Panther M100 & M120

Buyers guide

▲ This is how we like to see the big Panthers; a little battleworn, a little tired, a little oil spattered, but still lumbering on. This M120 example fetched close to £4000 when sold on eBay (September 2013) by Richard Gaunt at D R Classic Motorcycles. It needed a thorough overhaul, we hear, and was alive with interesting noises. But Cleckheaton's most famous son is famed for durability. Or is it simply that the Panther men
will see no evil, hear no evil and speak no evil of these quirky Yorkshire bred bikes, no matter how bad it gets?



Panther M100 & M120

Panther myths

Panther M100 and M120 ownership tips

Riding a Panther

1958 Panther M120 specifications

Living with a Panther

Top Panther specialists & links

Which Panther should you choose?

Panther M100 vices and virtues



▲ Panther Motorcycles were designed and built in Cleckheaton, Yorkshire by Phelon & Moore Ltd, Engineers. But the firm also operated a showroom at 324 Regent Street, London, W1 as proclaimed on this poster.


The Panther cat device appeared across a range of literature and gave the firm a slightly exotic edge that, some might say, bore no reflection to the typical Panther owner/rider. Except that Panther Motorcycles have covered huge mileages in some pretty exotic countries. Today we'd cal them adventure bikes.


This poster carries the slogan: Powered for Mastery. But more typically, the P&M tag (Phelon & Moore) was referred to as: The Perfected Motorcycle.


The design, however, was never perfected. Rather, Panthers were constantly evolving, and P&M was nothing if not one of the most innovative firms in the business despite the fact that they tended stay with a fairly constant fundamental design architecture.


The Panther name, incidentally, wasn't used until 1924, 20 years after the firm was founded. Early bikes were badged P&M.


Panther engine left side.


Check out the large wedge-shaped protrusion on the front of the crankcase. That's the Panther oil tank. It keeps the black stuff cool in the breeze, and maybe warms it a little in traffic thanks to the twin exhaust pipes. The engine is a single cylinder, incidentally, in spite of what those pipes suggest.

Panther M100 and M120 ownership tips


1. Consider switching to electronic ignition. Kirby Rowbotham will convert your existing magneto and update it with a Boyer Bransden unit. Expect easier starting, better pick-up, and a reliable spark.


2. Be careful not to overfill Panthers. Some owners report problems here due to topping up at a level between the two dipstick marks. The lower mark is, we hear, the correct one. The higher mark refers only to freshly rebuilt engines. If in doubt, consult whatever experts you can find.  Keep in mind that Panthers do tend to use a lot of oil, anyway. Engine mods are available, but the jury is still out over which ones are best, or even viable.


3. When buying, listen carefully to the owners advice on starting. These Panthers employ a half compression lever. The starting regime is a fairly complicated theatre of antics for the uninitiated. But once you've got the knack, it's no big deal. Wearing a ski boot is advisable for anyone with weak ankles.


4. Be prepared to experiment with spark plugs to get the best from these engines. Bosch, NGK and Champion offer different quality sparks, and Panthers can be fussy. So find the plug that works best for you (by experimenting across the heat range) and stay with it. Just make sure you use a plug with the correct reach.


5. When buying, ask about the gearbox adjusters. Threads strip easily—usually due to hamfistedness, but also simply down to age and wear and tear.


6. Check the gearing/sprockets. It might be set up for sidecar hauling or for solo riding. The difference on the road is considerable.


7. Exhaust port threads suffer badly for general corrosion and repeating heating/contracting. They crack and strip if badly maintained. And even when well maintained, you can run into difficulties. Use a copper based grease and avoid over-tightening. But don't loosen or adjust more often than is absolutely necessary. Just expect problems here occasionally—but solutions are easily available.


8. Get a copy of Barry M Jones's book:

The Panther Story. It's published by Panther Publishing and is jammed packed with Panther information and advice. Jones has also published another book entitled: The Story of Panther Motorcycles. Essential reading material for the Panther owner or enthusiast.


