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A.S.P. 'Series 3' .91 FS (Ring)

Just Engines Online | Engine Reviews & Comments | ASP Reviews |  A.S.P. 'Series 3' .91 FS (Ring)

By Malc Pinnock

By Malc Pinnock

RCM&E June 2001

The above title is quite a mouthful but A.S.P. “Series 3” rhymes doesn’t it?

OK, due for its UK launch in time for Sandown, the latest “Series 3” engines from Just Engines come in a full range of 2 strokes (.12-1.80) and 4 strokes (.30-1.20 + 1.60 twin and 64cc radial!).

I have already tested a small 15 size ABC stunt engine and was quite impressed with its power output and build quality. So, when the 91 FS arrived, I couldn’t wait to dive inside for a closer look after first running in and bench testing was completed. So read on and let’s see what hides inside.

Crankcase and Back plate

Crankcase and Back plate

A very intricate piece of pressure die cast alloy with its make looking from the front cast on a plate just below the cooling fins on the right hand side and finished by polishing. The rest of the castings have a matt grey finish which is achieved by fine bead blasting after all machining has been carried out. The country of origin is cast just below one of its very substantial engine mounting lugs. The size 91 is cast on the other side just below the fins. Overall the castings are of excellent quality with no flash or pockmarks.

The front end of the crankcase is very robust with three axially running tapered webs, one centrally each side of the case between the engine mounting lugs and front bearing housing with a further full web centrally at the bottom between the main case and front bearing housing. With further strength coming from the cam “box” and cam follower housing which are also webbed between the main case and front bearing. The left-hand side of the case is where the cam is removed. The cam is removed by undoing 2 x M2.5 allen capped bolts which retain the pressure die cast end cover which was a very smooth pressed in bearing and is sealed with a plastic gasket. Looking into the cam box, there is a further bearing pressed into the blind side of the cam box both bearings measure 13mm OD with an ID of 4.95mm, ensuring smooth running of the cam shaft. Rounding up this side, there is a crankcase breather nipple which is chromed and sealed with a fibre washer which allows the engine to breathe through the crankcase crankshaft passage.

Right round to the rear, the backplate is also pressure die cast from alloy. It is held in place with 4 x M3 allen bolts. On the top left hand corner it has a cast on lug which is drilled and tapped for the 2xM3 carb mounting allen bolts. The backplate sits deeply inside the crankcase and is sealed with a plastic gasket. Before looking inside there is a further drilling centrally just above the backplate which is for removal of the gudgeon pin after the tight fitting cylinder liner is removed, the crankcase had to be heated to remove the liner which makes for very good heat dissipation.

On removal of the backplate, we notice the maincase is recessed by machining to allow for the big end con rod swing looking down the crankshaft passage which has a variety of steps for the very complex four stroke crankshaft to which we will return later.

The crankshaft is supported by two good quality bearings which are pressed into the crankcase. The mainbearing has an OD of 30mm with an ID of 15mm whilst the front bearing measures 22mm OD with an ID of 10mm. Back to the outside and the top of the cylinder, which has five unevenly spaced tappings for the M3 allen capped head bolts. All machining for all the castings are crisp with no signs of pickup or burrs, showing the good quality alloy used in the engine’s construction.

Crankshaft assembly

Crankshaft assembly

The crankshaft is turned from a single billet of chrome steel with a very heavy counter balanced flywheel of some 9mm thickness which supports its substantial big end crankpin which has a diameter of 6.5mm.

The main shaft is quite complex due to having its valve gear drive taken from it. There are no less than six steps running forward from the flywheel, thank heavens for CNC machinery. OK, first step is for the main bearing of 15mm, then down to 11.5mm for a 3mm spacing to align with the crankcase breather vent, then up to 14mm. This is the section which is cut with the camshaft skew gear drive, then down again to 11.5 for the central part of the main shaft, forward and down to 10mm for the front bearing, then last of all, down to 7.8mm for the prop driver. This part of the shaft has been threaded 5/16 UNF for the prop nuts. Onto the prop driver assembly which is located on the shaft with a split alloy tapered collet.

The prop driver is from alloy bar stock which is wasted around the centre and has a dirt shroud at the rear and is helicaly knurled on its prop driving face. The prop washer is of heavy quality steel with a central thickness of 3.5mm, tapering outwards to 2mm at the edge.

Two prop nuts are fitted with the rear one recessing into the prop washer and the front nut being a lock nut which recesses into the rear nut and has been cross cut so it pinches to the shaft when tightened.thereby removing the chance of the prop flying off when the engine kicks which was an old four stroke hazard. Both nuts are tapped 5/16 UNF. To round up the crankshaft it has been chemically hardened and finish is by fine grinding.

