Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Another A65 oil pressure issue

Uh,......... Dave?
Your Trident feeds oil to the rods through TWO plain bearings. (Which are basically a type of split bushing.)

Rod bearings would be my guess. If the clearance is big the bearing is riding a thick wedge of oil. When there is no load it rides it evenly, when you apply just a little load it pushes it down on every power stroke pushing that wedge out to a degree. So there is less resistance for oil going through the bearing, and the pressure falls on the lightest throttle or goes back up when backing it off. When hot it's more pronounced because the oil is thinner.

My Firebird does it and it's end fed in roller bearings. It runs high oil pressure and the throttle reduces it when hot by 10psi usually 75-65psi it depends on the temp of the day and how hard it's working. It doesn't rattle on the bearings which can be heard in worn out cars rapping as you go on and off throttle. I intend putting new rods in it even though it runs perfectly. And I can flexi gauge the old and new bearing clearances and let people know.

Without a gauge you wouldn't know, I just figure it's indicating some more rod bearing clearance than other motors. The bush may play into that but it's not as heavily loaded as the rod bearings.

As do most all British engines. The triples just have extra plain bearings on the crank, it still feeds oil to the crank through the end, a process which many BSA owners pay a lot of money to implement.

Alternator on one end drive sprocket on the other, just the feeds through the mains. But it's well done and pretty trouble free with T160 size feed hose and fittings. They run decent pressure. The small T150 feed hose can starve them on very cold days by causing the pump to cavitate because it cannot pull the cold oil through the restricted hose. The bigger hose fixes that and worth doing at a rebuild. Less Williams used to sell kits.

I’d be asking who built those engines on the 3 bikes you own(ed). I’d also be asking who’s done what to the oprv? I could be wrong but I’m sure the blow off pressure on the triple is 70psi same as the A70.

Thing is, if the thing is working then why piss about with it? Unless the pressure is on it’s arse then I’d be more concerned about oil flow. The big ends could be grinding away and blocking the bleed holes on the crank with shell metal. But all is fine as at the oil pressure gauge it shows great pressure as it can’t flow and the rest is being dumped by the oprv to the sump.

Whilst a seemingly wild concept, it’s also pretty darn true.

In answer to some of the questions, it has a stainless steel SRM pressure release valve, which seems to work fine with no evidence of sticking. I also have the original manufacture release valve and both seem to show much the same behaviour. The timing side bush has gone and been replaced with an end feed system.

Pretty sure it will be rod bearings. If worn ones make a rap noise going on and off blipping the throttle it's because the load on them changes. If those motors had an oil gauge I'm sure the oil pressure would be changing. Some people with cars chase that phenomenon endlessly. A light would not show it.

I have good gauges on 3 A65s only one of the motors does that and all are roller mains and end fed. My 650 with std rods does exactly what you describe 10lb change going on and off throttle when hot. You could try a different gauge I guess. But one of the others has the same type gauge and doesn't do it at all.

The pressure relief is not effected by throttle being on or off. It's reversing load on the rods.

I'll be taking things apart in the near future with a view to investigating an end float issue. The big end bearings have something like 35k miles on them since the last overhaul involving a crank regrind. The oil pump has done well in excess of 65k miles but still delivers good pressure. The bike seems to run fine without issue doing typically a few hundred miles each week.

I couldn't say I'm 100% sure that pressure change is rod bearings. The guy with a hot car was doing everything to find what was doing it. pumps everything, Including moving where the gauge picked up oil pressure. It was a freshly rebuilt motor according to the seller. I'm just guessing. And come summer I should be able to pull the offender I have down and see if that's why.

As for the trident, oil is fed via the mains, they are split bushes like a car.

https://imgur.com/mke2nTq