Topic: HELP! low oil pressure light at idle, following engine rebuild

[NickD]

Hi all,
I recently rebuilt the engine on my '73 CB500. New crankshaft bearings, valves, rings, seals, gaskets, and head bolts. I inspected the oil pump prior to reassembly, and found that everything was well within the service manual-specified wear limits. I understand the relationship between the crankshaft bearing wear and oil pressure, which is why I'm so confused. I plastigaged the old crank bearings, and found them to be worn just outside of the manual specs. I replaced them all. Prior to the rebuild, there was no apparent oil pressure problem, despite an oil leak at the head. Now, following all of my hard work, the oil pressure light comes on at idle, after the bike warms up Oil is 10W-40, as it was before the rebuild. Am I missing something? I can't imagine the oil pressure sensor is to blame. The motor was assembled with a good amount of assembly lube, which I would think would increase oil pressure if anything.
Thanks in advance,
-Nick

[BenelliSEI]

Nick....... does it go out as soon as you raise the revs? Maybe a temporary gauge installation would help you understand what the pressure is at different revs? My experience is mostly with SOHC 750’s but they occasionally flash the light at low revs, when the oil is hot.......

[NickD]

Indeed the light goes off as soon as the revs increase. This puts me at ease a bit, but I still don’t understand what has happened to hurt oil pressure between pre and post rebuild. I never saw the oil pressure light illuminated with the motor running before :/

[NickD]

Does anyone know if there is an oil pump priming procedure I should've performed during the motor reassembly?

[bryanj]

Never needed to prime oil pump on a 500, its possible snder unit on pump is faulty.
Did you renew the O ring under the pump cover and if so what size did you use?

[dave500]

im going with the sender unit just get a new one,plenty fail now theyre old.

[bryanj]

One thing that does happen with the500 is when the primary chain wears it eats into the main oil galleries in the lower crankcase and has been known to wear right through which, obviously, means a drop in oil flow/ pressure to the crank. Normally if this happens it is very catastrophic very quickly so i doubt this is your problem but is worth considering.
You can actually feel for damage with the sump pan removed.

[NickD]

Bryan,
I had the cases split, and engine fully disassembled. I can’t imagine I wouldn’t have noticed chain wear like the kind you describe. I sure hope not, anyway. I have ordered a new pressure sensor, though some quick testing of the old one on the bench leads me to believe it’s ok (though I was testing it with pressurized air, and not hot motor oil). I did replace the o ring in the pump cover, using one from a high-quality Japanese gasket set. I pulled the cover off of the pump today, and everything seems to be in order. O ring still seated properly, no scoring inside the pump itself... what’s next?? I will of course install the new sensor, though I don’t want to let my hopes get too high on that front :/

[NickD]

A temporary gauge installation also seems like a good idea

[bryanj]

I dont know of any set that has the correct O ring for the cover which is 46 x 2 mm, any thicker would not let the cover tighten down enough and drop your oil pressure

[NickD]

Thanks for all of the input Bryan, really helpful. I pulled the cover back off this morning to be sure. The O ring measures just under 2mm thick, and everything seems fine regarding that. The o ring appears to be sealing properly, while still allowing the aluminum cover to mate fully with the pump body. I believe the gasket set was by Veshra, or something like that.

[uksparky]

The oil filter......could that be the problem, what brand did you use

[bryanj]

No problem, i have several to rebuild including No 36 in my avatar, plus they were my era w.hen i worked in dealerships.

[NickD]

Oil filter is a genuine Honda filter. I'll pull it off and inspect, but would be very surprised to find anything there :/ . I'm planning to hook up a mechanical gauge today. What sort of numbers should I be looking for? Anyone know at what pressure the sender closes the low oil pressure circuit, if working correctly?

[bryanj]

Not 100% sure but 7psi sticks in my mind

[rotortiller]

The engine has oil control restrictors at the cylinder base.  Those are in place right? 

[NickD]

Thanks for all the input everyone. As many of you suggested,  it does appear that the pressure sensor was the culprit. Strange, given that it seemed to be working properly on the bench. I was, of course, performing those tests with compressed air, and not hot motor oil. After hooking a gauge up to the sender port I got readings of 60psi with the engine cold, dropping to 50psi hot. What a relief.

[dave500]

try the old sender itll probably work again now?the compressed air might've freed it up?

[bryanj]

Was that psi at tickover? If not what was it at tickover.

[NickD]

Bryan,
Is tickover what we yanks call idle? If so, then yes, those numbers are all at tickover, around 900 rpm. The motor is running great after the rebuild and carb tuning, and I even dialed the idle down to ~500rpm for a bit to see how low the oil pressure might drop at super low rpms. Even then I didn't see it go below 40psi.

[BenelliSEI]

Your readings are excellent. Get another sending unit!

[bryanj]

 Yup tthats what i meant, in which case your pump is really good