Nederlandse versie

Introduction

This guide covers the two-stroke singles that Aermacchi built in Varese, mainly the models sold as Harley-Davidson (1965–1978). They fall into three families, each with its own engine.

The model line in brief

• Small town bikes: M-50 (1965–1966), M-50 Sport, M-65 and M-65 Sport / Leggero (1967–1972). Three gears, on the early models with hand change on the handlebar, flywheel magneto ignition.
• 125 family: Aletta and Rapido 125 (ML/MLS, 1968–1972), TX 125 (1973) and the Baja 100 (1970–1974) for off-road. The Baja is derived from the Aletta with a smaller bore and won its class in the 1971 Baja 1000.
• SS / SX / SST 125–175–250 (1973–1978): the last generation with oil injection, chromed aluminium cylinder, five gears and 12 volts. SX = off-road style with high exhaust, SS / SST = road version.
• Racers: the Ala d'Oro 125 and the RR250 twin are outside the scope of this guide.

How to recognise it: the new generation (SS/SX 175 and 250) has an upright cylinder and a double cradle frame whose top tube is also the oil tank. The SX 175 and SX 250 are almost identical apart from the bore.

The text is our own summary of public sources. Where sources contradict each other this is noted; check critical values in the workshop manual for your own type.

Safety: work on a cold engine, put the bike firmly on a stand and disconnect the battery when working on the electrics. Two-stroke oil and petrol are flammable: work in a ventilated space.

Technical data

All models have an air-cooled, piston-port single with gear primary drive and a wet multi-plate clutch.

Capacities SX / SST 250: tank 10.5 l (of which c. 1 l reserve), gearbox 1.2 l, oil tank 1.6 l. The SX 175 has an oil tank of about 1.7 l in the frame top tube.

A single source gives premix for the SX 175; most sources and the engineering itself (oil tank in the frame, pump on the left of the crankcase) point to oil injection. Check your own bike: if there is an oil pump on the left under a cover, you have injection.

How the two-stroke works

A two-stroke has no valves: the piston itself opens and closes the ports in the cylinder wall. The crankcase acts as a pre-compression chamber, which is why it must be airtight.

The cycle runs clockwise. Because the mixture passes through the crankcase, the crankshaft and bearings get their lubrication from that mixture.

Two ways of lubricating

• Premix (M-50, M-65, Rapido, Baja): the oil is in the petrol. Always mix in a jerrycan, never directly in the tank.
• Oil injection (SS/SX 175 and 250): a pump on the left of the crankcase adds oil from the tank in the frame. The pump cable is linked to the throttle; that adjustment is vital for the engine.

Consequences for maintenance

• Leaking crankshaft seals let in false air: the engine then runs unevenly, idles high or gets too hot.
• Carbon in the exhaust port and silencer costs power; decarbonising is part of normal maintenance.
• A piston that is about to seize often warns first with a brief tightening. Throttle closed and clutch in.

Periodic maintenance

On a two-stroke the most important oil is the one in the gearbox; the engine itself gets its oil via the mixture or the injection pump.

Mixing ratio (premix): keep to the ratio on the filler cap or in the manual. If that is missing, choose a good two-stroke oil for air-cooled engines and a ratio that applies to that oil; engines from this period are often run a little richer than modern engines. Never mix leaner than the oil manufacturer specifies.

Gearbox oil: SAE 20W-50 engine oil is specified for the SX series. Do not use oil with friction modifiers; it makes the wet clutch slip.

Changing gearbox oil

1. Warm it up, stand the bike upright and drain the oil through the plug under the crankcase.
2. Clean the magnet (if fitted) and check for metal particles.
3. Refit the plug with a new washer and fill to the level hole or dipstick.

Ignition

The SX / SST 250 sparks at 20.5° before TDC, equal to 2.5 mm of piston travel before TDC; the older models have contact points behind the flywheel.

Plug: for the SX/SST 250 the factory specifies a Champion N3 (14 mm) with an electrode gap of 0.6–0.75 mm. Cross-reference tables give an NGK B8ES for the SS 250 and a B9ES for the SX 250. Too hot a plug causes pre-ignition and piston damage; if in doubt choose the colder one.

