Topic: oil pressure question for the engineers...

[paulages]

i've been experiencing frustration in regards to potential low oil pressure problems with my '71 BSA thunderbolt. the A65 engine is infamous for its timing-side crank bushing, through which the entire bottom end is fed its oil. i frequent a BSA forum at britbike.com, where this topic has been discussed. however, i'm unsatisfied with a few questions, and i've found a few people on this forum to be a little better in the critical thinking dept.

the oil system works as such: there is an oil tank, which supplies oil to the oil pump, which sends to a main oil gallery. in this oil gallery is the oil pressure relief valve, which open at 50 PSI, returning oil to the tank to relieve pressure. also in this first gallery (on later models) is a port for a pressure switch, which switches a warning light at 5 PSI. beyond this is the crank main bush, then the rod big ends, then small holes which spray the cylinder walls. the other end of the crank is supported by a roller bearing.

so my questions is this: if a gauge is installed in the pressure switch port (between the pump and the first exit or pressure drop), how accurate will it be, and does the orifice size at which it reads matter? it is technically a dynamic system, so i don't know if the orifice size within this gallery will impact the reading.

this problem i 'm having is this: when cold, the gauge reads 50 PSI, and the light goes off. when it gets nice and hot, the light comes on well above idle, and it doesn't read   above 10 PSI on matter how high i rev. the BSA guys say to unplug the light and forget about it, but i can't help but be worried. i got this bike with a blown engine, obviously due to poor oiling. i don't want to see it happen again.

engineer guys??

[mystic_1]

Well, I'm not an engineer, but I do know that on CB750s its common to run a pressure gauge off the right side oil gallery access port above the points cover.  Those are plumbed through 1/8 NPT fittings.  Automotive oil pressure gauges often sit at the end of long delivery lines that have quite small internal diameters.

What gauge are you using and how do you have it hooked up?  Sounds like you have both the stock pressure switch and a gauge fitted.

mystic_1

[Bodi]

This sounds like a worn engine or oil pump. I'm not familiar with the BSA system, but that's the story for Hondas with low oil pressure at worikng temperature. What type of pump does it use?
If it can be inspected and service limits are published, you should do that first - a new oil pump is probably available (almost every part for these old Brit bikes is being made somewhere) and would cost a lot less than repacement bearings and such.
Check the bypass valve, most types can stick open slightly: with thick oil the leakage is trivial but hot thin oil will whiz through a small opening. Maybe you can disconnect the bypass line and see if there's flow when your motor is hot, obviously there should not be a any if your pressure is only 10 psi.
As far as the gauge goes, you can tee off the switch port for a gauge. The hole doesn't have to be very large at all since there's negligible flow to either unit. You get some flow when the switch diaphragm and gauge bourdon tube move, but minuscule. There will be some flow on pressure changes due to the trapped air compressing, again minuscule. It's common to use a "snubber" on gauge lines, this is a very small orifice that damps out pressure pulsations eliminating needle bounce to allow accurate reading - and make the gauge last longer. A pulsating needle means the tiny gears inside the gauge are working like mad, and they will wear their teeth off pretty quickly doing that.
The RPM v. pressure ratio tells you when stuff is wearing out. I don't know the "normal" situation with a BSA. Honda SOHC4 engines should make around 50 psi at highway speed, but are fine making 5-10 psi idling when really hot. The Honda oil pumps bypass at 60psi but with cold oil the bypass port isn't big enough to allow enough flow and gallery pressure can get up to 90 psi or so if revved with cold oil.

[bryanj]

Hi Mate years ago I ran a Triumph pre-unit 650 for a long time with only 5 psi showing on the gaugee whwn reveed hot and sod all at tickover---took the gauge off!!! It siezed a piston one freezing cold day but bottom end never went.
The expalanation i was given at the time by a lecturer at college where i did my OND in engineering was that as long as there is oil flow the spinning og the crank will increase the pressure at the shell bearings so that the big ends will cope.

As to the timing side bush its best to do a "Devimead" bearing conversion but with carriage that may be a bit pricey for you as they are in Wales UK

Secondly DO NOT use the aftermarket bronze bushes as they wear the crank get the original steel outer , white metal sleeved bush which used to be available in specified undersizes--sometimes you could even get the liner seperate!!

[paulages]

thanks guys. i should have mentioned that this is a fresh rebuild, with new bushes throughout.

bryan- yeah, i wish i could have done the roller bearing conversion, but man is it pricey. also, i had to go .050" under on the main bush, which BSA never offered. this engine was is close to its last rebuild with the parts that are in it, but i did everything properly and clearances shouldn't be an issue.

i am going to try to shut off the PRV and see what pressures i see then. my main question was in regard to the switch port and the orifice size feeding this cavity. some seem to think that poor machining causes false readings here (i did in fact tee off of this joint for the gauge and the switch). in my mind, though it isn't a totally closed system, the size of the orifice supplying oil to the switch/gauge shouldn't matter as long as it's on the pressurized side.

[bryanj]

Plz excuse dyslexic keyboaed, i type these at stupid o'clock by the light from the monitor so as not to wake the dearly beloved!!! then i need glasses to see the screen but cant see the keyboaed with them on---Hey Ho getting more geriatric daily!!!.

Back to the question, size of oriface wont alter pressure just the flow rate so readings will be accurate,
Cant remember is the BSA a gear pump? if so re-face the end plate to possibly increase eficiency

[kghost]

Couple thoughts from an Aviation perspective...

No the orfice doesn't restrict pressure readings. Just flow.

Often times the fitting tapping the line off a galley on aircraft engines is filled with Bronze then a very small hole is drilled thru the bronze. Keeps all the oil from pumping overboard if the line breaks.

In Winter or cold temps...it would take a while for pressure to show on the gauge due to the restriction.

Filling the line with Kerosene sped up the oil press. indications on a cold day.

Obviously not your problem with the BSA.......

Anyways.....

Assuming the gauge is accurate...as is the light.

I'd be suspicious of the bypass.

Sounds like it opens when the engine is cold.

Are you sure its closing completely when the engine warms up?

Watching the gauge as the engine warms up would be educational.

Does the pressure drop at a linear rate in relation to the rising temp?

Does a change in Oil viscosity make a difference? (assuming 30 or 40 Wt....does 50 wt make a difference?)

General rule of thumb with plain bearing engines such as the SOHC is...10 Psi for every 1000RPM increasing to the maximum of 60 or thereabouts (57 sticks in my brain for the 750)

[paulages]

kghost-

yes, the pressure drop clearly follows rising temperatures. i had nearly the same problem on my cb550 after building the engine up, and a stiffer spring in the PRV resulted in 70 PSI cold and 30 at idle/ 70 under load warm. i'll play around with the PRV spring and see what happens.

[kghost]

Yup.

I had that problem on a 750...

Pump spec'd out fine. All clearances listed in any publications measured within tolerances.

Weak PRV spring would'nt seal it off.

Very fustrating.

Stay after it

[crazypj]

best thing to do is the quill feed from end of crank.
 I didnt know Devimead were marketing it?
SRM were  originally in Penarth before moving to Aberystwyth  (that dates me  )
http://www.srmclassicbikes.com/
http://www.srmclassicbikes.com/location-and-map
 I would modify timing cover and crank end then plug feed holes in main journal, keep stock bushing until it wears out.
 feed line is clear?
 Had a C15 with 'crimped' oil feed pipe, under the rubber, restricted flow enough to blow out big ends in 1,500 miles

PJ