BMC

[Don Henderson]

I live in Fiji on A wharram cat with a BMC engine running a hydraulic pump to 2 prop's. My oil pressure has dropped to 10lb's lately. engine runs sweet, quick start with no pre heat. No smoke at all. Put 2 washers behind the oil pressure control spring to bring it to length, exact. Oil gets up to rockers . If I unscrew the oil pressure sender unit while the engine is running I get soaked in oil. Not to be recommended. is this the right site I'm on to ask this question????

 

[TQA]

If it is an electric oil pressure sender these are known to fail. Find a mechanical gauge if you can to check pressure From memory most BMC B units used to run around 70psi.

If you were only getting 10 psi just after start up I would expect a noticeable increase in mechanical noise.

 

[Don Henderson]

The engine has the same sounds, [sweet] as usual , no tappet noises, no big end knocking. Never had 70 psi, best was 40 psi. Tends to leak a lot out of the back main crank area. Oil seems always clean ??? though it does get replenished because of the leak. I think the rear is a splash guard & leather seal ?? I will fit a mechanical, bourdon gauge & see what the read out is. I have replaced the oil sender unit & short circuited & open circuited the gauge for range of movement. i'm a marine, auto elect. The engine & wiring is very tidy, oily though. Thanks for the info, input. I need more.

 

[vyv_cox]

Is the filter clean. When clogged the filter bypass will open, allowing oil through but at lower pressure to save the bearings.

 

[Don Henderson]

Bula thankyou for your reply. I changed the filter & checked the by pass ballbearing , spring assembly. The filter wasn't overly dirty & there was no residue in the bottom of the filter cylinder cover ie bearings damage.I will remove the filter housing off the engine block & check for blockages & build up. Will give the engine a oil flush. Someone suggested 1/3 kerosene not running the engine too long. I will put in a 30/40 oil. Cyclone season is nearly upon me & although I feel safe idling the engine up to 1500 checking the rockers for oil supply. I would have to sail mostly to get up one of my favorite mangrove rivers in a cyclone situation or go down to NZ &
give the engine a bottom end birthday. Don

 

[TQA]

If it is a B series either the 1.5 or the 1.8 it should definitely run at 70 -75 psi.

The oil pump is over specced and even with quite worn bearings the pressure at 2000 rpm would be 70 psi cold.

 

[Don Henderson]

Bula thankyou for that info. I usually run at 1500 to 1800 in a emergency. A bit slow, but my kids thrash it [bring the revs up] when they visit. I'm getting oil right up to the cam followers quickly & the rockers get oil about 7 minutes after start. the engine runs like a proverbial sewing machine. So I will just keep an eye on it & order a new oil pump or kit. Not sure how to do that or where to get it from yet. I can drop the sump real easy to get at the pump as I set the engine up to have any major work ensitue. [phonetic spelling] Any one know where I can get a new oil pump or kit????

 

[vyv_cox]

Asap-supplies.com keep a lot of BMC spares. They don't actually list oil pumps but could be a first contact.

 

[mtb]

I'd think it is a worn pump, the pressure relief valve is meant to stop excess oil pressure
thornycroft are at
Thornycroft EnginesAddress: PO Box 860 New LongtonCity PrestonCounty: LancashirePostcode: PR4 5GTCountry: United KingdomPhone: 01772 611144Fax: 01772 616364Contact Name: Mike Bamford Email: mike@thornycroftengines.co.uk
mick

 

[Don Henderson]

bula I took the sump off & removed the oil pump. It was in excellent condition. No metal residue in the sump either. All nice & clean. I figure it now to be the main bearings. Can't be too bad as the engine sounds good. Have assembled & will keep an eye on it as the cyclone season is approaching & I need the engine running, sorta. I should of dropped a main bearing & got a part no. to order them. Does anyone know what the part numbers of the main bearing shells are, or how to get them? This engine hasn't been overhauled ever . It's just sat in a packing crate for too long. Cheers.

 

[TQA]

If you tell us what engine it is and or the engine number there is a pretty good chance someone here can help you out.

If it is a B series the canal boys still use them and know where you can get bits.

 

[cliff]

If my memory serves me well there are a number of possible causes.
The pressure relief valve in the pump is jammed slightly open
The pressure relief valve in the pump is not seating properly (damaged seat)
Broken or tired spring in the pressure relief valve
Worn pump
Blocked or restricted pump inlet
Worn bearings (Mains and or big ends)
Worn rocker shaft(s)
Check the valve in the block - same as for the pump PRV
IIRC the rockers are fed via an external pipe between the block and the head - one of the banjo fittings has (or should have) a flow adjuster to control the flow to the rockers.
Oilways gunged up - thorough flushing reqd
Faulty oil gauge
7 minutes to get oil to the rockers? - should only be a few seconds - something is definitely amiss with the donk. If you cannot fix in-situ it is time to consider a full strip down and rebuild