Wednesday 20 May 2020

Repairing the Crankcases

Upon stripping the engine and vapour blasting the crankcases, the castings were found to be in very good condition except for the 3 pillars that support the bearing housing for the camshaft/magneto drive. These pillars, or what remained of them and which are part of the main timing-side crankcase casting, were effectively useless. One was just about intact, one was partially destroyed and the last one was completely destroyed and someone had tried tapping a hole “by eye” into the crankcase with disastrous results. The plan here was to machine these back to good material, fill in holes with lumiweld, machine a new base for a new tripod-like bearing housing and retap to take 5/16” high tensile studs.

The picture below shows the timing side crankcase on the milling machine at the start of machining.

The next picture shows the casting after the pillars have been machined back and with the historical attempts at redrilling and tapping now filled with lumiweld. I have inserted a dummy steel cylinder into the bearing housing to aid setup and to help avoid any distortion during heating/cooling.
Note what appears to be knurling around the bearing housing. I have found similar knurling on AJS K7 and K10 big end bearings and gearbox layshaft bearings. The bearing interference fits that AJS used seem to be a traditional .001” - .002”  – which works fine – so it is not clear to me why AJS did this “belt and braces” bearing retention as it seems unnecessary. Anyway, it’s a real nuisance, as it prevents the bearing being removed easily and the effective peening-over of the aluminium either needs to be removed carefully or the bearing cut out by a competent spark-eroder before the bearing can be taken out without damage to the housing.

Finally, the base for a new outrigger bearing housing can be machined on the rotary table and the holes tapped to accept retaining studs. A lip has also been machined so that the new bearing housing will positively locate concentrically.

The next task is to make the crankshaft pinion and the driven camshaft gear.

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