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MG MGB Technical - oil consumption

The engine in my 71 roadster is useing about a pint of oil every 100 miles.
I had it rebored winter before last and fitted new pistons.
The head was reconditioned a few years ago so I only fitted new stem seals to the valves, as the valves and guides looked OK they were not changed. Strangely before the rebore, even though the bores were well worn the engine burnt almost no oil.
I can at times see blue smoke coming fron the exhaust on acceleration, so I am convinced it is burning the oil.
I have not yet done a compression test but is there a way of deciding if the oil is getting past the pistons or valve guides.
Thanks for any help. Trev
Trevor Harvey

One pint every 100 miles is a bit excessive.

IIRC burning oil on acceleration implies piston rings. Valve guides usually blow smoke when decelerating.

A couple of thoughts.

How many miles has the engine done, since the rebore? I'm thinking about whether the rings have had a chance to bed in.
How did you run in the piston rings? Did you use running in oil? Some super lubes can be that effective that the rings don't bed in.

The other thing is your crankcase ventilation, particularly the oil separator. This is the front tappet cover, which has metal wool inside it to condense the oil, so it isn't sucked into the carbs. To check this disconnect the hose from the tappet cover and plug it. Fit another hose, with a catch bottle to the cover. If this "cures" your oil burning then that's the problem. Ways to fix vary from soaking the tappet cover in solvent and rinsing it thoroughly, to remove the gunk from the metal wool, to opening it and replacing the wool, but not with domestic steel wool, and welding it closed again. Maybe you can find a new one somewhere.

Herb Adler

Downshift while descending a steep grade, then leave the throttle closed. This will create a vacuum in the intake port. If you see blue exhaust smoke, then the oil is being sucked through worn valve guides.
Stephen Strange

One obvious, simplistic thought - presumably it is not simply leaking, do you get much of a puddle of oil under the car when you park it? Apologies for being so prosaic!
Piers Colver

DId you use running in oil? And what oil are you using now? If you didn't use a running in oil, or a running in additive, or, heaven forfend, you used semi or fully synthetic, you could well have glazed the bores and the rings won't seal to the bores.
Allan Reeling

Like Herb one of my suspects is the crankcase breather, and burning that much via that route I'd expect the hoses from the front tappet cover to the carbs to be wet with oil, so that's the first thing I'd check.

I know some people have problems with the covers, replacement being the only cure, and there were two types. One with a large square hole on the inside, and one with a series of much smaller holes.

Ordinarily you would have to insert a catch bottle in the vacuum pipe i.e. with an 'in' pipe from the cover and an 'out' to the carbs to get oil in it as a visible check though, as oil won't come up the cover pipe on it's own. It's possible you have you have had a incorrect non-vented oil filler cap fitted, the correct vented ones go down inside the oil filler hole with a small hole in the bottom through which should be visible a wire mesh filter. Non-vented are hollow inside.

As well as the cover itself allowing excessive oil to be consumed via the breather it could be blow-by. To check for this remove the oil filler cap with the engine idling and the note should change. You should hear gentle pulsing from the filler hole, and a sheet of paper placed over the hole should be sucked down onto it. If it's blow-by the paper will be blown off

As far as valve stems go leaving it to idle for several minutes then revving it hard will reveal leaks via this route, as a cloud of blue smoke, similar to a long down-hill on the overrun, perhaps not something Norfolk is well known for.
PaulH Solihull

Like Paul, I suspect the breather but in a worst case scenario - the bores were not properly honed.
Roger W

Thanks for your thoughts. The engine has done about 1500 - 2000 miles since rebore. mistakenly I used synthetic oil originally, now running mineral 20-50. So the bores could be glazed. Could this be rectified in some way, will this sort itself out by more miles on the clock.
The engine has much more power than it did, It goes very well.
I dont think it could be a valve guide problem as it did not burn oil before the rebore. Will have to check the breather system but I do recall cleaning the oil seperator filter at the time of rebore but maybe not thoroughly enough.
Trevor Harvey

Trevor - I suspect you have pinpointed the answer. Mileage will provide confirmation. This generation of ICE was designed to burn oil up to a rate of 3% fuel consumption - fuel dilution confuses measurement but a pint every 150 - 200 miles is not abnormal and the benefit of adding new oil in between changes needs no explanation.
Roger W

My 2000 Volvo V70 with 361,000 km on it uses no oil at all. What is it that permits this? It seems the only variables are materials of cylinder and pistons, ring design and clearances. As far as I know, cylinders are still cast iron and pistons aluminium alloy. And machinists today can match the manufacturers clearances, so why can't we expect good oil consumption from a rebuild of an old motor? I don't understand this "generation" argument.
Art Pearse

"My 2000 Volvo V70 with 361,000 km on it uses no oil at all" Don't you mean the electronic dipstick shows no variation between services?

