MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Big end knock ?

Hi all. Rita has developed a nasty noise and i'm at a loss as to what it can be. At approx 2500/3000rpm there is a discernable low end "rumble". However, as soon as I change gear, the rumble goes until I hit the rpm,s mentioned above. Any thoughts, ideas or helpful suggestions would be most welcome.

D J P Hewett

Big ends are usually worse under load. So does it change throttle closed to open? I have been a passenger in acar that ran a big end. There was no doubt about what had happened, it was like someone taking a club hammer to the crankshaft.
Stan Best

Rumbling is usually main bearings, a big-end is more felt as a deep thump at a slower rate. With either one would expect the oil pressure to have dropped as the noise developed.
PaulH Solihull

What is your running oil pressure, idling oil pressure and possibly most importantly how long does the pressure take to die away once the engine has been switched off
Iain MacKintosh

Hi all. The only thread I can answer with any confidence is the one from Iain. Oil pressure is spot on 50psi when first starting, constant 45/50psi when driving at dual carriage way speeds, 35/45psi in town and 25/30psi at tickover. With my limited knoweledge of mechanics, even I know that those pressures are far too varied. I hope this information helps with some sort of diagnosis.
D J P Hewett

These oil pressures do seem to be a bit low especially the dual carriageway pressure. Now it is very difficult to diagnose the problem without actually hearing the noise but from your description and the oil pressures it seems that the main bearings may be on the way out. Perhaps it is time to take it to a reputable mechanic and let him put an ear over it.
Iain MacKintosh

On second thoughts you could do well to take it to Chris Betson Octarine Services, 11 Byfield, Eastwood, Leigh on Sea, Essex who is very knowledgeable and frequently contributes to this board. I hope he is not too far away from you.
Iain MacKintosh

Lower indeed than what is usually considered 'normal', but apart from the 'dual carriage way' still within the limits given in the Workshop Manual of 50 to 80 hot running and 10 to 25 hot idling. FWIW mine starts at 80, runs consistently at 60, and normally idles at 40 unless stuck in traffic on a hot day when it can gradually drop to 25, which is caused by the oil cooler no longer cooling the oil. I'd put another gauge on it first, then check the relief valve. Both can contribute to low and fluctuating pressures.

Fluctuations can simply be from a specific combination of circumstances such as oil viscosity, temperature, relief valve characteristics and engine revs. It's always been an issue and a sintered plug was fitted to the gauge to smooth the worst of these out, but it still happens. Changing any aspect may stop it, move it to a different temperature or rev band, or not alter it at all.
PaulH Solihull

Yes Paul, but you forgot one thing, this engine has a rumble and it seems to me that it could be main bearings.
Iain MacKintosh

Hi Iain. Thankyou for the advice. Do you have an e,mail adress or tel number for Chris, I am only an hours drive from him so I could get down to him if he has the time to have a look/listen for me.
D J P Hewett

Can you post mpegs on this BBS, so we can all listen?
Art Pearse

Chris's mobile is 0780 1576731 I think you will find him very helpful.

Art, that may be a good idea especially if this site will accept mpegs or wav files.
Iain MacKintosh

Iain - not me, look at my first post where I say rumbling is usually main bearings more than big ends!
PaulH Solihull

The point I was really making is that you can still have a reasonable oil pressure and the main bearings can be shot. I believe that 45/50 is too low at dual carriageway speeds and that the main bearings could well be gone. I kind of think that the best course of action here is to let Chris Betson cast his ear over the car. At the end of the day he does not seem to be too far away.
Iain MacKintosh

Again, that's what I've said, i.e. 45-50 is lower than it should be and it could be main bearings!
PaulH Solihull

Hi all. Well, it's been a busy weekend checking out what i can on Rita and i'm more confused now than i was to start with. First job, check compression. All 4 cyl showing 170psi. Second, check timing, about 5 degress out, adjust, start car, oh, the noise now comes in at 2000rpm. To heck with it, i've phoned Chris and i'll go and see him soon in the hope that someone will finally diagnose the problem fully.
D J P Hewett

This thread was discussed between 14/09/2010 and 19/09/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now