1958 Panther M120 specifications

Type: Air-cooled OHV four-stroke single
Capacity: 645cc (650cc)
Bore & Stroke: 88mm x 106mm
BHP: 27bhp @ 4500rpm
Compression ratio: 6.5:1

Lubrication: Dry sump

Carburerttor: Amal 1-3/16th
Transmission: Burman 4-speed, multi-plate
dry clutch
Brakes: 8-inch sls drum front and rear
Electrics: 6-volt, magdyno
Front suspension:  Telescopic fork, hydraulically damped
Rear suspension: Swinging arm
Wheels: Front: 3.25/3.50 x 19-inch wire.

Rear: 3.50 x 19-inch wire

Frame: Tubular brazed, engine as stressed member

Fuel capacity: 4 gallon

Seat height: 31-inches

Wheelbase: 56 inches
Weight: 425lbs
Maximum speed: 80mph (approximately)


Top Panther specialists & links



Panther Owners Club


Andy Tiernan

Old Railway Station
Station Road
Nr. Woodbridge
IP13 9EE

Telephone: 01728 724321



Kirby Rowbotham

Ignition conversions


D R Classics

Talk to Richard Gaunt

07889 292536


There was a time when Panther motorcycles—at least, the large-capacity OHV models—were practically synonymous with sidecars. Which is to say that you might well see a sidecar without a ‘big cat’ attached, but rarely the other way around.


In fact, Phelon & Moore, the Yorkshire-based company which produced the famous slopers, once estimated that ninety-percent of all 594cc Model 100s were destined for three-wheeled harness. As for its successor, the last-of-the-line 645cc Model 120, all bikes were supplied with sidecar gearing and sidecar fork-trail as standard (although solo gearing was a no-cost factory option).


And it’s perhaps largely this fixed (or is that free?) association with sidecars that has kept prices of large-capacity OHV Panthers relatively depressed over the years. Who the hell, after all, wants a heavy, slogging, ponderous, noisy thumper for Sunday afternoon solo cruising?


Not many riders, apparently. Not when there’s so much other more suitable (read; sporting) stuff on offer in the classic marketplace.


Panther M120 instruments and steering damper.


Unfussed instrument layout albeit with a prominent steering damper that would never make it past a modern health and safety inspector. But Panther owners were, and are, a tough (or reckless) breed way beyond the fear of summary emasculation. Our advice is to pass down your genes before you try anything too clever on one of these.



Except that things are changing, and over the past ten years or so, with sidecars usage spiralling ever further out of fashion (certainly as far as practical, daily transport is concerned), there’s been a slow reappraisal of what was once the largest, single-cylinder air-cooled motorcycle in the world. Which is why the big Panthers are being increasingly appreciated as (hold onto your steering-dampers) solo rides. And in doing so, they’re slowly winning over a new generation of enthusiasts keen to climb aboard one of Britain’s most charismatic, and misunderstood, marques.


Trouble is, there simply aren’t enough bikes to go around—which, according to the laws of supply and demand, ought to be reflected in higher prices. But in fact Panther’s are still relatively affordable—leading you to suspect that many riders may well aspire to owning a ‘big cat’, but not to the extent of willing to shell out hard cash.


For instance, a decent, well-sorted 1959 645cc Model 120 (teles/swinging arm) will, at 2013 prices, set you back anywhere between £2800 and £5000—depending largely on whether it’s a trade or private sale, and, of course, how original it is. But there are still ratty examples for around £2000-£2500.


Or even less.


Meanwhile, a 1950 Model 100 (teles/rigid) in similar condition will cost around £6000 at the top end, but usually with a higher base price of, say, £4000. And, as ever, you can stick another £1500 or so upward as soon as you start poking around at the pre-war, girder-forked ‘exotica’.


Having said that, Panther buyers are a peculiarly fickle bunch, and its not uncommon to see good examples dip significantly below what might generally be thought of a reasonable asking price. Which means that as pure investments, you’re best advised to put your money elsewhere unless you’ve got a lot of patience.


And nerve.


 Panther M120 1964 engine close up.


1964 Panther M120. Note the interesting carburettor air filter.



Panther myths


But if you simply can’t resist a Panther (and plenty of people can’t), buy yourself a large rubbish bin too, because you’ll need somewhere to dispose of those preconceptions you’ve been storing all these years.


For example, contrary to popular opinion the big OHV Panthers engines aren’t, relatively speaking, particularly long stroking at all—although by using the engine unit as a stressed frame member (a Panther design feature since the beginning), they look it.