The quality of the materials, the tolerances and the oils in our modern ICEs have all improved but the extended service intervals ensure that they wear out just as quickly/slowly. (The Volvo 5 cylinder 2.4 is one of the greats).

The oil specs tell the same story - try using synthetic SAE 5 - 30 in a standard 1800 MGB.

Roger W

Roger, I check it by eye and it hasn't moved in 3000 km (as long as I have had it).
But my point was what has changed in 50 years that explains why engines can be put together with low oil consumption, that could not be done before? "Better materials" does not explain it. Oil escapes through gaps and being forced by pressure. Pressures are generally higher these days with turbos. Are the clearances tighter and if so why not in the past?
Art Pearse

One issue you didn't mention is oil pump performance. MGB's pump is better suited to 20w50 mineral oil.
Trevor, put some running-in additive in and give it a few hundred miles on that, it MAY bust the glaze.
Allan Reeling

It may be allowed that the B engine could use oil, but it isn't a given. On my trip to Utah and back (4127 miles), I didn't add any oil, and it was probably less than 1/8 inch lower on the stick when I returned. I figured maybe a cup.

Roger, I'm glad to hear you say that about the 2.4 Volvo. I just bought a 1999 V70 wagon last week with 197k miles on the clock. Pic below.


C R Huff

Nice Charley, it will go forever!
Art Pearse

Thanks, Art. I hope so. I'm still sorting and detailing, but so far I like it. My 83 300SD Benz diesel was getting pretty rough and ugly.

C R Huff

I do not know of a running in addative, can you reccomend something.
Just a thought, is it to late to drain the oil and put in a running in oil, treat it as a just rebuilt engine.
Trevor Harvey


In the old days it used to be commonplace to seat stubborn rings by removing the air cleaners and drifting some Bon-Ami into the carb intake while the engine was running.

I don't know if you have Bon-Ami over there. It is a mildly abrasive powdered cleaning product. Their slogan, along with a picture of a fresh-hatched chick, was "Hasn't Scratched Yet". It is less abrasive than something like Comet (if you have that to know the difference).

You may want to solicit other opinions before you try it.

C R Huff

Also, Trev, I don't think anyone suggested doing a dry / wet compression check. It might tell you something.

C R Huff

I have some stuff called "Comp Cams", I think i got it from Real Steel. Called engine break-in oil additive (it's yank!!).
Might be worth a try, but not sure if it's too late now! Maybe worth a word with Peter Burgess. If it's any comfort my re-build, done by me, used a bit of oil (pint every 500) to start with but has gradually reduced, as one would expect, with time.
Allan Reeling

My oil consumption depends on usage. Pootling around lanes it uses none, a 100 mile or so sprint down a motorway and it will use some. Modern MGs were said to use oil until they were given a good hard drive, because until then the modern multi-grades are too good at reducing wear and the rings never bed in. Several sources recommend a classic i.e. early spec straight 30 e.g. Castrol XL30 for running in to help bedding in, then the oil of your choice.
PaulH Solihull

Just about everything has changed in the last fifty years, materials,design,tolerances,lubrication and importantly - build quality. From the start, we knew that better materials were available and that build quality was variable. Charley, I suspect someone has very carefully restored yours and that it has new pistons and new US rings ( and I also doubt you you thrashed it on your long journey).

A surprising amount of what comes out of a modern engine at the oil change did not come out of the can at the last change .

Separately on another thread, Trevor has set out data which casts doubt on the quality of the rebuilt of his engine. So long as he keeps it topped up with new oil - it still has a long life ahead.

Roger W


You are quite right that I did not thrash it on the trip. I don't know the history since I didn't get a paperwork file with the car. But, since it had only 50k miles on it when I got it, and virtually all of the paint was gone off of the engine, I guessed that it hadn't been apart.

C R Huff

You have got a good one - keep changing the oil - cheap stuff with a good detergent rating. The paint can come later.
Roger W

Thanks Roger,

Actually, I already did the painting of the engine & engine bay. Pic Below.


C R Huff

I had severe oil consumption issues after my engine was rebuilt. By all accounts it appeared as though the cylinders had not been honed before the engine was reassembled. I disassembled the engine, took it to another machine shop and had them use a machine to hone the bores with a specific cross hatch (60 degees?) pattern, and replaced the rings. The engine oil level has not moved in the 6 years since, other than as a result of oil changes:)

I also did a running in procedure at 500 miles post-second-rebuild where I went onto the highway when it wasnt busy, drove 40 mph in 4th, and put the accelerator to the floor till I hit 65mph, then coasted back to 40mph, a total of 10 times. Make sure your timing isnt too advanced if you try this.
Erick Vesterback

It's also better to use an 'old fashioned' oil like Castrol XL 30 for running in as it helps the rings bed in much faster than modern oils. Even modern MGs like the Z series tended to have high oil consumption until they were given a good ragging, for that reason.
Paul Hunt

This thread was discussed between 21/04/2013 and 10/05/2013

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