At last, a single cylinder four-stroke of over 120-size… at a decent price, too. On close inspection it’s the proverbial quart in a pint pot, just 4mm longer than a 120 four-stroke but with the same engine bearer width and bolt spacing. This one should hit the spot with anyone suffering from a wheezing 120 in a 120 – 140 size ARTF that really could do with a bit more get up and go. In this situation the next step in the quest for more power is to fit either a two-stroke or a larger, very expensive four-stroke. The introduction of the ASP180 makes the four-stroke option a far more attractive proposition, with a launch price of £199.99 including silencer it won’t break the bank and this test indicates that it’ll deliver power in spades.
First to come off is the carb’, achieved by removing the two M3 caphead bolts that secure it to the backplate. The carb’ is then gently eased from the intake manifold, which is in turn removed from the cylinder head after extracting two further M3 caphead bolts. A degree of care has to be exercised here in order not to damage the fibre gasket. The manifold is a very nicely pressure die cast part and is fitted with a chrome plated steel pressure nipple that’s sealed by a fibre washer. This is connected to the timed crankcase breather nipple via a piece of silicon tube (supplied), in order to recycle any excess oil from the crankcase back through the engine.
The pressure die cast alloy rocker box cover is the next part for removal, retained to the cylinder head by two M2.5 caphead bolts. Removing this gives access to the two machined, hardened steel rocker arms, adjustable tappets and locknuts. The central M4 caphead bolt and its locking washer can now be removed, releasing the rocker arm assembly. With this out of the way the two hardened steel pushrods are recovered, clearing the way for removing the cylinder head.Article continues below…
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The head is secured to the crankcase by six M4 caphead bolts, which have to be removed diagonally (working across and around the head) a couple of turns at a time so as not to distort the casting. Incidentally, the same rules apply in reverse for reassembly. Our next job is to remove the two machined bar stock alloy pushrod tubes, along with the four rubber ‘O’ rings that seal the tubes to the crankcase and cylinder head. This just leaves the alloy gasket that seals the head to the crankcase, and the hardened steel cam followers, both of which can be extracted at this stage.
The 180’s pressure die cast alloy cylinder head is of a very high quality; the unit has cast-in bronze valve seats and valve guides and has very smooth, gas flowed porting with a wedge-shaped combustion chamber that’s bored and tapped for a long-reach four-stroke plug. Two hardened steel valves are fitted, the inlet being slightly larger than the exhaust. Each is equipped with a single valve spring, cap and pair of steel retention collets. Do note that great care must be taken when removing the valves as the springs, caps and collets can fly everywhere!
CAMSHAFT & FOLLOWERS
Two M2.5 caphead bolts retain the die cast alloy cam box cover, which is sealed by a plastic gasket and fitted with a ball race bearing to support the hardened steel camshaft (a second cam bearing is fitted deep inside the crankcase cam box). Looking into the cam box to study the camshaft reveals a timing mark on the face of the cam gear cog. Cam timing for the ASP180 sees the piston at TDC (Top Dead Centre) with the timing mark in line and to the right-hand edge of the exhaust cam follower. You’ll note that this puts the exhaust cam lobe in the 2 o’clock position.
If the cam followers weren’t removed at the head removal stage described above, they can be recovered now by turning the engine upside down and tapping the case into the palm of your hand. If they still refuse to come out, rotate the engine a few times until the followers are at the top of their stroke, then grasp the cam with a pair or tweezers, rotate rearwards and gently pull; once the camshaft is removed the cam followers can then be gently pushed into the cam timing chest and removed through the cover aperture.
The pressure die cast backplate is the next part for removal, this sits deeply into the crankcase, sealed by a plastic gasket and retained by four M4 caphead bolts. The backplate has a flange cast at the top for mounting the carb’, tapped for the two M3 retaining bolts removed earlier.
LINER & PISTON
As is usual with ASP engines, the liner is a very good fit into the crankcase, which makes for excellent heat dissipation. In order to extract the liner I applied a small amount of heat evenly around the crankcase cylinder fins. This expanded the area enough to enable subsequent rotating of the crankshaft to raise the liner, whereupon it could be pulled out by hand. The liner has a retaining lip at the top and is made from case hardened steel, which is finished by fine grinding and lapping.
In order to extract the piston and conrod assembly, the gudgeon pin has to first be removed. At the centre rear of the crankcase, just above the backplate, there’s a hole through which the gudgeon pin can be viewed. With the gudgeon pin and hole aligned the rear of the case is given a firm tap into the palm of the hand, and the fully-floating hardened steel gudgeon pin can then be removed. The pin is fully bored for lightness and retained at the piston front by a semi-blind hole (a hole that’s of smaller diameter than the gudgeon pin) and is fitted with a PTFE wear pad at the rear to ensure no damage is done to the liner when the engine’s running.
