Topic: Low oil pressure - what's dangerous?

[Aceon]

Does your crankshaft have all the steel ball bearing oil plugs in place..?

If you mean the big rubber seals, then yes. Not sure what you're referring to if not.

What are your oil pump measurements?

How do they compare with what Honda wants…?

Looks like you had some hard pieces go through…?

What are your rod and main bearing clearance..?

It’s hard for everyone to arm chair diagnosis without critical clearances…

I can grab the measurements tonight, it definitely does look like some chunks went thru.

Unfortunately I didn't measure the rod and main bearing clearances when I had the engine out. I figured it ran fine when I took it apart, it'll be alright. Maybe that's not the case  :'( Could out-of-spec rod bearings cause such low pressure?

HM, the rust spots felt pretty superficial to me but I'll take a closer look tonight. The fact that the center seal uses pressure could certainly cause the issue though, as mine was very hard.

[Tracksnblades1]

If your oil tank is empty before your start your engine it’s a tell tale sign your oil stopper or your oil pump seal(s) is/are leaking..

I don’t prescribe to the “30psi BS” on the oil seal between the supply and scavenge sections of the oil pump..
if Honda used a pressure dependent shaft seal, their engineers would have placed the oil stopper in a position to prevent the oil tank from draining down from the seal also. If your stopper is good it can only be one of the other oil pump seals draining the tank..If your oil tank stays full for days, weeks, months,  it’s telling nothing in the oil pump is leaking..

Look at the position of the oil stopper in the oil flow diagram. The shaft seal is under static head of the oil tank when the engine is off. The oil stopper shuts off oil tank’s static head pressure from the oil galleys so your tank oil doesn’t drain into the sump by dripping out through your main and rod bearing clearances. resulting in an empty oil tank for the supply side every time you start the engine.

Look at the dimension of the scavenge pump compared to the supply. It’s bigger and has more potential capacity to keep the sump empty. Hence the term “dry sump”.. with your oil shooting back to your oil tank, I think you have a big leak…i would hook a 30psi oil pot to where your oil gauge is, and with the sump cover off, oil tank cap off,  turn the pot on and try to see, listen where the oil comes out of the sump from..it looks like the oil stopper will prevent oil pot pressure from back flowing through the pump, but it wasn’t designed to stop oil in this direction from coming back from the main galley so I’m not sure if it will. If it does prevent back flow, normal main and rod bearing clearance will only allow oil to drip out from the cold oil pot pressure. If you hear or see a stream of oil coming from a single bearing or bearings you found your problem…if the oil tank overflows or oil comes pouring out of the oil pump screen, consider the oil pressure pot test results a failure because the oil stopper is not preventing oil pot pressure to the oil pump. But remember it wasn’t designed to do that in this direction…

Blocking off the oil pump supply port would be the only sure way to use a pressure pot. But it’s still going to be hard to see anything except how much oil collectively is running out the sump…a bad leak is instant…

[Tracksnblades1]

The steel ball bearing are driven in the crankshaft’s oil passages to dead end them and are peened over to prevent from coming out due to centrifugal forces  and oil pressure. Reputable machinist remove these balls and clean all the 40 year old centrifuged carbon/grit out and sometimes install threaded plugs instead..

The way Honda drilled the crankshaft oil passages (from the outside) required the use of these plugs..very seldom do they fail unless someone serviced them and didn’t do their due diligence..

[Aceon]

I was reading some other threads and read one about low pressure due to kinked oil lines. I just doubled checked mine and the send/return lines look fine, but the oil tank / crankcase breather was kinked. That wouldn't cause that sort of oil flow issue, though would it? Just higher crankcase pressures?

[Tracksnblades1]

I was reading some other threads and read one about low pressure due to kinked oil lines. I just doubled checked mine and the send/return lines look fine, but the oil tank / crankcase breather was kinked. That wouldn't cause that sort of oil flow issue, though would it? Just higher crankcase pressures?

I agree with your conclusion. There should be vents from the valve cover and engine case too…
I’ve read the inner part of the supply hose can delaminate and cause problems and restrict but I’ve never seen a Honda hose do that..

[HondaMan]

Does your crankshaft have all the steel ball bearing oil plugs in place..?

If you mean the big rubber seals, then yes. Not sure what you're referring to if not.

Could out-of-spec rod bearings cause such low pressure?

HM, the rust spots felt pretty superficial to me but I'll take a closer look tonight. The fact that the center seal uses pressure could certainly cause the issue though, as mine was very hard.

