Very true, Trevor.
Being as the vast majority of boring I’ve done (as part of my rebuilding work) has been on automotive or marine engines, I’ve always set up on the cylinder / head face. Did that with the few motorcycles I’ve bored as well, when really it would be best to bore those from the cylinder / c’case flange. Just habit, I guess – plus it’s easier. Last one I did (my A10) I came across something I’d never seen before. Cyl was + .020” so I bored to + .040”. First cyl was fine, 2nd one ran off to one side to the extent it didn’t clean up. In hundreds of bores over many decades that was the first time I’d seen that. Clearly whoever bored it previously – and maybe the factory before that, though more unlikely – hadn’t set up properly. Naturally I thought it my fault – perhaps I’d allowed some crud to get between the cyl face and the boring plate, but further checking proved not. Regardless, I had to bore both again, this time to + .060”. Damned nuisance, as I had NOS Hepolite pistons ready. Had to get modern rubbish pistons instead - and + .060” were hard to find.
Whilst that was an extreme example, I think sometimes we expect too much from yesterday’s production lines. People tend to think manufacturers make things as well as they possibly can. Wrong, to do so = more expense = less competitive v other manufacturers = going out of business. The aim is to make as cheaply as possible, ie just good enough. Materials chosen are the cheapest capable of doing the job. Tolerances are as wide as possible, as that takes less time and ensures fewer rejects. For example, we like our conrods and crankshafts clean and polished. When I first worked on an American v8 (car engine) in the 60’s I was surprised to find the crank was so rough I had to be careful not to cut my hands on the flash. Yet that engine ran smooth as silk. Take a look at Jaguar XK conrods – a rather spindly and very rough casting yet those engines run smoothly, perform well and I’ve only ever known one rod exit the crankcase – and that was while racing.
I think many people restoring this type of machinery expect too much and go too far. Different if you’re building a race engine, but entirely unnecessary for ‘cooking’ engines that will usually only henceforth do limited, gentle mileage. Unless you’re fitting a billet crank and Carrillo rods, why work to a tenth when building an engine?