The single ring, slipper-type piston is a permanent alloy die casting, fully finished by machining. Two large internal webs support the gudgeon pin. CNC machined from high tensile alloy, the heavy-duty conrod is bronze bushed at both ends with a single oil hole centrally at the top for the small end and two oil holes for the big end.
Let’s start at the front by removing the chemically blackened steel prop’ locknut, prop’ nut and prop’ washer. Next to come off is the prop’ driver, which is turned from alloy bar stock with a helically cut driving face and dirt shield at the rear. It’s also cut internally with a slot running axially for the steel woodruff key that retains it on the crankshaft.
With these parts out of the way the crankshaft can now be removed with a firm push.
Machined from a single billet of steel, it’s a heavy-duty unit that features a large, fully counterbalanced flywheel and a 9mm diameter big end bearing. Moving forward, the next area of interest is the section of the shaft that takes the rear bearing; this has a diameter of 17mm and is bored with a timed breather / oiling hole that exits into the centre of the shaft. Here the bore is blind at the front but opens at the rear into the main case, the purpose being to relieve bottom end pressure and excess oil as the piston descends. Said oil travels through the aforementioned breather hole and lubricates the camshaft, valve gear and bearings before exiting the case via the pressure / breather nipple, with any surplus being recycled through the carb’ induction manifold.
Forward from here the shaft is cut with the skew gear drive for the camshaft, and a further plain bearing section of 13mm diameter. The shaft then steps down to 10mm diameter for the front bearing, and a little further on is cut axially for the steel woodruff key that retains the prop’ driver. Finally, the shaft is cut with a UNF thread for the prop’ and locking nut.
The 180’s crankcase is a totally new casting and is of high quality, pressure die cast from alloy with a clean finish. All details are clearly defined and all machining is to a very high standard.
The front end of the crankcase is very stiff, thanks to three axial webs running between the main case and the front bearing housing, which has been recessed for the prop’ driver and acts as a dirt shield for the front bearing. Further stiffening comes from the cam timing chest, which is also webbed to the front of the cylinder fins. There’s a very substantial mounting lug either side of the crankcase, with the left (exhaust) side of the case containing a chrome-plated steel breather nipple that’s sealed by a fibre gasket. Internally the case is fitted with two quality heavy-duty crankshaft bearings, the front item being of the semi-sealed type to give protection against dirt ingestion.
The main body of the carb’ is pressure die cast from alloy, using a rubber ‘O’ ring for sealing. Finished by fine grinding, the steel throttle barrel has a choke bore of 11mm and is very smooth in operation. Internally the barrel is fitted with a brass slow running jet that’s sealed by a rubber ‘O’ ring, and an adjustable steel throttle lever that’s retained to the barrel by an Allen grub screw. The barrel is secured in the body of the carb’ by a cheese head screw.
Finished by chemical blacking, the steel main needle valve assembly is fitted with a very good ratchet device for the fuel needle and is sealed by a rubber ‘O’ ring; the needle is bored centrally for an extension and fitted with a retaining grub screw. The carb’ is equipped with a chrome plated steel fuel nipple; sealed with a fibre washer, this is fitted in a vertical position to give the option of reversing the carb’ to suit individual requirements.
This typically simple four-stroke exhaust system consists of a chrome finished steel manifold that has a locking nut at each end and a two-piece silencer made from turned alloy. The front part of the silencer is fitted with a chrome finished steel pressure nipple and has an internal baffle, with the tail pipe press-fitted into the front part. ASP use a 14mm thread, which is the same as O.S., so there’s a wide range of pipes, manifolds and flexi hose readily available off the peg – Just Engines can help here.
ON THE BENCH
When the ASP180 arrived on my doorstep it had already been given a brief evaluation run by Just Engines, so I was quite surprised to find it was still very tight and unable to be hand-started. As such a quick dab with the electric starter had it running instantly, and it soon settled down to a nice, steady idle. Instant starting was the norm for the whole test from either hot or cold, with excellent throttle response throughout the rev and prop’ range. During the test it was clear that an O.S. type F plug gave the best results, offering better starting, lower idle speed and a smoother throttle response than other varieties.
The ASP proved to be a good performer and turned propellers of a larger pitch than I expected, enabling me to try out a couple of super new Hawk wooden props, that will soon be available from Flair Direct. The engine was given a very thorough workout over some four hours (during which time it freed up sufficiently to be hand-started) and when dismantled it showed no signs of wear.
At last – a budget, single cylinder, 180-size four-stroke… it’ll surely prove to be a top seller thanks to being excellent value for money and physically small enough to add much-needed ‘grunt’ to underpowered models. There’s nothing undesirable in terms of its build quality, either. Thoroughly recommended.