Measuring timing with a dial gauge (contact points)

1. Remove the spark plug and screw a dial gauge holder into the plug hole.
2. Turn the engine to TDC (highest reading) and zero the gauge.
3. Turn back until the gauge shows the specified value before TDC.
4. Connect a test lamp or buzzer across the points. They must open exactly at this point.
5. Turn the points plate until that is right, tighten and check again.

CDI systems: above all check the pick-up gap and the marks. No spark? First check the wiring and the earth; the electronic parts themselves fail less often than the connections to them.

Carburettor, decarbonising and cylinder

All models run on a Dell'Orto: 18 mm on the M-65, VHB 27 on the SX 175 and PHB 32 on the SX 250. On a two-stroke too lean a mixture is dangerous: it does not cost power but a piston.

Tuning the carburettor

1. Always start with a clean carburettor, a good air filter and sealed inlet rubbers.
2. Turn the air screw lightly closed and then 1.5 turns open as a starting point.
3. Set the idle with the engine warm, using the slide stop screw.
4. Turn the air screw a quarter turn at a time until the speed is highest and then reset the idle.
5. Judge needle and main jet by the plug colour after a stretch at full throttle: light brown is good, white or grey is too lean.
6. Oil injection: after the throttle cable also adjust the pump cable, so the mark on the pump lines up with the reference at idle.

Decarbonise exhaust and port

• Remove the exhaust and scrape the carbon out of the exhaust port with a piece of wood or soft aluminium; do not scratch the chrome or the cylinder.
• Position the piston so it covers the port, so no dirt falls into the cylinder.
• Many silencers have a removable baffle. Do not burn it out with an open flame near flammable materials; cleaning in a degreaser also works.

Checking cylinder and piston

• The SX/SS models have a chromed aluminium cylinder. You cannot rebore it; when worn it needs replating (nikasil or chrome).
• Check that the piston rings are not stuck in their grooves and measure the end gap in the cylinder.
• Scoring on the exhaust side of the piston points to overheating: too lean, too advanced ignition or too little oil.
• When opening up, always replace the base and head gaskets, and if in doubt the crankshaft seals.

Clutch, chain, brakes and fault finding

The clutch is a wet multi-plate clutch in the gearbox oil; maintain the chain and brakes as on any classic.

• Clutch: 2–3 mm free play at the lever. The SX series has primary kick-starting, so you can start in gear with the clutch pulled in.
• Hand change (M-50, early M-65): the gears are changed via a cable from the twistgrip. A stretching cable gives false neutrals; adjust it until the gears engage properly.
• Chain: 20–30 mm slack halfway along the lower run, off-road models a little more. Check with someone on the bike; suspension travel tightens the chain.
• Brakes: single-leading-shoe drums (SX 175: 140 mm front and rear). Adjust so the brake bites in the first quarter of its travel and clean drums without compressed air.
• Electrics: the small models have a flywheel magneto, sometimes without a battery (the lights only work with the engine running). The SS/SX series has 12 volts with a battery.

A two-stroke that suddenly seizes: clutch in, throttle closed and coast to a stop. Let the engine cool and check piston and cylinder before you ride on.

Sources

Consulted on 17 October 2026; all text in this guide is our own work.

• Aermacchi Motorcycles: specifications SX-250 / SST-250
• Aermacchi World: SX 175
• Aermacchi World: M-50 Sport
• National Motorcycle Museum: SX175 1976
• Motorcycle Classics: Harley-Davidson SX250
• Motorcyclespecs: Harley-Davidson SX 175
• Moto-data: SST 250
• Wikipedia: Harley-Davidson Baja 100
• Wikipedia: list of Harley-Davidson models
• Cybermotorcycle: M-65 Leggero
• Cybermotorcycle: Aermacchi models
• Cybermotorcycle: SX-175 1976–1978
• Myron's Mopeds: M-50/M-65 specifications and parts
• Bob's Cycle: Harley spark plug cross-reference

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