It is not usually so much the rod bearings as it is the main bearings that cause lowered oil pressure - IF the pump is good. Interestingly, the rod bearings don't seem to space themselves nearly as much as the main bearings as the miles add up. Once the main bearings reach 0.0020" the PSI will start to drop noticeably, and when they reach out past 0.0024" it is hard to get hot oil pressure above 30 PSI even with 20w50 oil. Honda's clearance limit is 0.0032", but this will make for really low oil PSI, even with a good pump. I've only seen 1 CB750 this bad, and only on 2 crank bearings, and it did indeed have really poor oil PSI, which galled the rod bearings - they looked like someone had 'polished' them with Scotchbrite or something.

I just 'refurbished' a 750K4 bottom end in November where the rest of the engine was rebuilt about 6-7 years ago, but suffered marginal oil PSI even then. The Brown and Black bearings were not available then and I could not get him a set of them until early this year (at the same time that he could bring me the bike: he is a world traveler, big-time): he brought the bike back last Fall to have them installed. His hot oil PSI went from 35 PSI at 3000 RPM to 62 PSI at the same engine speed, same oil type (Bel-Ray EXL Mineral 20w50) by changing the main bearings to Black and the rods to Brown, yielding 0.0012" on all bearings but one (#5) main bearing, which came in at 0.0014".

[Aceon]

The rotor diameter and thicknesses were within spec, save for the scavenge rotor which was 11.94mm thick - .04mm out of spec wouldn’t make that pressure difference though.

My tank seemed to stay full over multiple days, or at least it did before the rebuild. I haven’t had oil in this one long but it stayed full for 2 at least.

I’ll give this pump a rebuild and try again. After that I’ll probably order one from CycleX to rule it out. If that doesn’t fix it, I guess I’m changing bearings…

[Tracksnblades1]

The rotor diameter and thicknesses were within spec, save for the scavenge rotor which was 11.94mm thick - .04mm out of spec wouldn’t make that pressure difference though.

My tank seemed to stay full over multiple days, or at least it did before the rebuild. I haven’t had oil in this one long but it stayed full for 2 at least.

I’ll give this pump a rebuild and try again. After that I’ll probably order one from CycleX to rule it out. If that doesn’t fix it, I guess I’m changing bearings…

You’ll win..Just stick with it and don’t give up…
You’ll like Kenny…he’s a diamond…he’s making his own living doing it right…He’ll  know it inside and out…
I like reading his responses when he has to follow up after the internet experts…

[Aceon]

Yeah I’ve spoken with him on a separate issue I was having, super helpful guy! Also, you guys weren’t lying about the center seal being a PITA to get out. It’s cemented in there, I’m gonna have to buy a large tap in order to remove it. I’ve been prying for an hour and it hasn’t moved a millimeter.

[scottly]

 Based on the other threads, it seems like the issue could be my main bearings

During your "rebuild" did you replace the rod and main bearings? Worn bearings are the #1 cause of low oil pressure, not oil pumps, crankshaft balls, or kinked vent hoses.

[Aceon]

During your "rebuild" did you replace the rod and main bearings? Worn bearings are the #1 cause of low oil pressure, not oil pumps, crankshaft balls, or kinked vent hoses.

I did not, I mentioned above that I figured since it was running fine beforehand, the bearings were probably fine. Im now realizing that was a combination of naivety and laziness. Anyways, I’ll have an update soon enough on the whole situation.

[PeWe]

Oil pump relief valve correctly assembled?
The one with a weak 14mm alu plug, possible to remove where pump sit.
If spring is weak/wrong spring.

I noticed that stock spring gives 80PSI at start, cold engine 20W-50 oil.
Oil rebuild kit's shiny spring give 70PSI, same engine, oil and pump. A few days between.
Only spring replaced.

The pump was rebuilt earlier with all o- rings, seal and new rotors.

[willbird]

Oil pump relief valve correctly assembled?
The one with a weak 14mm alu plug, possible to remove where pump sit.
If spring is weak/wrong spring.

I noticed that stock spring gives 80PSI at start, cold engine 20W-50 oil.
Oil rebuild kit's shiny spring give 70PSI, same engine, oil and pump. A few days between.
Only spring replaced.

The pump was rebuilt earlier with all o- rings, seal and new rotors.

Some of those shiny new springs in oil pump rebuild kits coil bind and do not relieve pressure, or so I read here

[HondaMan]

Oil pump relief valve correctly assembled?
The one with a weak 14mm alu plug, possible to remove where pump sit.
If spring is weak/wrong spring.

I noticed that stock spring gives 80PSI at start, cold engine 20W-50 oil.
Oil rebuild kit's shiny spring give 70PSI, same engine, oil and pump. A few days between.
Only spring replaced.

The pump was rebuilt earlier with all o- rings, seal and new rotors.

Some of those shiny new springs in oil pump rebuild kits coil bind and do not relieve pressure, or so I read here

I noticed one didn't fit from a "kit" I bought, but I can't remember where I got the kit. ?