Check for yourself. The 1947 634cc Norton Big Four, for instance, swings in at 82mm x 120mm (a ratio of 0.68:1). Meanwhile, a late-model 591cc BSA M21 has a bore and stroke of 82mm x 112mm (a ratio of 0.73:1). This compares with 88mm x 106mm for the 645cc Panther 120 (0.83:1) and 87mm x 100mm for the 594cc Model 100 (0.87:1). And if you’re still not convinced, compare those figures to, say, a 1938 500cc Speed Twin which has a bore and stroke of 63mm x 80mm (0.78:1)—and then ask yourself which bike, relatively speaking remember, has the longest stroke.


Hint: It isn’t the Panther.


But that huge, sloping (and very much structural) single-cylinder engine, enlarged in 1959 from 594cc to a then fashionable, if arguably inadvisable, 645cc does pack a hell of a wallop (as you’ll discover if it kicks-back during an abortive start-up). And it’s this low down grunt that, understandably, is often mistaken for a long stroke punch.


Next up is the myth that heavyweight Panthers thump along at around one revolution per lamp post. Instead, they tend to burble along without any undue shock to the system at between 2000 and 4000rpm, with maximum revs at slightly under 5500rpm (Model 100) and 4500 (Model 120). Maximum torque comes in at around 3500 and 3000rpm respectively.


With regard to the smaller OHV models in the range (the 348cc Model 75 and the 248cc Model 65, for instance) Panthers rev to levels similar to most other contemporary British singles.


Panther120 engine, right side.


The engine as a stressed member? It's actually something of a misnomer. You might just as easily call the Panther engine an unstressed member, because they lope along with impressive indifference. This one's a M120 650cc unit. The 600cc M100s are, unsurprisingly, a little softer and more gentle. That tiny lever on the timing chest is for half-compression. See the main text for more on this.



Riding a Panther


When it comes to solo handling, the Big Panthers are competent if, okay, not exactly inspiring mounts. Panther’s own-make of two-way damped telescopic forks, introduced in 1954, are about average (if a little noisy) and are certainly no worse than those that Triumph, for instance, was offering during the same period.


Panthers fitted with the earlier Dowty Oleomatic air/oil forks (1947-1953) provide an extremely comfortable ride, but often at the expense of variable damping (due to heat-driven air expansion in the forks). Theoretically, we’re talking about rising-rate suspension, of course. But in practice, it can get a little more uncertain than that. However even then, unless you’re riding hard on a seriously irregular surface for long periods, you’re unlikely to notice much difference as long as the forks are in good condition.


Which is the rub, both literally and metaphorically because 1940s fork-seal technology left much to be desired. Meaning that it doesn’t take much to wear them out, and when they do wear, they’re prone to collapse—and with no integral springs (as standard, at least), you’re pushing it home.


One theoretical advantage of the Dowtys is that the spindle caps (licensed, incidentally, from BSA) are reversible to permit solo or sidecar trail. But in practice, that makes little difference to the average rider who’ll set his ride up one way or the other and forget it. But it is worth keeping in mind if you want to tweak the forks for sharper or more lazy steering.


Overall, you’re unlikely in any case to want to go ‘scratching’ on any of the post- or pre-war Panthers. But as long-distance tourers, or even as Sunday runabouts, they’re highly capable machines which are nothing if not solid, yet docile, and (welcomely) predictable.



▲ Panther flirted and experimented with other engine configurations including twins and two-strokes, but the 40-degree sloping single is the one that made Phelon & Moore famous.



Living with a Panther


What is true about Panthers is that they are hacks—which is to say that they’re extremely robust machines which, with regular maintenance, will return thousands upon thousands of miles of satisfying motorcycling.


Most will weep oil (pushrod tube, cylinder base, primary chaincase). Many will throw up interesting foibles (over-oiling, gearbox, clutch, spinning main bearings). But just about every Panther problem has been well documented, and solutions for most are available—which is unsurprising when you consider how little the fundamental design has changed in over four decades.


Also, spares for late-model bikes are actually quite good—largely through the efforts of the excellent Panther Owners Club (POC) which for years has been involved in buying up old stock and remanufacturing, where possible, to original specifications.


