Identifying Magnetos on Early Petter M Engines

Early Petter M engines are familiar sights in Australian collections, from machinery sheds outside Toowoomba to display lines at country agricultural shows. Their magnetos can look broadly similar while differing in mounting, drive arrangement, timing and internal construction. Correct identification therefore depends on several clues rather than a single badge.

A replacement magneto may have been fitted during the engine’s working life, and many surviving engines have passed through several owners. A Petter sold new in Britain could have received a different unit after arriving in New South Wales, while a magneto rebuilt in Melbourne might carry a local repairer’s plate instead of its original label. The aim is to identify what is present now, then assess whether it is appropriate for that engine.

The following method combines visible details, historical probability and practical checks. It is useful when cataloguing an engine, preparing a restoration, comparing photographs or searching for parts through Australian collectors and vintage machinery clubs. It also helps prevent an attractive but unsuitable magneto from being treated as an original fitting.

Why the magneto matters

A magneto generates its own ignition current from mechanical rotation, so an early Petter M does not need a battery to produce a spark. Permanent magnets and a rotating armature create a high-voltage pulse, which travels through the high-tension lead to the spark plug. The engine’s timing gear or magneto drive determines when that pulse occurs.

On an old stationary engine, the magneto is part of the identification evidence. Its position may indicate the original crankshaft arrangement, while its coupling can reveal whether the engine was designed for impulse starting. The magneto also affects starting behaviour: a sound impulse mechanism gives a strong spark at low cranking speed, whereas a worn direct-drive unit may produce little or nothing until the engine is spun quickly.

For Australian owners, this has a practical consequence. An engine that starts readily at a winter rally near Ballarat may fail to fire after sitting in a humid coastal shed in Newcastle. Condensation, oxidised points, deteriorated insulation and a weak magnet can all imitate an incorrect magneto. Identification should come before dismantling, but basic operational checks remain important.

The main families found on Petter M engines

Period Petter engines could be supplied with magnetos from more than one manufacturer. Names encountered in surviving examples and parts literature include Wico, Bosch, Eisemann, Lucas, ML and Dixie, although availability varied by production date, market and engine specification. A Wico EK is frequently associated with small British stationary engines, but its presence alone does not prove that every M engine originally used one.

The casing shape provides an initial sorting clue. A compact magneto with a black or dark cylindrical body, an exposed impulse coupling and a separate terminal cover has a different visual character from a longer rectangular-bodied unit with a prominent nameplate. Bosch and Eisemann examples often have distinctive casting forms and terminal arrangements; Lucas and ML units may be recognised by their stamped plates, pole-piece layout or service fittings. Paint colour is weak evidence because repainting was common.

Do not confuse a magneto with a coil-and-battery ignition system. A battery coil may have a separate condenser, switch and wiring loom, whereas a magneto normally has an integrated armature, contact breaker and high-tension outlet. Some conversions retain an original-looking drive bracket while replacing the ignition system, so trace the wiring and inspect the generator body rather than relying on the bracket alone.

The independent Ashworth Parker engine list is a useful reminder that unusual engines and non-standard fittings often survive in small numbers. Comparative photographs from related stationary engines can help, but a similar-looking magneto from another maker is not automatically interchangeable.

Reading the mounting and drive

Begin with the mounting plate, studs and drive coupling. Measure the bolt spacing, shaft diameter, keyway and the distance from the mounting face to the drive centre. Record whether the magneto is mounted upright, horizontally or at an angle, and note whether its drive shaft points towards the crankshaft, camshaft or a separate timing gear.

A correct replacement must rotate in the required direction and reach the correct firing position. Reversing a magneto may produce a spark but can leave the impulse coupling working incorrectly or the contact-breaker timing outside its usable range. Some units are designed for clockwise rotation and others for anticlockwise rotation when viewed from the drive end, so always describe the viewpoint in notes and photographs.

Look closely at the coupling. An impulse coupling usually has a spring-loaded mechanism that winds, holds and releases as the engine turns slowly. It may make a sharp click during hand cranking. A plain flexible coupling, fibre drive or rigid dog indicates a different arrangement. The presence of an impulse device does not establish the magneto brand, but it can narrow the likely period and starting system.

Australian restorers often encounter engines mounted on replacement steel skids rather than their original timber bases. A bracket may have been slotted or redrilled to accept a later magneto, particularly where a scarce British unit failed. Measure the original-looking features separately from later modifications, and photograph witness marks, unused holes and cut metal before cleaning anything.

Using markings and construction details

Search for a maker’s name on the face plate, rear cover, terminal block and impulse coupling. Stamps may be faint, hidden beneath several layers of enamel or obscured by a repair plate. Use raking light and a soft brush rather than aggressive wire brushing. Numbers on the armature or body can be more valuable than a partial logo because they may identify a series or service specification.

The high-tension outlet is another useful clue. It may be a threaded brass terminal, a moulded black cap or a side-mounted connection. Count the low-tension terminals and inspect the switch arrangement. A magneto with an internal earth-stop terminal differs from one that uses a separate contact-breaker lead, and the wiring method can reveal whether the present unit was adapted.

