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A.S.P. 'Series 3' .70 FS (Ring)

Just Engines Online | Engine Reviews & Comments | ASP Reviews |  A.S.P. 'Series 3' .70 FS (Ring)

By Malc Pinnock

By Malc Pinnock

(RCM&E April 2004)

The new ASP FS 70 AR Series 3 will eventually replace the ASP FS 65. This is a new engine from China’s longest serving engine manufacturer and has the latest type crankcase, with the timed breather exiting into the intake manifold giving the fresh fuel charge a slight boost, from the engines bottom end pressure pulses, It also redirects any unused oil back into the engine to improve internal lubrication, not all over the inside of your cowl. Further improvements come from the cylinder head and intake manifold, with better shaped and enlarged porting for improved gas flow. Read on and lets see if the new 70 really is an improvement over the earlier and very popular 65.

ASP FS 70 Strip

ASP FS 70 Strip

Where to start? Lets start the strip by removing the carb, choke and intake manifold, this is done by removing the 2xM2.5 carb / choke retaining, cap head bolts. The carb and choke can then be gently be eased from the intake manifold. The pressed steel choke device, with its rubber sealing pad and extendable operating lever is a very good thing to have on a four stroke, it makes life much easier when priming your engine, but do not operate it along with the electric starter as flooding will soon occur. All that’s needed is a couple of hand turns before the glow is connected.

The pressure die cast alloy intake manifold, is retained to the cylinder head by 2xM2 cap head bolts, be careful when removing these so as not to damage the fibre gasket. This is a very nicely cast part, and it contains the chrome plated steel pressure nipple that’s sealed by a fibre washer. This is connected to the timed crankcase breather nipple by a supplied piece of silicon tube

The rocker box cover is the next part for removal. Retained to the cylinder head by 2xM2 cap head bolts, and pressure die cast from alloy. Once this is removed we have access to the 2 machined hardened steel, rocker arms, adjustable tappets and lock nuts. Tappet, setting gaps are .1mm from cold. The central M2.5 cap head bolt and its locking washer, can now be removed releasing the rocker arm assembly.

The 2 hardened steel pushrods can now be removed, leading you on to the next job of removing the cylinder head

Cylinder Head

Cylinder Head

5xM2.5 cap head bolts retain the cylinder head. Remove these diagonally, working across and around the head, a couple of turns at a time so as not to distort the head.

As this is still under pressure from the 2 machined bar stock alloy, push rod tubes that are sealed to the head and crank case by 4 rubber O rings.

Model engine cylinder heads; never cease to amaze me. This one is a gem. Pressure die cast from alloy, with it’s cast in bronze valve seats and valve guides. Intricate porting, shaped combustion chamber and cooling fins. It’s been cast, bored, tapped, and machined, yet we just take it for granted, as yet just another part, that’s fitted to the engine.

OK, back to the head, bored and tapped for a long reach four stroke plug, and fitted with 2 hardened steel valves, the inlet being slightly larger than the exhaust. Each valve is fitted with a single valve spring, cap and pair of retention collets. Great care must be taken if removing the valves, as the springs caps and collets can fly every where. Sealing of the head to the crankcase is by alloy head gasket. Well that just about wraps up the head. Apart from mentioning that the exhaust manifold is tapped with the same size thread <Actinic:Variable Name = '11x75'/> as an OS. So there are lots of silencer and tuned pipe options available.

Cam box cover, Cam shaft and Followers

Cam box cover, Cam shaft and Followers

2xM2 cap head bolts retain the die cast alloy cam box cover. The cover is sealed by a plastic gasket, and is fitted with a ball race to support the hardened steel camshaft, with a further bearing for the cam being fitted to the crankcase.

Looking into the cam box at the camshaft, on the face of the cam gear cog there is a timing mark.

Cam timing for the ASP70 is by having the piston at TDC [top dead centre] with the timing mark in line, with the centre of the exhaust cam follower. If the cam followers can not be removed by turning the engine upside down, and tapping the case into the palm of your hand, 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 rear wards and gently pull, once the camshaft is removed the hardened steel cam followers, can be pushed into the cam timing chest and then removed.