Top speed is around 80-85mph—although you’re unlikely to want to cruise at much above 60mph. You’ll (barely) hold your own in everything except motorway and fast dual carriageway conditions, which is why you’ll probably find yourself more often on gently curving ‘A’ and ‘B’ roads enjoying romps of around 30-40 miles. But keep in mind that these are bikes which have a well earned reputation for covering huge distances and that you’ll wear out before they do. And even when Panthers wear to within an inch of their lives, they seems to have plenty of miles left in them. Some machines are like that.


Before buying, our advice is that you join the Panther Owners Club and get some specific, practical advice. Also—and regardless of whether you plan to buy a Panther—you’re strongly recommended to buy a copy of  The Panther Story by Barry M Jones. It’s one of the best researched tomes on the market and is a fascinating insight into Phelon & Moore, the makers of ‘The Perfected Motorcycle’.


And when you have made your selection and bought your machine, set aside a couple of maintenance hours each weekend and be prepared to get your hands dirty. It’s odds-on you’ll be quietly charmed by these strangely graceful—and yet still somehow dowdy—‘big thumpers’.


Panther engine cutaway.


▲ Panther engine cutaway. Simple, rugged, understandable. The spares availability from the Panther Owners Club is pretty good, and generally you can keep one of these on the road without too many parts supply issues. But you'll be constantly on the lookout for those hard to get items.


Which Panther should you choose?


As ever, the first rule is to buy with your eyes. If you’re going to ride it at all, you have to like what you see. But generally speaking, the later models (swinging arm/teles) are the most practical.


And comfortable.


Working backwards, the 645cc M120 was introduced in 1959 and ceased production seven years later. With 27bhp, a 6.5:1 compression ratio, a four-speed Burman BAP gearbox and 426lbs beneath you, you’ll have a lazy, effortless ride and will enjoy good fuel economy (approximately 70mpg under steady 55mph cruising conditions). But you won’t break any speed records and will learn to live with modest acceleration.


All M120s are teles/swinging arm.


Ignition and charging is through a Lucas magdyno, while an Amal 389 carb meters the mixture. With a wheelbase of 56-inches (1422mm) and a fork trail set-up for sidecar lugging, your cornering will be slower rather than faster. But whatever else, your ride will be fairly predictable.


Brakes are full-width, eight-inchers. When set up right, they’re adequate at the front, and adequate-to-good at the rear. But, because the brake cams run in iron bushes, they are prone to rusting. Regular usage, as ever, is your best bet.





The wheels, both 19-inchers, are, incidentally, interchangeable.


The first weak mechanical point is likely to be the three-plate, five-spring clutch. That huge single-cylinder engine delivers a correspondingly large chunk of torque. It also provides a fair amount of engine braking which means you’ll be burning your candle at both ends. Clutch spares and mods are available. Just keep everything well-maintained—especially the gearbox adjuster bolts which are pathetically inadequate and are prone to strip—and you shouldn’t encounter too many surprises.


Keep in mind too that a lot of the ‘lousy clutch’ stories relate to sidecar outfits which, by their nature, take a lot of extra hauling. Solo bikes tend to have an easier (if still slightly troubled) life—so, where possible, check the provenance of the machine you’re interested in buying.


The gearboxes, as with all Burman boxes, are slow and unhurried. But they are rugged and tend to improve (up to a point) as they warm up and with thoughtful use, meaning that despite all the legendary engine torque (or perhaps because of it) it’s important to use the gearbox properly and not let the engine labour in top gear at speeds below 35-40mph—or for that matter in any wrong gear.


Check too, where possible, if the bike you’re interested in has sidecar or solo gearing. The difference, as you’d expect, is significant. Solo gearing should have a 26 tooth engine sprocket. Sidecar gearing has 24 teeth.


And forget the ‘pull down the side of a house’ claptrap. Just treat the transmission (which has an integral clutch cush drive to help even out those firing impulses) with care and consideration and leave the demolition to the experts.


Starting is usually fairly straightforward thanks to a half-compression lever on the right-side timing chest (not to be confused with a hand operated valve lifter). However, plenty of retard and a good, firm ‘swing-through’ is essential. When riding, make sure the half-compression lever has been returned or—although the engine will run—you won’t be going anywhere in a hurry.


