Oil pressure.

[GBW]

I didn't expect a miracle but, with a completely rebuilt engine, I expected more than I have.

Cold, 40 p.s.i. which, I think, is ok.  However when hot and working, this drops to around 20 even at 2000 rpm.

I have replaced the pressure relief valve (which made no difference) and the pump is new.

Should I stop worrying?

It burns little oil and that in the sump is very clean after around 150 hours of running.

 

[Kingdom Isambard Brunel]

Pressure is a bit low especially on a rebuilt unit. I would expect 70psi cold, a little less on tickover with 40psi hot dropping to 20-25 at tickover.

Are you using 15w-40 or 20w-50 mineral oil?  Not synthetic?

The biggest cause of dropping oil pressure on an otherwise sound engine is the injection or lift pump leaking fuel oil into the sump. Even a small amount makes a big difference to the oil viscosity.

[GBW]

Not reassuring!

[Tony Brooks]

Who did the rebuild, and do you know exactly what they have done? Unfortunately, "full rebuild" means different things to different people. 

 

As @Kingdom Isambard Brunel alluded to, was the injector pump overhauled and new seals fitted. If not, there is a good chance the main shaft seal is leaking. Luckily the pumps are not too difficult to take off, refit and re-time. Ditto the lift pump, the diaphragms can pinhole/split and leak. 

 

A full rebuild should really have all the engine auxiliaries at least overhauled or checked/tested, but usually do not.

[alan_fincher]

I agree with the others that your pressure figures seem low, particularly if it is a fully rebuilt engine.

 

One of our previous boats had a "well used" BMC that had almost certainly seen virtually almost no work beyond basic servicing in a decade.  All its psi numbers were appreciably better than those you are recording after a rebuild.

 

You do know that the pressure gauge is accurate, do you?

Edited June 4 by alan_fincher
embarrassing spelling error.

[Tony Brooks]

I wonder if the rebuilders measured the oil pump for wear - they should have done, but often they do not.

[GBW]

New pump.  The crank was very stiff to turn after rebore and regrind which gave me some confidence.

I do believe the pressure gauge.

Let's hope we get through the season without a major failure.

I guess I'll lift it out again in the autumn - sigh.

I did have a suspicion that I lost oil at one point through the rocker cover. I had no audible warning then but do now.

Is there any point lifting the engine without complete removal and dropping the sump for bearing examination or is it not worth the faff?

[bizzard]

If the crank was very stiff to turn maybe the reconditioners only used new piston rings on the old pistons and didn't scrape out and clean the ring grooves before fitting them, causing the stiffness. I've come across this a couple of times in the past.

[Tony Brooks]

19 minutes ago, GBW said:

I do believe the pressure gauge.

 

Has it been checked against a mechanical gauge?

[GBW]

On 04/06/2025 at 11:44, bizzard said:

If the crank was very stiff to turn maybe the reconditioners only used new piston rings on the old pistons and didn't scrape out and clean the ring grooves before fitting them, causing the stiffness. I've come across this a couple of times in the past.

The block was over top limit and fitted with liners, bored, new pistons and rings.

The mechanical oil gauge is screwed straight into the block.  I have had trouble with long pipes in the past and wanted to avoid them.  The remote gauge (in the cockpit) is electric.  

I have noted that, when cold, the oil pressure rises with revs and then abruptly stopps.  I assume this is the pressure limiter (which is new).

[Tony Brooks]

1 minute ago, GBW said:

I have noted that, when cold, the oil pressure rises with revs and then abruptly stopps.  I assume this is the pressure limiter (which is new).

 

That sounds like the oil pressure PRV working.

 

Have you checked the PRV seat in the block?

Has the length and tension on the PRV spring been checked?

 

I think that I would be tempted to get the engine hot and revving, and then stop it and carefully remove the PRV hexagon and spring. Then use a stick/cane/pencil to push on the PRV valve. It should not budge, but may move a little. If it did, I would expect swarf from the machining has got into the oil/oil galleries and may be causing it to stick, so it sticks closed for too long and then jumps open, but does not reseat. Not likely, but possibly worth half an hour to check.

 

If the oil is correct (20W50, 15W40, or SAE 30 the only things that are likely to cause it are worn oil pump (bit you say it is new) or excess clearance in the bearings. I think that the joint where the inlet strainer can is attached to the oil pump used to have a gasket on it, but I think that in most cases it would below the oil level, making it difficult for the pump to suck air instead of oil.

