Panther 1935 Model 50. 7. The Engine

 As noted before this 500cc engine was incomplete and badly mauled. The crankcases and rocker box are the only parts I can reuse and the former needs some work.

On the timing side the lug that holds the dynamo chain cover had been sawn off. I carved one from a piece of aluminium and screwed it to the case, finishing off with some aluminium solder. The magneto drive bush was badly worn so this was replaced.

Missing lug and broken out mag stud holes.

I made two stepped studs for the magneto fittings on the drive side case where the holes were badly damaged. These were tapped out oversize and screwed in place. The seating for the U-bolt saddle was rather mashed and uneven; this was filed flat.

The original flywheels were scrap as they were cracked between the two tapered holes. I considered them irreplaceable, or at best very hard to find as they have a throw of 90mm for this 500 cc motor. I decided to build it as a 600cc and use flywheels from the M100 wreck that I had previously rebuilt. Although rusty they cleaned up well. They also had balance holes in what appeared to be logical places, unlike other pairs of flywheels I had to hand.

Usable flywheels? Of course.

I dismantled the rusty flywheels and soaked them in vinegar to kill the rust before wire brushing and checking for cracks. The shafts were removed; the timing side shaft is shorter on this engine compared to the later motors so I fitted that from the Model 50 flywheels. The driveside shaft was damaged and replaced with a new one.With clean flywheels I fitted the new big end pin without the conrod to check for endfloat and found that the cases would not close. This I expected as the pin has slightly oversize tapers to allow for stretch in the flywheels. I machined the faces of the wheels around the bearing bosses to achieve an endfloat of about 20 thou before taking it down again.

The bigend race was pressed into a standard M100 conrod. (The original M50 rod is the long type for a two ring piston and I intend to use a three-ring slipper piston. The original rod is also scrap as the race is so loose it just drops out.) The bearing was then assembled and turns sweetly with no tight spots. The flywheels were assembled, the nuts torqued up  and the lock washers bent against the nuts. I had come across a stand for truing the crank and this proved to be ideal. With judicious belts of a heavy copper mallet I felt satisfied with a run-out of 2 thou.

Crankshaft on the truing jig.

Although as found this engine had two ball races on the crankshaft I decided to fit single-lipped rollers as specified in later Panther engines.These MRJA11/8 bearings can be very expensive but I found some on the internet made by NKE of Austria. The use of cheaper bearings brought forth much lip-sucking in engineering friends but they look fine although the true test is in how they perform. Time will tell; certainly their website looks quite impressive but that is of course no guide. 

The driveside case was heated to drop out the old race and fit the new. On the timing side the race just fell out so the new one was fitted with Locktight bearing fit. With the cases closed and tight an endfloat of 12 thou was obtained and the crankshaft rotated quite sweetly, dropping  to neutral under its own weight so all seems well.

With the bottom end in its cradle I made up the missing engine studs in stainless before final assembly with sealant, I also bent up some new U-bolts in 3/8" stainless.

Bottom end in its cradle.
Note the new lug at the top.

I put the assembled bottom end into the frame and this allowed me to assemble the clutch and primary drive starting with the chaincase back. The one that came with the bike is for the post 1935 crankcase where a large opening in the chaincase fits over the boss on the crankcase. This early crankcase has no such boss so I turned up an aluminium adaptor, fitted it with an oil seal and screwed it to the chaincase with sealant. Normally these cases are bolted to a lug on the engine but here the lug has been broken off so I plugged the redundant hole with a nut and bolt. Finally the rear case was bolted back to the engine plates.

 

Chaincase rear with adaptor

Adaptor from the back

The clutch was then assembled and the engine sprocket (26 teeth for solo gearing) and primary chain fitted.

The slipper piston I had to hand was +30 so I chose the best cylinder I had and got this rebored to suit. With new rings gapped to 10 thou the piston and cylinder were fitted to the cankcase and bolted down with spacer tubes on the U-bolts so I could turn the engine over without the cylinder lifting.

The reprofiled cam followers eventually arrived (thanks Foxy) and I could assemble the timing chest and time the engine. I rigged up dial gauges on the piston crown and on the inlet pushrod and bolted a timing disc to the engine sprocket. These come with two handy threaded holes for a puller but have a secondary use here. A wire pointer provided a reference and top dead centre could be found with accuracy.

Timing disc and dial gauges

Dial gauges

The book gives valve timings for this engine starting with inlet opening 10 degrees before tdc. I set the cam to this and put the crankshaft pinion on after, using the keyway which gave the best fit to the camwheel teeth. The two keyways give you half a tooth leeway. I checked the rest of the valve timings and they were miles out. I tried again using the inlet setting for later engines, 25 degrees before tdc and everything fell into place, with all the other timings close to published figures.

I had no magneto with the bike and approached a friend who builds magnetos and has a vast stock. He offered to build me a BTH from bits and soon wished he hadn't. Apparently there are three different lengths of armatures in these instruments and he rewound the wrong ones twice. Eventually though he produced a very smart BTH magneto; not cheap but showing a very healthy spark. This needed.a 10mm thick mounting plate and another friend made one for me on his mill, copying one off my Dowty rigid. He made a lovely job of it but sadly it didn't fit as my pattern was for a Lucas mag and the mounting holes were in the wrong place. I set to with a lump of 12mm aluminium plate, the lathe, angle grinder, hacksaw and files and made one that does fit. I could't bear to ask him to make another.

The BTH has a small taper and I didn't have the magic POC adjustable coupling so had to revert to adjusting the timing on the taper. I timed the magneto to TDC at full retard and when checked on full advance gave 38 degrees before TDC. This is the quoted figure for 1935 but P&M reduced this to 35 degrees after some big-end failures. This though is controllable at the handlebar lever. Later I replaced the dog with the adjustable type which makes timing the ignition so much easier.

I now turned to the cylinder head and rocker box.

The head that came with this machine is the wrong one, and is for an earlier model. I have passed this on. I had the right head to hand and another one on the spare engine I have for my 1936 M100. I also had two rocker boxes, one that I had sold to the previous owner of this bike, this has come back, and one along with the head on the spare engine. I had a good look at both.

Both heads were fine so I cleaned up one, painted it and fitted the valves. These are open and use large diameter springs and associated fittings. I had most of this but was short of one spring seat so I turned one up.

Cylinder head.

Both rocker boxes were ok but the shafts and rockers less so. The push rod end rocker is part of the shaft but the valve end is separate, it is keyed to the shaft and retained with a nut and spring washer. The shafts are hardened and tend to break away at the keyway. I had a mixture of valve end rockers, three late type and an early one which has a screwed tappet adjuster, this runs against the valve. As this engine will have adjustable tappets at the bottom of the pushrods I need the later type rockers without an adjuster. One of the pushrod end rockers has been broken and welded so my choice is narrowed as I don't want to use that.

Rocker box

Broken keyway on rocker shaft

Welded rocker, pushrod end.

Rocker parts, note early type adjuster

Picking all the best bits I assembled the rocker box. I had no pushrods so took those from my spare engine and fitted head and rockers to the cylinder. 

One part I didn't have was the oil filler cap which sits on the rocker box but I recalled that I had given a broken one to Sherb when he bought my 1933 M100. He generously sent it back and I fitted a new threaded part which I had clumsily broken off years ago. This completed the build of the motor so I filled it with straight 40 oil, fitted a temporary exhaust and a small fuel tank and fired it up. It started easily and ran well so I could now sort out the final stages of the build.

Oil filler cap

The completed motor.