Examine the contact-breaker cover, condenser position, pole shoes and ventilation openings. Original fasteners, factory paint beneath a bracket and undisturbed timing marks strengthen the case for an early fitting. Modern screws, soldered wires, silicone cable and a freshly machined adapter suggest later work. These signs do not make a magneto historically wrong; they simply separate factory evidence from restoration evidence.

When documenting an engine, record the full combination: Petter M model and serial information, magneto maker, body number, mounting orientation, coupling type, rotation and spark-plug lead position. A photograph of the complete engine is valuable, but close images of the drive end, data plate and terminal side are far more useful when seeking advice.

Distinguishing original fittings from replacements

Surviving M engines rarely have an untouched history. A farm operator may have replaced a failed magneto with whatever was available through a machinery agent. In Australia, imported parts could be expensive or slow to obtain, encouraging owners to adapt a Lucas or local stock item rather than wait for a matching British unit. A later magneto can therefore be mechanically sensible even when it is not factory original.

Compare the magneto with the engine’s age, casting style and controls. If the engine has early hand-finished details but the magneto carries a post-war service label, treat it as a probable replacement. Conversely, a period magneto from another manufacturer may have been fitted when the engine was new, especially if Petter offered alternative ignition equipment for different export markets.

Look for adapter plates and altered timing marks. A plate with two sets of holes may show that one magneto replaced another. A shortened shaft, brazed coupling or drilled flange is strong evidence of adaptation. If the drive gear meshes cleanly and the timing can be set within a sensible range, the conversion may be worth preserving rather than returning automatically to an assumed original arrangement.

The Baby Blackstone engine guide provides a useful comparison because related British horizontal engines also demonstrate how ignition equipment, mounting details and restoration decisions must be read together. Similar external dimensions do not guarantee the same timing or drive specification.

A practical inspection routine

Start with the engine stopped, fuel isolated and magneto disconnected from the spark plug. Clean only enough of the casing to expose markings. Photograph the magneto before removing it, including the direction-of-rotation arrow if present. Mark cable positions and keep shims, keys and unusual washers in a labelled container.

Turn the engine slowly in its normal direction and observe the coupling. A healthy impulse unit should engage and release consistently. Check for excessive shaft movement, cracked insulation, loose terminals and oil contamination. Do not hold the high-tension lead while testing; use an appropriate spark tester or insulated tool, and remember that a magneto can deliver a painful shock even on a small engine.

If the magneto is removed, inspect the mounting face and drive timing without forcing the coupling. Check whether the points open at the expected crank position and whether the impulse trip occurs close to top dead centre. Magneto rebuilding should be entrusted to a specialist when coils, magnets or impulse springs require attention. A visual identification guide cannot establish insulation resistance or magnetic strength.

Record these identification clues

Avoid these common mistakes

A sound record makes parts searches much easier. Include metric and imperial measurements where possible, since Australian restorers may compare British workshop data with locally machined replacement parts. State whether measurements were taken from the engine or the magneto, and mention any missing cover, damaged coupling or altered bracket. For owners attending events around Sydney, Adelaide or Perth, clear photographs and measurements are more useful than a vague description at a swap meet.

Finding reliable evidence in the Australian market

Australian examples can preserve details that are uncommon in Britain, but local history also introduces uncertainty. Engines were imported through dealers in Melbourne, Sydney and Brisbane, then moved through farms, workshops and municipal departments. A Petter discovered in a Queensland shed may have a locally sourced replacement magneto and still retain an unusual export-market bracket.

Search by combinations of terms rather than a single phrase: “Petter M magneto”, the magneto maker, the body number, “impulse coupling”, and the mounting dimensions. Vintage engine forums, club newsletters, auction catalogues and photographs from field days can reveal recurring patterns. Treat claims such as “original magneto” carefully unless the seller explains the evidence.

The prime contractor resource is not a magneto catalogue, but it illustrates why provenance and documentation matter when assessing equipment histories. For a Petter, the equivalent evidence may be a dealer invoice, farm ledger, repair tag or photograph showing the engine before restoration. Such records can settle questions that appearance alone cannot answer.

Local buying customs also matter. At a country clearing sale, a magneto may be included with a dismantled engine and described simply as “Petter parts”. On Gumtree or a club noticeboard, dimensions and photographs may be incomplete. Ask for the drive-end view, the nameplate and the rotation direction before travelling, particularly when the item is several hours from Canberra or regional Victoria.

Correct identification is a balance of mechanical fit, historical likelihood and surviving evidence. A magneto that starts the engine is valuable, but a carefully recorded replacement is more useful to future owners than an unsupported claim of originality. Preserve unusual adapters and repair marks, service the ignition safely and add your findings to the growing record of Petter engines in Australia.

Share measured details, clear photographs and documented corrections with recognised vintage machinery clubs, specialist forums and archival websites. Each accurately recorded magneto helps another owner identify a rare fitting, locate a compatible component or understand how these early engines continued working across Australian farms and workshops.