Liner piston conrod assembly

Liner piston conrod assembly

Very simple liner being a four stroke, machined from a steel billet with a wall thickness of 1.55mm and a taper of .04mm top to bottom. At the top there is a flange of 1.4mm thickness which locates into a recess in the head and has a mating surface of 2.8mm. Down to the bottom there is a 30o taper for easy insertion of the ringed piston. The liner is finished by case hardening with the bore ground and lapped to a very fine finish.

Onto the piston which is die cast from silicon alloy and finished by machining, the piston is fitted with an iron ring of 1mm thickness 1mm from its crown. It has an overall height of 21mm and is heavily relieved front and rear to clear the flywheel and backplate. The long sides help to eliminate piston slop. Looking from underneath, there are two large gudgeon pin webs with the piston skirt being lightened by machining the lower walls to a thickness of 1.25mm. The fully floating gudgeon pin is inserted from the rear of the piston through the hole in the crankcase as mentioned earlier, after the conrod has been located on the crankshaft.

The front of the piston has a step in its outer wall to retain the gudgeon pin. There is nothing to retain it at the rear but it has been fitted with a P.T.F.E. pad. The gudgeon pin is from chromed steel with a diameter of 5.95mm and a length of 23.65mm and is bored right through for lightening. CNC machined from quality alloy, the conrod is bronze bushed at both ends with a single oil hole at the small end and the two oil holes for the big end and is quite substantial for this size of engine. That just about sums up the liner piston assembly.

Cylinder head

Cylinder head

Once again, a complex piece of pressure die casting from alloy with its various fins, ports, drilling and tappings. Where do we start? OK, the combustion chamber which is fitted with a recessed 2.8mm wide light alloy head gasket. The chamber is of the wedge type with squish areas to the front and rear and has its two poppit valves mounted in the flat area across the middle of the head. The head has been drilled and tapped for a long reach plug which inserts from the rear of the head and protrudes into the wedge between the two valves for optimum burning of the fuel gasses.

The inlet port follows normal four stroke practice, being from the rear right hand side of the engine looking from the front, with a large ovular inset port with a die cast and machine finished inverted “L” shaped carb manifold which is bolted to the head with 2 x M2.5 allen capped bolts and sealed by fibre gasket.

The exhaust exits on the left and is tapped for the exhaust manifold.

From the top, the rocker box has an internally cast tower for the rocker shaft which is retained in the head by an M3 allen bolt and locking washer. There are two holes drilled to the front of the box for the pushrods, with a tapping to the front and rear for the die cast rocker box cover which is from alloy and retained with 2 x M2.5 allen bolts.

The Valve train

The Valve train

Right, let’s start at the bottom and work our way up. The camshaft, miniature engineering at its best, CNC machined from steel and finished by chemical hardening.

The inlet cam lobe being on the right or, when fitted, in the blind end of the cam box. Next to it in the centre is the cam drive skew gear which takes its drive from the crankshaft as mentioned earlier. Then we have the exhaust cam lobe on the left. The cam skew gear has a timing dot on the left hand side of the gear wheel and is supported with a quality bearing at both ends of the shaft.

Easy to time, this is the first job on re-assembly, with the big end at top dead centre , the dot aligns with the pushrods. It just takes a little tooing and frowing and a slight rotation of either the crankshaft or the cam to get the correct alignment.

Next in this case its onwards and upwards to the cam followers, which are from chromed steel and slightly radiused on their bottom edges with a diameter of 5.95mm and a length of 13.05mm. They are counter bored at the top for location of the push rods which are from hardened steel of 2mm diameter and are 56.5mm long and run in separate alloy tubes which are sealed at both ends with rubber ‘O’ rings.

Over the top we have the 2 CNC machined hardened steel rocker arms which are mounted on a hardened steel rocker shaft which is retained in the head with an M3 allen bolt and locking washer. The rocker arms are retained on the shaft with a circlip at each end.

Each rocker arm is fitted with a steel tappet adjuster lock washer and M3 locking nut for adjusting the valve clearance which is 0.1mm. Both valves are from hardened chrome steel with a length of 27mm with a maximum O.D. of 3mm with various tapers for its retaining collets and where the valve passes through the ports. Two different sized valves are used. As common practice the inlet being the larger measuring 11.15mm and the exhaust being slightly smaller at 10.62mm. All that remained are the steel wire type valve springs which are fitted with steel caps and the two per valve retaining collets which are great fun on re-assembly causing many expletives, so unless the valves need attention, leave them alone.