[willbird]

Oil pump relief valve correctly assembled?
The one with a weak 14mm alu plug, possible to remove where pump sit.
If spring is weak/wrong spring.

I noticed that stock spring gives 80PSI at start, cold engine 20W-50 oil.
Oil rebuild kit's shiny spring give 70PSI, same engine, oil and pump. A few days between.
Only spring replaced.

The pump was rebuilt earlier with all o- rings, seal and new rotors.

Some of those shiny new springs in oil pump rebuild kits coil bind and do not relieve pressure, or so I read here

I noticed one didn't fit from a "kit" I bought, but I can't remember where I got the kit. ?

Whatever I read here said the spring fit but it coil bound before any oil was allowed to bypass.

[PeWe]

Oil pump relief valve correctly assembled?
The one with a weak 14mm alu plug, possible to remove where pump sit.
If spring is weak/wrong spring.

I noticed that stock spring gives 80PSI at start, cold engine 20W-50 oil.
Oil rebuild kit's shiny spring give 70PSI, same engine, oil and pump. A few days between.
Only spring replaced.

The pump was rebuilt earlier with all o- rings, seal and new rotors.

Some of those shiny new springs in oil pump rebuild kits coil bind and do not relieve pressure, or so I read here

I noticed one didn't fit from a "kit" I bought, but I can't remember where I got the kit. ?

Whatever I read here said the spring fit but it coil bound before any oil was allowed to bypass.

I have not seen any issues. Both my bikes have an oil pressure gauge.

I think I know which thread where the springs were up..

It was a theoretically coil bind, not a real seen one. I contributed a photo into that thread, stock vs pump restore kit spring.


EDIT:
Here the photo I added. Exacly one year ago. May 3 2023.

Thoughts about spring will not compress as much stock due to a coil extra.
This means higher pressure which never happen with that spring from seen kit

There are people that place a shim on spring to increase pressure.



Cold start pressure from 80 to 70 PSI.
Pressure when hot is below 70PSI anyway.

How much pressure will filter take until it will move to allow unfiltered oil or maybe release a dump of collected dirt to crank and top end?

I have never seen pressure over 70PSI with either springs when engine is hot.

Checked my brown K2 yesterday at around 8000rpm on 3rd gear. 65 PSI something after an hour ride and more.
Will the spring open up earlier when hot than cold?

I did not look down when 8000rpm on 5th gear Plug cleaning
(17:48 sprockets, rear tire 4.0-18 a little bit more than half rubber left.)

[newday777]

Per
I like your plug cleaning technique 😎

[PeWe]

Per
I like your plug cleaning technique

I liked it too.
Verified rpm vs gearing, 188kmh. OK for the brown K2. I could press it more.

Plugs after latest ride vs plugs before with 120 mains.
64A carbs.
Tricky to get photo correct vs the reality. Looked similar on my phone's screen.

[Aceon]

Hey guys, back with some disappointing news. I rebuilt the pump, primed it, threw it back in and started it up and I got the same oil pressure as before. Only about 10psi COLD at 3000ish rpm with 10w40 oil.

It's looking like the main bearings and rod bearings need a swap, but I'm really trying to avoid this as I spent all winter rebuilding and I'm dying to ride again. 

Do you guys think that a fully refurbished pump from CycleX, 20w50 oil instead of 10w40, and other misc. improvements could give me an extra 10-20psi? I really don't think it would cause too much wear to run this engine for one season with less than optimal oil pressures. Hell, it's been running at this pressure ever since I've owned it, I'm sure. Obviously it's always better to have higher pressures and you all will give the "correct" answer, but if we're not being perfectionists, could I get it ridable?

EDIT: Ken gave me a call and told me to shim the spring and report back. Will update.

[Aceon]

So I shimmed between the cap and the pressure relief spring with 2 washers, totaling to about .12in/3mm. When I started it up, the pressure was exactly the same.

Here is a video showing the startup, oil tank flow, and oil flow at the tappet covers - it's hard to see on camera but it's just small flicks coming off, definitely not a ton of oil. The carbs need to be re-tuned after the new parts, so ignore the rough running.
https://drive.google.com/file/d/1ouGmeQOu3goeceJl1eJS18HiqSx_qtJt/view?usp=sharing

Any ideas? I assumed the shims would have at least a bit of an impact...

[HondaMan]

Hey guys, back with some disappointing news. I rebuilt the pump, primed it, threw it back in and started it up and I got the same oil pressure as before. Only about 10psi COLD at 3000ish rpm with 10w40 oil.

It's looking like the main bearings and rod bearings need a swap, but I'm really trying to avoid this as I spent all winter rebuilding and I'm dying to ride again. 