Back plate

Right folks, it’s time to open the back door. The pressure die cast back plate is retained to the crank case by 4xM2.5 cap head bolts, and has the carb mounting flange cast on at the top. The back plate sits deeply into the crankcase and is sealed by a plastic gasket. And once this is removed its time for the next job, and that’s the removal of the liner piston assembly.

Liner and Piston assembly

The liner in the ASP is a very good fit into the crankcase. This makes for excellent heat dissipation so a small amount of heat, from my trusty gas torch was applied evenly around the cylinder fins. The crankshaft was then rotated; when warm enough the liner will raise up enough to pull out by hand. Upon inspection the liner is made from steel. Case hardened, then finished by fine grinding and lapping.

Now it’s round to the rear of the crankcase, in the centre of the case Just above the back plate there is a hole. Looking into the hole, align the gudgeon pin with the hole, then give the rear of the case a firm tap into the palm of your hand, and the fully floating hardened steel gudgeon pin can then be removed.

The single ring piston is a permanent die casting from alloy, is of the slipper type and fully finished by machining. Internally it has two large webs to support the fully floating gudgeon pin. This has been fully bored, for lightness and is retained at the front by a semi blind hole in the piston. It is fitted with a PTFE wear pad at the rear, to ensure no damage is done to the liner when the engine is running.

This leaves us with the heavy-duty con rod. CNC machined from high tensile alloy. 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. That rounds up the liner piston assembly.

Now it’s time to remove the crankshaft.

Crankshaft assembly

Crankshaft assembly

To remove the crankshaft a puller is required, don’t be tempted to belt the shaft with a 5lb lump hammer, as this destroys the shaft threads, prop driver and front bearing. Yes this is done and I have seen the results <Actinic:Variable Name = 'SCARY'/>. Remove the black chemically finished, steel prop-locking nut, prop nut and prop washer. Then use the puller to remove the alloy prop driver and split tapered collet. The crankshaft can then be removed with a firm push.

The heavy-duty crankshaft is machined from a single billet of steel. The ample big end bearing has a Dia of 7mm, and is supported by a large fully counter balanced flywheel.

Moving forward the next part of the shaft is for the rear bearing, this has a Dia of 15mm, and is bored with a timed breather / oiling hole, into the centre of the shaft. This boring is blind at the front but opens at the rear into the main case. this is to relieve bottom end pressure, and excess oil as the piston descends. This excess oil lubricates the camshaft and valve gear, it then exits the case via the pressure / breather nipple as mentioned earlier.

On to the next part of the shaft, this has been cut, with the skew gear drive for the camshaft, and has a plain bearing portion of 12mm Dia. Forward then to the front bearing part of the shaft, this has a Dia of 10mm, and leads us on to the 8mm prop driver, and out put part of the shaft. That has been cut with a 5/16 UNF thread for the prop nut and locking nut. That’s it, now for the crankcase.



What a super casting, as clean as engines of twice the price. An excellent piece of pressure die cast alloy. All details clearly defined no flash or pockmarks and any machining carried out, is to a very high standard.

The front end of the crankcase is very stiff. With 3 axial webs between the main case and the front bearing housing, that has been recessed for the prop driver. That acts as a dirt shield, for the front bearing. Further stiffening comes from the cam timing chest that is also webbed to the front of the cylinder fins. Each side of the crank case has a very substantial mounting lug, with the left side or exhaust side of the case, containing a timed, chrome plated, steel breather nipple, that is sealed by a fibre gasket.

Last but not least, the case has been fitted with two quality smooth running bearings, the front bearing is of the sealed type to give protection against dirt ingestion, and that’s all folks, its off for a peek at the carb, and a quick mug of coffee.

ASP Carburettor

The main bony is pressure die cast from alloy, it is of the push on type using a rubber O ring for sealing. Fitted with a very smooth operating, steel throttle barrel. The barrel has been finished by fine grinding, and has a choke bore Dia of 7mm. This is fitted with a brass slow running jet that is sealed by a rubber O ring. The barrel is fitted with a steel adjustable throttle lever, and is retained in the body by a cheese head screw. The carb is fitted with a steel main needle valve assembly, that has a very good ratchet device for the needle that is also O ring sealed, and has provision for a needle extension. Sealed chrome plated steel fuel nipple, in a vertical position, giving you the option of reversing the carb, to suite your models requirements.

