Rebirth of a Lightweight Panther. 8. The Engine

       Upon inspection it was clear that the engine was a bit of a mess. The head and barrel were off and it was very dirty so without more ado, and without really looking at it, I pulled it apart. This was fairly easy as the crankcase was only loosely bolted up. Clearly someone had been inside it as the crankshaft gear was missing. The other timing gears appeared to be ok and the cam followers came out easily.

Engine as received

     With the cases cleaned up I checked the big and little ends for play and found none. Also the timing side bush and the drive side roller bearing appeared to be  in usable condition. I therefore put sealant on the joint faces and gaily went ahead with assembly. WRONG! To my surprise the cases would not close. I was surprised as I hadn't changed anything and I had expected the crankcase would go together easily

    After cleaning off the jointing compound with difficulty I bolted the cases together against three feeler gauges spread around the perimeter, adjusting their thickness until the crank would turn sweetly with a little endfloat. This was achieved with a 70 thou gap between the cases! But how had this come about?

     Possibly a previous owner had fitted a new bush with a thicker flange and gave up when the cases wouldn't close but that's largely irrelevant, the problem was how to cure it. I didn't fancy removing the bush but the drive side roller race looked to be improperly seated so I heated the case in the oven, dropped out the race and reseated it.  This made no appreciable difference. I removed a shim from under the roller race, which luckily was a sliding fit on the crankshaft, again without improvement. Talking to Sherb I decided the only answer was to skim the flywheel faces behind the bearings. I put the crank assembly in the lathe between the 3 jaw chuck and a revolving centre and checked this would turn slowly and safely with the conrod sliding on the saddle. At my slowest speed of 43 rpm I skimmed the faces, 10 thou each side before trying in the cases. It took many goes before the cases closed with a satisfying clunk, giving me between 5 and 10 thou endfloat. I fitted all the bolts and tightened up dry to ensure the endfloat did not disappear before taking it down, cleaning everything off, and using Locktight Bearing Fit on the roller bearing. I then reassembled the cases for the final time, this time with sealant. The crank turns sweetly but I still wonder where that 70 thou came from. I put the engine bottom end into the frame and tightened up all the mounting bolts. The engine still turned over sweetly.

      The cylinder had lost a chunk of the top fin but many years ago I had put some pieces of fin from an old barrel in a drawer and to my surprise I found them straight away. I trimmed a piece to fit and brazed it on. The cylinder is on standard bore and a quick whizz with a hone cleaned it up nicely. I sprayed it with heat proof paint and popped it in the oven to cure.

Repaired fin before painting.

      The piston is of standard size but was dirty and without rings. I found a set of Hepolite rings in a box. These are labelled 350 Stroud but are the correct size. I checked the end gaps at the bottom of the bore and they all came in at 10 thou. I oiled the grooves and fitted the rings before heating the piston with a hot-air gun before sliding the gudgeon pin into the small end and carefully locating the circlips. Retrieving the barrel from the oven and with a new base gasket I bolted the cylinder down onto the crankcase.

Hepolite ring set.

Bottom end in the frame.

  

      I cleaned up the timing gears choosing the best of several heavyweight crankshaft pinions to replace the missing one. After close inspection I am confident all the gears are 20PA although not marked. I then put them into the timing case temporarily, they will need to come out to set the valve timing. The replacement pinion was very tight on the key so I eased that down a bit and the pinion slid on tightly to the shaft.

In fact my confidence re the crankshaft gear was totally misplaced. A document in the POC library shows that 20PA gears were not fitted until 1949 although KE805 material was used the year before and some gears are so marked. 

Ref, POC Library

I turned to the internet to see if there was a way to check the PA simply. The size of Imperial gears is defined by the diametrical pitch, DP. Panther gears have a DP of 20. A table on the internet gives the base pitch for a range of DP and PA values. For a DP of 20 and a PA of 14.5, the base pitch is 0.15208 inches, for 20PA it is 0.14761"

Use a digital caliper and measure across three teeth placing the anvil of the caliper on the tangent to the involute and not on the fillet or across the tooth corners. Note the reading then measure across four teeth. subtract the values  to give the base pitch.

Measure across the teeth about mid-height.

On a crankshaft pinion from a Red Panther bottom end I had on the shelf I got the following values. Over 3 teeth,  0.552" over 4, 0.400" difference  0.152" = O.152" = 14.5PA. For the late heavyweight pinion, taking similar measurements on the teeth, 3 teeth 0.5475", 4 teeth 0.3995" difference, 0.148"  close to 0.1476" = 20PA.

Close comparison of the gears shows that the 14.5PA gears are thinner at the root than 20PA.

14.5PA top, 20PA bottom

Gears may be found stamped 805 14.5PA. These were probably made as spares for early machines after the change to 20PA.

I removed the heavyweight gear and fitted that from the Red Panther engine.

Timing gears and oil feed pipes

Timing gears assembled in the case.

       The piston was set to top dead centre using a dial gauge on the piston crown and a timing disc was fitted to the drive side crankshaft. The engine was turned backwards to 10 degrees before TDC and the camshaft fitted so that the inlet cam was just beginning to lift the follower. As the cam is one piece for both inlet and exhaust all other settings follow naturally. I checked them anyway and they conformed more or less to the P&M data sheet I was using. This sorted the valve timing so the engine was turned a complete revolution to set the ignition timing at 38 degrees before TDC on the compression stroke. The ignition cam cannot be fitted until the timing cover is in place.

P&M Data Sheet

With the timing cover tight against a new gasket I fitted the cam to the camshaft and set it to just open the points on full advance, using a resistance meter to confirm the exact opening of the points. I then turned to the head.

The valves were ground in and the head painted before bolting it down onto a copper gasket. I had no pushrods for this engine although there were a pair in my other engine I could use as patterns. I had a spare pair from a heavyweight so I removed one end of each of these and cut the tube to the length set by the pattern. These tubes were of aluminium alloy, some earlier pushrods used steel tubing. The rockers are very similar to those in a late heavyweight engine, in fact the rocker spindles are identical.

The exhaust valve lifter and spoke are almost identical to those in a heavyweight engine although the arm is slightly shorter. I found a cable and installed that but it is poorly situated, coming up against the underside of the fuel tank. The engine now just needs an advance/retard cable and a lead to the points from the ignition coil.  Also a carburetter must be sourced and jetted correctly.

Completed engine.