PANTHER SPANNERING
For a thousand miles, Nick Adams’ 650 Panther ran flawlessly. And then the oil started seeping – and so did his ongoing struggle to stem the tide…
For a thousand miles, Nick Adams’ 650 Panther ran flawlessly. And then the oil started seeping – and so did his ongoing struggle to stem the tide…
I’ve been fighting with my Panther ever since I got it. Panther? Not those fierce, desperately cute black leopards that terrify you even though you want to give them a hug. The Panther I mean is hard metal. I wrote about it in RC170 shortly after I’d acquired it from a friend. Astonishingly we’re still friends even though, truth be told, the Panther has been a little vexing. This is somewhat surprising since the Panther is ‘The Perfected Motorcycle’ (note the cunning use of P & M), a slogan vigorously supported by the international Panther Owners’ Club whose members frequently refer to ‘lesser marques’ when describing non-panther motorbikes.
By the late 1960s, when I first became interested in motorbikes, Panthers had a well-established reputation as basic, robust, reliable old sloggers – the kind that cloth-capped plumbers and window cleaners might use to drag around a sidecar box full of tools from job to job, and young families could use for their annual trip to the seaside, the kids stuffed into a huge double sidecar.
For kids like me, lusting after Norton Dominators and BSA Spitfires, old sloggers like the Panther were hardly the stuff of dreams, but as families abandoned the combo in favour of Minis and Anglias, they were cheap, available and generally reliable as long as you didn’t expect blistering performance.
My own love affair with Panthers started with a 1950 Model M100 Redwing. This was a 600cc single with no rear suspension but with sophisticated Dowty Oleomatic air forks. Despite the abuse I subjected it to, it carried me for many happy miles until I abandoned it one rainy day along a Scottish roadside. Shortly after, it was replaced with a 1964 Model M120. This was the ‘big one’. 650cc of thumping single cylinder power which, as
legend would have it, fired at every lamp-post and could drag a heavy sidecar up the side of a house.
Then, all of a sudden, a mere fifty years later, I had another one. Initially everything was rosy. Within a few days of getting the bike home I was happily riding the back roads, relishing the delightful boom from the twin silencers and the tractor-like pulses from the big, lazy engine. Obviously, it’s not a bike for major arterial highways, but it can plug along at 5560mph quite happily.
For the first thousand miles everything went smoothly. It was comfortable, fun to ride and even the handling and brakes performed acceptably, as long as I remembered I was riding a 60 year-old bike, designed in the first part of the 20th century. Then, one day, as I was chugging along one of my favourite roads, returning from a lovely 100 mile ride, I suddenly noticed that the whole outside of the engine was slathered in oil. Up to this point, the Panther had been remarkably oil-tight, with only the occasional drip from the primary chain-case to remind me that it was, after all, a British motorcycle of a certain vintage.
I was horrified. Not only was most of the oil on the outside, but it was clear that so much had made its way into the valve cover that it was being forced past the valve guides, into the combustion chamber and ejected as thick grey smoke through the exhausts.
I checked the oil level. There was still enough in the sump so I carefully limped home, my right boot shiny with oil dripping from the pushrod tube. Clearly, something was causing excess pressure to build up inside the engine, forcing the oil out through the gaskets, seals and O-rings which had hitherto been keeping the oil where it was supposed to be. Why the deluge had happened so suddenly and what had caused it was a mystery. I had no choice but to start a long, tedious and unsettling process of investigation, analysis and bewilderment.
My grubby little Panther rider’s handbook suggested that the oil pump had a rudimentary method of adjusting the flow of oil. It consists of a hole in the oil pump, a ball bearing, a spring and a screw-cap to hold the spring in place. Tightening or loosening the screw increases or decreases tension on the spring, either increasing or decreasing the pressure at which oil gets to squeeze past the ball bearing. Cool, thought I. I’ll just adjust the
screw-thingy and that will cure the problem.
No such luck. No matter what I tried, it didn’t make a scrap of difference. As soon as the engine was warm, the oil would start to ooze.
In response to my requests for help on the Panther Owners’ Facebook forum, I was told that my flappy valve was almost certainly stuck. ‘I have a flappy valve?’ I thought, wondering whether they were talking about the bike or my heart, and set about finding it.
The big nut that holds the drive sprocket onto the end of the crankshaft is usually hidden behind the primary chaincase. The nut contains a little disc of metal, about the size of a sixpence, which is held loosely in place by a split pin. When the piston is rising, the disc is sucked against a flange so air can’t enter the crankcase. When the piston descends, the disk is pushed off the flange allowing crankcase pressure to escape. The idea is that if the air can escape, crankcase pressure will be relieved and the oil won’t be forced out of every seal and orifice like mine.
So I investigated the flappy valve. I cleaned it. I checked it. I even took the nut off and sucked and blew just to make sure it was working properly. I put it all back together and like an oil tanker stranded on the rocks – oil everywhere again. It was time to go in for a coffee and a think.