Silencer and Exhaust manifold

The remaining parts, the exhaust system, This consists of a steel manifold, with a locking nut at each end, for it,s 11x75 threads. The same as those on OS engines so you have an excellent range of after market options. And the final part to cover the silencer. This is of a two parts pressed together. The larger front part is machined from alloy bar stock, and contains a steel tank pressure nipple. The rear part, the tail pipe, is also machined from alloy bar stock and is pressed into the front.

Right that’s the strip completed, all I have to do is reverse my strip on the engine, to see if it works, then go and bench test it. But before I do this remember to use a good quality oil for assembly, as most damage to an engine is caused in the first run by lack of lubrication.


Having first read the excellent English instructions, the engine was bolted to the bench connected to the throttle servo, and the fuel pipe and pressure nipples connected to the tank. Fuel recommendations are for fuels containing not more than 5% Nitro.(10% Nitro can be used, but if pre ignition occurs, fit an extra head shim, or just go back to using 5%). With an Oil content of 15% to 18% of witch contains a minimum of 2% castor oil. Or if using pure synthetic use one containing 20% oil. A good rule is, if the contents are not stated don’t buy it. A lean run could be fatal, for your engine and could also void your warranty.

Ok, my choice for the test, Flairs Yellow / Gold, 5% Nitro, 5% Castor, 10% Synthetic (ML70) Just Engines also recommend a range of plugs to use, the supplied plug was the very good ASP ‘S, (for FS), But my preference was the OS ‘F‘(FS) as these seem to give a slightly improved performance.

Right Engine primed glow connected 3 turns open on the needle, 12x6 prop fitted for running in, its all systems go.

The new ASP 70 took no more than a couple of flicks by hand to start, and was kept on a very rich setting for the first couple of 5 minute runs. Then allowed to cool between runs, this is my preference as I like to bed ringed pistons with steel liners to bed in carefully. This pays dividends long term, being a scrooge I expect a long life from my engines. After 4 tanks approximately, 30minutes running time I started to increase the throttle openings, and lean the engine out until it was able to hold full throttle, without showing any signs of distress. I always check the exhaust to ensure that it contains un burnt oil, this helps guard against that fateful lean run when running in. The ASP was run in for a much longer period than was required as this helps to get proper test figures on the bench.

How did it perform? Faultlessly, just a couple of hand flicks hot or cold, was all that was required. The only adjustments made during the test, were to check all bolts for tightness after running and check the tappet clearances, before the very punishing full throttle test was carried out.

The 70 was found to be a improvement over the old 65, with more torque and higher rpm. After the exhaustive test, the engine was stripped with no signs of wear apparent, as the photos will show. Highly recommended for anyone wishing to purchase a new four stroke, and complements the ASP range which now comprises; .30, .52, .61, .70, .80, .91, 1.20, and the 1.60 flat twin.

Data File

Engine ASP FS70AR

Configuration O,H,V Single cylinder 4 stroke, with 2 valves, Steel Liner with single ringed alloy piston. and twin ball race crankshaft

Displacement .70 11.6cc

Stated Power 1.1 BHP @ 11,000 RPM

Bore and Stroke Bore 25.9mm Stroke 22.0mm

Weight 580g, 640g including silencer

Plug used in test O.S ‘F’

Fuel Used in test Flair Yellow / Gold, 5% Nitro, 5% Castor, 10% Synthetic

Average noise test 82 db @ 7mtrs.

Supplied with Full English Instructions, 2 x Allen Keys, 2Year warranty

Prop tests

Size Type Idle Max
11x7 Radio Active [ RAM ] 2,400 11,600
12x6 RAM 2,400 11,400
12X8 APC 2,400 9,800
13X6 RAM 2,400 10,400
14X6 APC 2,400 8,900

More Reviews

You can find more ASP engine reviews here.

Just Engines Online | Engine Reviews & Comments | ASP Reviews |  A.S.P. 'Series 3' .70 FS (Ring)