Do you guys think that a fully refurbished pump from CycleX, 20w50 oil instead of 10w40, and other misc. improvements could give me an extra 10-20psi? I really don't think it would cause too much wear to run this engine for one season with less than optimal oil pressures. Hell, it's been running at this pressure ever since I've owned it, I'm sure. Obviously it's always better to have higher pressures and you all will give the "correct" answer, but if we're not being perfectionists, could I get it ridable?

EDIT: Ken gave me a call and told me to shim the spring and report back. Will update.

My first advice is the same as I always give for the CB750: use 20w50, non-detergent oil. The 10w40 oil nomenclature was a mistaken (and arrogant) error on the part of American Honda in 1975 that changed the manuals (incorrectly) forever. Look into any one of the Shop Manuals as published by Honda: they only suggest using 10w40 oils below 40 degrees F outside - that's some cold riding!

[Tracksnblades1]

Ace,

Did your oil pump delivery(supply) section have the recommended clearance 0.0015-0.002, I did some rounding for this post..delivery housing depth - delivery rotor thickness….?

Nearly all pumps, gear, rotor, vane, etc have more volume with hot thin oil when the end play clearance is on the minimum..I personally like all the clearances on the oil pump at the minimum side.. There was a MessoMoto guy making billet trochoidal    parts for the 750 oil pumps. This would be the best new unused, if his tolerances are on the tight side.. Otherwise your left with looking for the best of what no one else wanted..😩

Back in the old days a lot of tired CB750s and 836s ran Vavoline  or Castol straight 50w.. I put a lot of miles on the F1 riding with my two buddies who ran straight 50w. One used it in a 75K and the other in a custom 836 chopper.. I never used anything heavier than Vavoline 20w-50 (summer) in my F1 and changed it every 1000-2000 mile depending on the temperature and how hard we ran them..

[HondaMan]

So I shimmed between the cap and the pressure relief spring with 2 washers, totaling to about .12in/3mm. When I started it up, the pressure was exactly the same.

Here is a video showing the startup, oil tank flow, and oil flow at the tappet covers - it's hard to see on camera but it's just small flicks coming off, definitely not a ton of oil. The carbs need to be re-tuned after the new parts, so ignore the rough running.
https://drive.google.com/file/d/1ouGmeQOu3goeceJl1eJS18HiqSx_qtJt/view?usp=sharing

Any ideas? I assumed the shims would have at least a bit of an impact...

If the pump isn't reaching nominal pressure first, shimming the relief valve won't change anything.
When these pumps were new they generated up to 80 PSI momentarily when the engines started cold, fully primed (from previous running) and with 20w50 oil. Then the relief valve would begin flowing and the PSI would drop to something more normal, like 65 PSI cold. This was like, 1 second at 80 PSI and then the needle would slew to 60-ish. Shimming the relief valve would then result in the 60-ish number staying higher, like 65 or 70 PSI. Mostly, though, above 70 PSI the oil filter's housing O-ring starts weeping oil, making a mess. Some roadracers would shim fully up to 80-ish PSI for 'all they could get', but it made for messy engines.

[Aceon]

If the pump isn't reaching nominal pressure first, shimming the relief valve won't change anything.

That makes sense.

Did your oil pump delivery(supply) section have the recommended clearance 0.0015-0.002, I did some rounding for this post..delivery housing depth - delivery rotor thickness….?

Nearly all pumps, gear, rotor, vane, etc have more volume with hot thin oil when the end play clearance is on the minimum..I personally like all the clearances on the oil pump at the minimum side.. There was a MessoMoto guy making billet trochoidal    parts for the 750 oil pumps. This would be the best new unused, if his tolerances are on the tight side.. Otherwise your left with looking for the best of what no one else wanted..😩

Yeah, the rotors in all sections looked to be at pretty optimal specs according to the sheet you sent before. Even the gouges out of the rotors were pretty minimal IMO.

I had another chat with Ken and he mentioned burping the leak stopper by loosening the screws and turning the engine over. He was suggesting that the oil pump was not fully primed and an air bubble could be causing the lower pressures. I'll try doing that and hope for the best.

Any other suggestions? Is it worth checking the countershaft O-ring? I distinctly remember installing it, but I guess it's possible that it got dislodged.

[Aceon]

I ended up pulling the pump out again to focus on re-priming it, then stuck it back in. Still no pressure on the starter, but I didn't start it up.

Here is a video of the oil coming from the oil gallery on the starter.
https://drive.google.com/file/d/1-9FjLydIp2kLwjmZwIz0tZlZ1dyExj9c/view?usp=sharing

It looks like I have significant flow but finding this leak is such a PITA. I might still try a new pump but I'm doubtful that'll fix anything... looks like I might be taking this thing back apart... nooooooo