That think lasted quite a while. I had other bikes to ride and places to go. Weeks passed before I was back out in the garage
pondering the Panther. While idly investigating potential sources of the problem I kept myself busy by making sure all the exterior nuts and bolts were fully tightened.
The Panther’s cylinder head is attached to the rest of the bike by four long bolts which thread into the crankcase and two smaller ones which protrude between the fins at the front and rear of the engine. I gave them all a tweak. The ones on the right side and at the front and rear snugged up nicely, but as I tightened those on the left side I could tell that the steel studs were working their way out of the aluminium case. Arghhh. That is not what I wanted.
A few miles from where I live resides a man with special machining skills. Could he put some new, threaded inserts into the crankcase? He could? Cool. So I stripped the engine, removed it from the frame, carted it over and more-or-less forgot about it over the winter.
Eventually spring arrived and with it thoughts of chuffing along pleasant back roads on the Panther, so retrieving the engine, I hurried home and stuffed it back in the frame.
This is going to sound too good to be true, because old, single cylinder bikes can be difficult to start, especially when they’ve just been in pieces, but on the very first kick, the bike roared into life and, after a couple of minutes, settled into a nice steady idle. I let it sit, chugging away quietly.
At first all was well. Even as the engine warmed up there were no signs of oil leaks, but as soon as I had ridden a couple of miles, the top of the engine case was awash in oil dripping steadily from the pushrod tunnel. I’m not usually given to flinging tools across the garage or other external signs of annoyance and disappointment, but I have to admit, a few choice words might have been heard, had anyone been within earshot.
Once again I queried the knowledgeable folks on the Panther forum and combed the internet for any clues to explain the excess crankcase pressure that was clearly at the root of the problem.
‘There must be too much blow-by past the piston rings,’ wise and experienced Panther gurus advised me. I knew that the piston and rings were new and had been fitted to the cylinder before I got the bike. It had travelled less than a hundred miles before I got my hands on it and still only had just over a thousand. Oh well, I’d better have a look.
I pulled off the fuel tank, loosened the headsteady, removed the valve gear, unbolted the cylinder head, and, with a little jiggling and loosening of engine bolts managed to slide the barrel off the piston.
Hmm. That’s curious! I could see the tiniest
scoring on the cylinder walls and piston skirt but, more worryingly, the piston wasn’t moving freely on the gudgeon pin. Why on earth folks in the UK call it a gudgeon pin is beyond me. A gudgeon is a small freshwater fish which looks nothing like the tube of shiny metal that holds the piston to the connecting rod. In other parts of the world it’s called a wrist pin – a far more informative, if less intriguing descriptor.
Weird terminology notwithstanding, my gudgeon wasn’t doing its job. The piston should have been able to flop smoothly on the end of the conrod. Mine would move but it was stiff. Perhaps the stiffness was causing the piston to angle slightly in the bore, allowing gases to escape past the rings and into the crankcase. Although all the engine internals were new, I decided to bite the metaphorical bullet and get a new piston with the cylinder rebored or honed to fit.
Once he’d had a good look at my parts, Karl at the machine shop declared that the old piston was from a Harley Shovelhead. Replacements were readily available. Did I want the cheaper cast version or lay out the big bucks for a Wiseco forged piston?
Under normal circumstances I would probably have opted for the cheapest alternative, but I’d just been reading Des Molloy’s inspirational books ‘The Last Hurrah’ (from China to Holland) and ‘No One Said it Would be Easy’ (in South America) about his monumental travels on his aged Panther, and I was full of ambition to ride my own Panther to exotic and distant parts.
‘Let’s go with the Wiseco, Karl,’ I said, gulping slightly as I thought of the extra $100 that would add to the bill, while desperately hoping that I’d need to worry less about major engine troubles down the line.
Eventually the piston arrived, the barrel was cleaned up to suit and I was ready to put the bike back together. I diligently massaged the gudgeon pin and its mating surfaces until the piston moved smoothly and easily, slid the barrel over the piston rings, jiggled the cylinder head into position, then bolted everything back in place. Once again it was time to hear it run.
The time-tested way to start these big singles is to pull in the decompression lever, which opens the exhaust valve slightly, ease the piston past top-dead-centre, then, with the ignition on (I should mention here that my bike has a coil and Thorspark ignition), give a mighty kick. Once again the bike obliged by starting immediately. Once again, it idled nicely until well warmed up. And once again, oil started working its way past seals and
O-rings as soon as the bike had done a couple of miles. Darn it! What on earth was I missing?
Months passed. Either I was hiding inside avoiding those nasty, spiky Covid bugs or I was out on one of the other bikes enjoying one of the few activities one could engage in with little likelihood of infection. The Panther languished in the corner of the garage until the wet autumn weather started to impede my riding and I’d gradually built up enough enthusiasm to take another crack at it.
