I have not yet fitted one to a BSA preunit twin, so I cannot specifically say, But on any bike thats a rider or one of my keepers I am a big fan of a Oil filter and a Oil pressure gauge. ( dont wish to get into an oil debate, but modern oils behave much differently than the oils in the marketplace back in the 1940s to early 1960s) Used to be my online imaginary friend (Friends for 10 years but never met in person) was Gerry Bristow (RIP) who worked for many years for Duckhams and later Castrol oils and would insert some common sense into the many oil wars that would rage in online forums.
But I HAVE fitted Liquid filled oil gauges to many British bikes and I believe IMHO that it makes you a better owner and rider when fitted. It reminds you to properly warm the engine, and to be sensitive to its needs with a very visual amount of feedback. On a Triumph plodder I will service and reuse the standard Triumph plunger stock pumps. With anything hinting at extreme service or use,,I fit high volume 4 valved late style pumps, or the Morgo aftermarket pumps.
On all my chevys (and I used to build a lot of hot rod chevy motors) I always used without fail a Melling high volume pump. Remember,, You want VOLUME,, NOT Pressure. High pressure is indicative of high friction or resistance. You want adequate film strength and appropriate amount of flow moving to cool AND lubricate. (Not TOO much mind you).
So, a Typical Triumph twin (500-650-750) or Norton Twin (500-600-650-750-850) With a modern 20-50 multi grade or (10-30w in winter) I would expect the cold pressure to peg around 70-80 PSI on startup and first few minutes (Which is why warm up and gentle use is critical),, Normal cruising speeds and high idles (momentary stops) would see avgs around 20-30 PSI, and when fully hot and on anything other than frigid winter temps at Idle barely ticking over a Triumph or Norton can flicker as low as 5 psi and typically around 7-8 psi. Keep in mind a well sorted machine should be able to idle at very low RPMS.
I use a braided stainless line, or for stock appearances use a black fibre weave material for German and European cars for lines... I run a Brass T fitting hidden and one side goes to the gauge, and the other to a sensor for an Idiot light. I use off the shelf US industrial or commercial automotive grade sensors. I typically pick one that triggers at 7 psi but you can buy any spec you want. (Pressure point) The gauges are typically aftermarket gauges for HD and other bike applications and pick one that goes to 100PSI as some only go to 40-60- or 80 and if you overclock them, they are never accurate again.. The one problem is they typically dont have back lighting. (I have a fix for that). On a stock Triumph Bonnie or TR6 with Smiths tacho and Speedo they can be attractively mounted between the gauges and not stick out like a sore thumb. Nortons we always typically made a Alloy plate out of 1/4" or 1/2" T6 plate.
For Triumphs I ran the gauges off the timing covers mostly except a few I tapped into the pressure relief valve ports,, For a Norton I ran double banjos both sides off the heads and ran the feed off the 2nd banjo. For a BSA Preunit twin I would have to carefully study where I would tap in,, But on my keeper projects, You bet,.. A oil gauge WILL be fitted. I will work to make sure its not out of place or obviously intrusive. Im okay with a hot motor and the idiot light flickering at Idle,, tells me its working. But I know personally riding, I was glad to have the gauge, and I know of several shop customers back when I ran one who felt the gauge saved their motors.
It is beyond the scope of this forum, but worth thinking about,,, On many of these old plodders its well beyond anything to consider, but in a number of race engines and several Hot rod projects we used back in the 1980s and 1990s when I was more active,, There's a company called MOROSO and they supply the street rod and racing trade and they used to stock oil system accumulators. These are std fair in a wide variety of aerospace applications and coming from that career field they were very familiar to me. The Accumulators have various capacities depending on model (Volume) But how they work is one side of the accumulator is a charged chamber with pressurized Nitrogen gas. The other side has a reservoir of oil and we plumbed these into the main oil gallery feeding the mains and rods. At high RPMS,, High G cornering, or catastrophic oil loss (Holed sump) the accumulator toggles a pressure sender and Idiot lights go off to warn the driver, But you have a safety setting in place that if Oil drops below X value in pressure the accumulator feeds oil into the system so no lapses occur. After exiting the chicane at the end of Turn 9 at PIR the oil pressure returns to normal from normal feed from the pump and accumulator recharges. Again in the next High G event, instead of cavitating and air in the system the accumulator steps in again & again. Oil related engine failures are almost unheard of, or non-existent. If you spend a lot of time rebuilding engines, many people tend to be a bit paranoid about things like over temp and oil failures.
Ignorance is NOT bliss... I have 2 friends who have crashed motorcycles because they were so distracted by noises or monitoring the engines/drivetrains they were not adequately paying attention to riding safely and both cases exited the road ways in a very embarrassing manner. Both of these guys were very experienced riders and not their first rodeo. I will admit to 3 different very close calls myself over the years as well.
