped for a STD long reach glow plug.

Just Engines Stunt Venturi

The ASP 15 comes fitted with a ‘J’en Venturi, turned from alloy barstock with an O.D. of 14mm stepping down to 9mm for the mounting boss. It is fitted with a rubber ‘O’ ring for good sealing.

Looking from the top, it is tapered downwards to its choke bore of 4.90mm. The fuel needle and spray bar assembly are of O.S. type with a positive twin ratchet from pressed steel with an adjustable banjo type fuel nipple. Overall finish is by chrome plate with a chemically blackened ratchet.

Silencers - Your Choice

When ordering you have a choice – a short dustbin type cast from alloy with an adjustable tail pipe and fitted with a brass pressure nipple. Or the usual expansion type cast in two pieces, once again with an offset adjustable tail pipe (or a quiet baffled silencer). The front portion is heavily finned both on the manifold and on the front can top and bottom, once again fitted with a brass pressure nipple. Both tail cones have been drilled and tapped for the X head through bolt, which also has a locking security nut so that it doesn’t come apart in flight. Both come with their own X head mounting bolts and manifold gasket and both offer a good noise reduction for just a little loss of power.

How it performed

A.S.P. ‘series 3’ are a new range for Just Engines this year but have been available in the U.K. for some time as R/C engines.

Fitted with the ‘J’en Venturi which is manufactured to the chart used by “PAMPA” (The U.S. stunt organisation) which ensures good 4-2-4 engine operation.

Just Engine’s offer this conversion for all A.S.P. engines from .12 to .53 and they also make them for most other manufacturers engines. Trust me, they work extremely well and are easy to set up.

O.K., back to the A.S.P. 15. This came with the advise of “run it in carefully as the liner piston fit is tight”. Tight! It squeeked louder than my wallet according to Paul Landels, so read the instructions carefully complied by Paul and your engine will last a long time. Running in took about an hour. using Flair Yellow Spot 5% Nitro, 20% Castor fuel. This was done by the 5 minute run, allow to cool method.

The first four tanks were run very rich, then gradually leaning out until maximum R.P.M. was achieved without showing signs of distress, but I had to deviate slightly from the instructions, substituting the recommended 7x6 prop with an 8x4 because of the tightness of the engine.

Overall testing takes about 6 hours, using both silencers, various props and, of course, the noise meter which will help to save our flying fields. The noise test is taken from the front to the rear and both sides at 7 meters, then an average is given.

When all tests are complete, it’s back to the workshop where the engine is stripped and examined for wear and measured. What we found surprised me. No wear to any part, in fact, it doesn’t look like its been run. The only evidence is a matt ring just below the piston crown showing the nip point at top dead centre. Not even discoloration to the piston crown or combustion chamber.

Throughout the test the engine was hand started. This usually took no more than half a dozen flicks from hot or cold, even in the freezing conditions at the flying field.

When used in the U-Key 15 Stunter, it was easy to tune and settled into its 4-2-4 settings, or when asked to scream, it did this readily. All in all, this little engine, not much bigger than an 0.9cc and a lot smaller and lighter than most 15’s, performed admirably and was very easy to use even for a beginner offering excellent value for money and if run in carefully, following the excellent instructions, a long trouble free life.

Engine test data

Prop Tests

and it works.....

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