[GBW]

24 minutes ago, Tony Brooks said:

 

That sounds like the oil pressure PRV working.

 

Have you checked the PRV seat in the block?

Has the length and tension on the PRV spring been checked?

 

I think that I would be tempted to get the engine hot and revving, and then stop it and carefully remove the PRV hexagon and spring. Then use a stick/cane/pencil to push on the PRV valve. It should not budge, but may move a little. If it did, I would expect swarf from the machining has got into the oil/oil galleries and may be causing it to stick, so it sticks closed for too long and then jumps open, but does not reseat. Not likely, but possibly worth half an hour to check.

 

If the oil is correct (20W50, 15W40, or SAE 30 the only things that are likely to cause it are worn oil pump (bit you say it is new) or excess clearance in the bearings. I think that the joint where the inlet strainer can is attached to the oil pump used to have a gasket on it, but I think that in most cases it would below the oil level, making it difficult for the pump to suck air instead of oil.

The oil is Smith Allan as recommended by the rebuild company.

I had considered wear to the PRV seat (and then subsequently forgot about it).

Is that a replaceable item or is into be ground in situ.

Good idea to push the valve in.

On 04/06/2025 at 11:44, bizzard said:

If the crank was very stiff to turn maybe the reconditioners only used new piston rings on the old pistons and didn't scrape out and clean the ring grooves before fitting them, causing the stiffness. I've come across this a couple of times in the past.

The block was over top limit and fitted with liners, bored, new pistons and rings.

The mechanical oil gauge is screwed straight into the block.  I have had trouble with long pipes in the past and wanted to avoid them.  The remote gauge (in the cockpit) is electric.  

I have noted that, when cold, the oil pressure rises with revs and then abruptly stopps.  I assume this is the pressure limiter (which is new).

[Tony Brooks]

27 minutes ago, GBW said:

had considered wear to the PRV seat (and then subsequently forgot about it).

Is that a replaceable item or is into be ground in situ.

 

I think it is just a machined taper in the block. Personally I doubt it is even ground at the factory, but as long as you can clean all the compound out there is no reason that you can't lap it in. I was thinking more about cracks or grooves.

 

31 minutes ago, GBW said:

The oil is Smith Allan as recommended by the rebuild company.

 

Yes, but what is its SAE viscosity and API spec. Without those two bits of information, we can't rule incorrect oil out.

[alan_fincher]

12 hours ago, Tony Brooks said:

Yes, but what is its SAE viscosity and API spec. Without those two bits of information, we can't rule incorrect oil out.

 

Yes, please say exactly which Smith and Allen oil.

Their web site is a bit overwhelming, but whist I can see viscosities that are a good match for your engine, most seem to have an API spec that is not.

Note a "more modern" oil is often an unsuitable oil in these older engines.

[GBW]

On 06/06/2025 at 21:06, Tony Brooks said:

 

I think it is just a machined taper in the block. Personally I doubt it is even ground at the factory, but as long as you can clean all the compound out there is no reason that you can't lap it in. I was thinking more about cracks or grooves.

 

 

Yes, but what is its SAE viscosity and API spec. Without those two bits of information, we can't rule incorrect oil out.

HD30 monograde.

CF-4/SF/SG.

[Kingdom Isambard Brunel]

3 hours ago, GBW said:

HD30 monograde.

CF-4/SF/SG.

Though these were built to run on 30 motor oil, I would always use 20W50 or at a minimum 15W40 oil.

The pressure release valve has a tendency to get worn into ridges which stop it seating properly. It should be a smooth cone shape on top of a smooth hollow cylinder with the spring inside.

 

What did the reconditioner say about the low oil pressure?

Edited June 9 by Kingdom Isambard Brunel

[Tony Brooks]

What  KIB  said above, but it does not sound good - throw the problem back at the re-conditioners to sort out.

[GBW]

2 hours ago, Kingdom Isambard Brunel said:

Though these were built to run on 30 motor oil, I would always use 20W50 or at a minimum 15W40 oil.

The pressure release valve has a tendency to get worn into ridges which stop it seating properly. It should be a smooth cone shape on top of a smooth hollow cylinder with the spring inside.

 

What did the reconditioner say about the low oil pressure?

The prv is new.

Life interfered with fitting the engine in the boat and I am sure the rebuilder would struggle to find any interest!