Hi,
I have never had a vehicle with a by-pass filtration system, so I didn't realise the whole content of your post. Guess I'm probably too young to have experience the by-pass system system as used on road vehicles.
I do however, know that the 500cc Triumph Grand Prix had a external oil filter built in as standard, situated in front of the gearbox, , with regards to take off point presumably, it would have be plumbed on the return side before the oil tank.
Some specialist Clubs have had greater capacity oil tanks designed & fabricated to fit, to give the old motors more of a chance. You can get thermostatically controlled oil-coolers too.
A Guy running a Speed Twin in India has posted on a site... discussing running old bikes in hot climates,.......He writes..."The flow is from the pump to the cooler to the filter to the tank return point, which feeds the rockers. Having the filter at the coolest point of the oil flow means that more debris should settle out... the oil pressure gauge is a cheap after-market car one. I bought it because it harmonised with the speedo colour scheme. It is attached to the standard test point at the lower front of the timing case. It reads 60lbs a few seconds after startup, then when the bike is at operating temperature and cruising it reads about 1lb per mph, so 60lb on the highway, 40lb around town. If it gets near or below 40lb on the highway I know I'm in trouble and near cooking the motor."
With regards to filtration:
Spring '85 issue of Classic Mechanics:
Royce Creasey wrote an article on filtration for motorcycles...The particles causing the most damage are those in the 15-30 micron (0.004in.-0.011in.) range. Wire mesh filters are unlikely to catch these sizes. They are especially poor at catching the long, thin slivers due to their very small cross-sectional area, which happen to be just the thing to bugger gear type oil pumps and jam non-return valves........with regards to Automotive filters he states...... This type of paper filter will stop all particles down to 12 microns, and a diminishing proportion down to 5 microns. They are very good at catching the long thin ones mentioned above, that can do most damage.
In the article it had a photo of a gear shaft lifted from the Ducati and a cam spindle from Royce's Velo. The Ducati bit has covered a claimed 8600 miles, the oil having been 'filtered' through Ducati's then, joke filtration set up. The Velo spindle has run 60,000 miles on his bike, having been fitted as used off another. The Velo. spindle was worn out, but the surface was visibly better then the Ducati item, i.e. properly filtered oil = much lower wear rates, as we would expect.
As for Brand makes, I remember the Purolator brand, had an excellent name as a good upmarket filter.
Citroen 2CV, Peugeot and Talbot filters can be adapted for Norton's not sure about Triumphs, but I suspect they are adaptable.
There's also a lot of talk on the net about the necessity of a good modern filtration system when taking advantage of modern Synthetic oils. Through the changing ingredients in modern oils they behave differently to older purely Mineral-based oils, because modern oils in particular Synthetics keep particles in suspension, through using higher detergent levels; they therefore,( if a good filtration system is not in place), cause the suspended particles to just keep going through your engines components, leading to potentially more engine wear taking place!
However, some people swear by the advantages of using Synthetic oils on old air-cooled motors as the Synth's have a lower potential to disintegrate at high temperatures, but they have to be used as stated with a modern filtration system. If you can stomach a twitching needle, I think an oil pressure gauge is a good safety device for covering most situations when on the road, particularly when running engines of the kind of almost 70 year old vintage we are talking about.
Somewhere I have what was my Uncles 1940s Pitman's Book of Triumph, with all the technical diagrams in it, model by model, but at the moment I can't find it!
Cheers
JBW.