Sometimes our minds build relatively simple tasks to monstrous proportions. Taking the Panther to pieces for another look was such a task. In reality, it’s an easy, straightforward motorcycle to disassemble. Tank off (I hadn’t even fully bolted it on since last time), carb off, remove the whacking big castings that hold the top of the engine to the frame, and unscrew and drop the exhaust pipes. Unbolt the valve cover, taking care not to damage the gasket I’d so carefully manufactured out of gasket paper last time around, and remove the rocker arms and pushrods. Unbolt the four big engine bolts and the two smaller ones under the head and pull the head off. It takes a little jiggling to remove the cylinder head but eventually that heavy cast iron lump is lying on the floor.
With the spark plug out, gently rotate the engine until the piston is at the bottom of its stroke and slide the barrel off. Man, it weighs a ton. Stick a rag in the top of the engine case to stop the conrod flopping around, and we’re done. Nothing to it. Why had I turned it into such a major event? It took me less than two hours.
With the engine partially disassembled I
started looking for troubles. The most obvious source of crankcase pressure would be if air was squeezing past the piston rings. This would leave dirty smears on the cylinder wall – but no, the walls were squeaky clean. What else could it be?
The rear of the cylinder has a little oil-feed channel that squirts oil through a tiny hole in the cylinder wall below the piston skirt. Could this be the source of the problem? I measured the height of the hole and compared it with the up and down movement of the piston, but it seemed to be in the right place. Perhaps too much oil was getting through. I decided to block the hole, figuring that the thrashing flywheels would be throwing up more than enough oil mist to keep the cylinder wall properly lubricated.
During one of my many re-assemblies I’d noticed a slight bubbling at the cylinder head gasket. This is a thin copper ring. I’d read that if they have been in use for a while they tend to get stiff and don’t seal well, so I annealed it by throwing it on the kitchen stove, and turning up the heat until it was nice and red. (Yes, my wife was out.)
Everything else seemed to be working properly. The piston moved smoothly and evenly in the barrel, so I put the engine back together, kicked it into life and went for a short ride. Bad idea!
I was less than a mile from my house when I felt the bike slowing. I instantly pulled in the clutch, cut the engine and pulled to the side of the road. After giving it a moment to cool and with the ignition off and the valve lifter open, I tried the kickstart. The piston slid up and down with no nasty noises, so I went through my usual starting routine, turned on the ignition and gave her a spin. Once again the Panther burst into life and sat happily ticking over while I put on my gloves.
Getting home was going to be a trick. There’s a long, steep hill between where I was and where I was heading, so I took it easy, stayed in the lower gears to avoid any lugging and gingerly headed up the hill, my fingers hovering over the clutch lever. The instant I felt the engine balking I squeezed the clutch and let it die. This could turn out to be the longest mile for quite a while.
Another few minutes passed. She started up again with no untoward noises and I was able to crest the hill and coast home without any further trouble. Clearly, thinking I knew more than generations of Phelon & Moore engineers was a delusion. It was time, once again, to strip the engine and open up that oilway again. Oh well, we live and learn.
Saturday morning arrived and with it the time and enthusiasm to once again get to grips with my truculent feline. By now I was a practised hand at taking the Panther to pieces. In less than two hours I had the cylinder head off and the barrel loose on the main engine studs. This time I didn’t remove the barrel – I rotated the engine until the piston was at the top of its travel. This gave me enough space to unblock the oil channel I’d so stupidly blocked off, without sliding the barrel off the piston or disturbing the piston rings.
With the outer primary chaincase off, I scrupulously cleaned and reset the ‘flappy’ valve. Then, making sure the little hole in the base gasket lined up with the oil channel, I stitched the whole thing back together.
I wheeled the bike outside, gave it a kick and to my enormous relief and pleasure, it fired up immediately. No nasty noises, and once it warmed up a bit, a nice steady tickover and all the right mechanical sounds. The flappy valve was doing its thing – puffing air out as the piston descended and sealing up tight as the piston rose. Time to go for a spin.
I left the primary chaincase cover off so I could keep check on the puffing action of the flappy valve and enjoy the risk of grinding my heel on the rotating clutch nuts. My short test circuit takes me through my little rural subdivision, down a hill towards Collins Lake, along the lakeshore road for about a mile, then back up the valley edge on a little unpaved track through the woods. It’s just long enough to get the bike properly warmed up and has enough variety that all the gears get used at various revs. A beaver had caused a small washout at one of the culverts, so I had to navigate a foot-deep gully across the track. I stopped momentarily to think about it, but deciding it was well within the Panther’s capabilities I plugged on.
Back at home in my driveway I was thrilled to find no nasty signs of excessive leakage, a flappy valve that was cheerfully chuffing air from the crankcase and an engine that felt ready for some real road miles. And then…
…it snowed! Oh well. The winter will give me a chance to tidy things up a bit and I may be able to sneak out for some longer exploratory rides. I’m not entirely convinced that the battle is over but I’m determined to be victorious.