Cylinder heads are twin-port, but some bikes may have since been fitted with single-port (which requires a slight modification to the push rod tube fitting). The single-port was an option designed to facilitate sidecar attachment (later made redundant following revised sidecar fittings). But, as you might expect, single-port engines tend to be a little noisier and run a little hotter (although Panthers tend to keep their cool overall). The debate about whether single- or twin-port engines are best is, in the true Panther tradition, ongoing. Keep a close watch on cylinder-head port threaded inserts too which are prone to corrosion.




Swinging arm bushes need regular (and copious) greasing. And check for free movement. A certain amount of lateral swinging-arm freeplay is acceptable, but some owners mistakenly over-tighten the spindle and wreck the bearings.


Dampers tend to be reasonably durable, but keep an eye open for leakage and sagging. At the other end of the bike, check for cracks or other damage at the cylinder-to-headstock interface.


One major problem of the Model 120 was over-oiling. Panther’s long established oil-scavenge system incorporates a primitive, but (until the Model 120) effective knife-edge weir in which the flywheels scoop oil from the engine cases into the integral oil tank (wedge shaped chamber forward of the timing chest). The clearance is only 60 thousandths of an inch.



However, when the earlier Model 100’s engine stroke was increased from 100mm to 106m (to create the Model 120), the flywheels were trimmed to provide clearance for the longer stroke piston—which in turn reduced the scavenging  effect and led to over-oiling.


Some owners have extended the oil-weir to rectify this problem, so ask if it’s been done on the bike you’re interested in. Also, some Panthers have been fitted with a Volvo car piston (see Panther Owner’s Club for details) which has a better oil control ring. It’s an easy mod, but a specially machined gudgeon pin bush will be required to facilitate it. Keep in mind, too, that a Volvo piston will also give slightly higher compression with increased risk of pinking. So use the best fuel you can get.


Early main bearing housings were prone to fracturing and necessitated a hasty revision. To help combat this, and to reduce overall engine stress, Panther also revised the engine mounts by replacing the previous U-bolts with four separate studs (the left hand ones via adaptors; the right pair screwing directly into the crankcase). But do check for cracks in this area and for damage to the exposed forward sump.


Other changes include a recessed spark plug and ‘squish’ chamber for improved combustion, and a smaller exhaust valve (Barry M Jones, Panther guru, says the exhaust valve was actually increased in size). The M120 offers a slightly harsher ride than its smaller brother, the 600cc M100. But there are a couple of extra horses on tap by way of recompense.


Watch out for the centre stand too. As many Panther men will tell you, it’s a lot harder to get it up than it is to get it off.


But that’s age for you.






Panther M100 vices and virtues


The Model 100—and for that matter the earlier 498cc Panthers—have much the same vices/virtues as the Model 120, excluding the aforementioned oiling problem. But generally speaking, the M100s are generally considered to have sweeter running engines—and what they give away in torque, they (almost) make up for in pick-up.


Earlier, rigid framed bikes, give a more predictable ride (the Dowty forks notwithstanding)—albeit at the expense of some (but not much) creature comfort. As with the M120, check for cracks, corrosion and alignment between the cylinder head and frame lug—especially if a sidecar has been attached at any time.




This restored 1959 Panther M120 twin-port fetched £5432 when sold in 2010 by Mid American Auctions.




Swinging arm M100s were introduced in 1954 and were improved in 1958. Bikes built during this period have half width brakes front and rear, the rear being a Royal Enfield design (which incorporates a supplementary cush drive). Post 1958, full width hubs were an option.


Dowty (air/oil) forks were fitted between 1947 and 1953, but once again these may have been substituted for later Panther own-make conventional telescopic forks. Keep in mind that the Dowtys are prone to wear and require special care and attention.



Pretty 1958 Panther M100 (600cc) sold by Andy Tiernan of Framlingham, Suffolk. Panthers.are very much an acquired taste, but if you're seriously hankering for one, Andy Tiernan usually has one or two in stock and knows all about their vices and virtues.




Early Model 100s (up to 1956), will have either an Amal 289 or a 376 carb. Later models, will have the 389 monobloc, with or without air filter. Up until 1951, ignition will be by Lucas magneto, charging by separate chain-driven dynamo. Post ‘51, the model switched to Lucas magdyno.


Keep in mind that many, if not most, Panthers will be a mish-mash of models, partly through necessity, and partly simply to improve what’s improvable. Above all else, Panther owners like to keep riding, and if that means adapting, bodging or converting, then so be it.




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