Carb assembly

Carb assembly

Yet another piece of fine die cast alloy. The carb body has two holes for the M3 allen bolts which retain it and the very natty sprung choke assembly to the engine back plate. The carb is fitted with a finely ground steel throttle barrel which has a choke size of 7mm. It is of the twin needle type with the slow running jet turned from brass and running through the throttle barrel.

The throttle arm is steel and adjustable with an allen grub screw. The main needle valve assembly is steel with a blackened finish and is fitted with a positive pressed on spring steel ratchet device. The main needle has been bored centrally for an extension if required and has been fitted with an allen grub screw for holding the extension. The carb body is fitted with a chromed fuel feed nipple and is sealed with a fibre washer. Further sealing to the carb needle and jets are by rubber ‘O’ ring, including the ‘O’ ring which is recessed in the carb body for the machine finished die cast induction manifold.

The handy choke device is from blackened pressed steel, with a bonded-on rubber pad for sealing when choking. It is held in the open position with a wire spring and has a brass hexagonal bar fitted with an allen grub screw for the wire choke operating extension.

Silencer and Manifold

The exhaust manifold is from steel, bent to an angle of 30o and threaded at both ends and fitted with two locking nuts, one for the head and one for the silencer and is finished by chrome plating.

The screw-on alloy silencer is of the absorption type and is turned from bar stock. It is fitted with a chrome plated pressure nipple (very simple) and it works very well. That about rounds up the engine and its fittings, so on to the test.


The first thing with a “big thumper” four stroke is to get the fuel right, so after some discussion with Paul Landels from Just Engines, who recommends 15% oil (15-17%) which contains not less than 2% castor and Phil from Flair, it was decided to use Flair Yellow/Gold which contains 5% nitro, 5% castor, 10% synthetic and 80% methanol. This proved a good choice as the engine was very tight and the castor offers good protection for the moving parts, whilst most of the synthetic is burnt off.

So, mounted on the test bench, tank filled and all lines connected, engine fitted with a 14x6 master airscrew so as not to overstress the engine, but the heavy blade giving a bit of extra flywheel. I primed the engine using the choke, found the compression stroke, connected the glow, four flicks and its running. Opened the throttle and tuned in to a rich running setting, throttled back to just over ¼ throttle, then settled down with a newspaper (early morning start) using the five minute run. Allow to cool method for running in, gradually leaning out until the engine shows no signs of distress at full throttle.

This took just over an hour, then the test began by using various props and the manufacturers dread – the noise meter. The engine performed very well in the test taking no more than a couple of flicks to start hot or cold. Soon settling down to a reliable tick over with throttle response being instant. So, it’s back to the workshop for the strip and inspection and, in my case, the dreaded writing.

On strip down, the engine showed no signs of wear with very minimal signs of being run, just some carbon and discolouration around the valves and exhaust port – not bad for over 6 hours running.

Externally the exhaust had discoloured with burnt oil on the pipe and silencer. Warning: the manifold gets very hot on a four stroke, so beware.

Once again, for what is described as a budget priced engine, it has all the characteristics of its more expensive counterparts and with a little care when running in, it will reward the owner with a long, trouble free life. This engine, in my opinion, offers excellent value for money.

Prop performance

13 z 6 Radio Active Ram 11,200 RPM
13 x 6 APC 10,900 RPM
13 X 6 Master Airscrew 10,400 RPM
14 X 6 Master Airscrew 9,400 RPM
14 x 6 APC 9,800 RPM
15 x 8 Master Airscrew 7,600 RPM
16 x 6 APC 8,400 RPM
16 x 6 Master Airscrew 7,200 RPM
16 x 8 APC 7,000 RPM
16 x 10 APC 6,200 RPM

Data File

Engine type: Single cylinder 4 stroke O.H.V.

2 valves steel liner.

Ringed alloy piston. Tappets .1m.

Bore: 27.7mm

Stroke: 24.8mm

Capacity: 0.91

Power Output: 1.55 B.H.P.

Weight: 640 grms

Supplied with: 2 Year Warranty and specification sheet.

English instructions – parts explosion.

Plug used in test: O.S. Four Stroke

Fuel used in test: Flair Yellow/Gold

Noise at 7 metres: 60DB idle 2.200 RPM

84DB Max on 12x6 prop at 11.200RPM

Average 81DB on STD Silencer

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Just Engines Online | Engine Reviews & Comments | ASP Reviews |  A.S.P. 'Series 3' .91 FS (Ring)