Mona’s Isle
Great article Flat Twins by Stuart Francis OBA 98 pages 66-72. I was particularly drawn to the wonderful coverage on the 1915 Quirk’s Mona (“Mona’s Isle” Latin for Isle of Man) restoration project by Harvey West. So why am I responding to it? Because Harvey has struck a problem with the cracked aluminium crankcase which had been previously weld-repaired and failed again. Harvey’s final comment is “currently considering what to do next”. Well Harvey let’s have a look at the issue.
First why did it crack in the first place and then again? Either bad design or manufactured from substandard material, or physical damaged or wear and tear. There is a lot of stressful activity going on down there in the bottom half during working hours contributing impacts on parts reciprocating, rotating, vibrating, expanding, friction, distortion and thermal gradients causing cyclical and alternating stresses. For one example, a cyclical stress such as a big end in good nick can combine with a hammering alternating stress after excessive wear. While everything is still holding hands, stresses are transferred to the world of the bottom half causing strain until a spot somewhere says “I’ve had enough”; that’s when the fight starts between stress and strain. Something’s gotta give, and it will. Aluminium has very low damping capacity, unlike cast iron which has very high damping capacity, so the Aluminium crankcase shares all the stress forces to all the locations which don’t want them. The many stressful factors cooped up in a crankcase are trying to get out or rearrange the general layout of the thing. Adding an internal strengthening plate probably antagonise the stress planes. All the above is additional to dimensional distortion caused by the welding. Many times I have examined the shrapnel of a crankcase to determine the start point culprit. Using my magnifying glass, the aluminium crankcase casting looks a bit ordinary but considering its vintage and QC practices of the era it’s not surprising. If the original crack and weld was of that era it was probably infused with oxide ► inclusions
and hydrogen porosity sticking it back together which is why it developed a stress crack again in the same place. Hidden disaster rides again. The aluminium foundry operations around 1915 would have been basic practices using coke-fired crucible melting furnaces and natural chocolate sand loam to make the moulds with oven baked sand and linseed oil cores. The metal was scrounged scrap metal instead of today’s purchased specification ingot. The working conditions were terrible. These practices had predominately disappeared by the 1950s.
What to do? Make a new crankcase casting by one of the many foundries in business today. You could get Neil McMillan to make a pattern and use the foundry he used to cast it, probably in Al601 heattreated. With today’s CAD CAM operated machine shops all over the place it wouldn’t be a problem digitising off the old crankcase for dimensions. I know – I know, I hear you; “Christ I’m not made of money”, but from my experience in these circumstances it will be cheaper, quicker and personally more satisfying in the end for you with your restored bike.