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MG MGB Technical - using oil

I rebuilt the engine in my 72 roadster last year which included a rebore out to 60 plus and fitted new pistons etc. the engine runs great and has a lot more power.
The head was worked on by a previous owner about 6-7 years ago and had the inserts fitted and new guides and valves fitted, these still look to me to be fine.
The problem I am having with it is the oil consuption.
The car has now done about 1500 miles since the rebuild and I have had to put about 2-3 litres of oil in to keep it topped up. There is no oil leakage, and even though the bores previous to the rebore were quite badly worn it used very little oil.
I have done a compression test and am getting good even readings.
The engine must be burning the oil but is there a way of determining if it is coming down past the valves or past the pistons.
Your veiws would be appreciated. Trev
Trevor Harvey

Are you using valve seals?
See following website for some options.
http://www.mgaguru.com/mgtech/engine/ch202.htm
Mike (visiting from the MGA forum)
Mike Ellsmore

Hi Trevor
It's easy to see if it's the valve guides let the car over run going down a hill then accelerate, if you see a puff of blue smoke it's being sucked down the guides by the vacuum caused by the over run. If it's the bores someone following you should see blue smoke when you accelerate away normally.
My car improved it's oil consumption considerably over the first several thousand miles after a rebore, I was quite bothered at first about the amount it used but after about 10,000 miles it settled down to less than a pint per thousand miles.
Ron
R. Algie

What oil did you use for first running? Opinion is that modern multi-grades don't allow the rings to bed in very well and modern synthetics are even worse. Modern MGs like the ZS had a reputation for high oil consumption until you gave them a bit of a ragging. Some recommend using older formulations and straight grades like Castrol XL30 after a rebore.
PaulH Solihull

Thanks for the replys.
Mike, Yes, new valve seals were fitted,a cupped type replaced the old dougnut type.

Ron, I will try your method of testing for smoke.
A local mechanic said the same thing that it could settle down after 10,000 miles or so, maybe it will improve.

Paul, I am using a semi synthetic 10/40 GM oil that is supplied by my local garage. I have used it in the car for the last 5 years. I know it is thinner than a straight 30 oil but this gave no problems before the rebore. Might be worth trying a 30 grade at the next oil change.
Trev
Trevor Harvey

To expand on the synthetic use. I've used synthetic motor oil in my other cars for many decades. I switch from brand to brand, depending up sales. 5w-20 or 5w-30 in winter and 10w-20 or 10w-30 in summer. I never run the oil for the "advertised" life or mileage. I change oil and filter at regular interval, and with it being on sale isn't expensive.

I starting adding 1 quart synthetic (5w-20, 10w-30) to my regular Castrol 20w-50 a year or so ago (3000 miles ago of so). Essentially to slowly start cleaning the engine. I'd rather change oil than rebuild the engine. I used more oil last year too. But things were cleaning up.

The old rule of thumb was one burns less with synthetic, but one leaks more with synthetic.

This spring I switched to all 15w-50 synthetic, can't remember if it was Castrol or Valvoline or ? Oil pressure is just fine. Oil is staying cleaner, and I'm definetely using less oil. I have not quantified this yet.

I realize this isn't addressing the whole zinc additive issue. But my 2cents on synthetic, so far in my MG.
R.W Anderson

Hi Trevor,

Any or all of the above.
Possibly also your crankcase ventilation. Check your hose, where they connect to the carbs and see if they or full of oil. If so did you clean / replace the side cover with the vent tube?

Herb
Herb Adler

The point Paul is making is that modrn synthetic oils work too well in the B series engine. The piston rings are never allowed to contact the cylineder walls and bed in together. His point about working the engine is good as well, you need to generate some BMEP to shove the rings into the bores. You may now have glazed bores which need honing to get them to seal. Once you have properly run it in then all the above comments about sythetics, or semi_synthetics are true. Our everday Fiat holdall is 11 years old and I got it ex daily rental so it was caned from new, it never uses a drop of oil beteween services becsuse the rings were forced to bed in.
Stan Best

Your break in oil was too slippery. Change the oil to a cheap brand and run the engine up, in 2nd gear, to 4,500 rpms. Then back off completely on the throttle. Do this at least 4 times. It should seat your rings. RAY
rjm RAY

There used to be running in oils, if not a simple 20W/50 may work, or as noted even an economy one. What happens when bores "glaze" is that intsteas of the rings and bores lapping in to smooth surface that holds compression and allows the oil scrpaer rings to work the irregularites are just burred over leaving pockets where oil will stick and gas can escape. Its worth trying the trick above before resorting to honing the bores.
Stan Best

Trevor,

When I rebuilt the 3 main engine in my ī63 MGB six years ago, it was bored to +.060 like yours. I put straight 30 oil in it, and ran it for 20 minutes at about 2000 rpm to bed in the valve lifters. After 3-400 miles I changed to the Comma Sonic mineral 20w50 that I have used since. Oil consumption remained quite high, and I started worrying. But after 5-6.000 miles or so the excessive consumption suddenly stopped, and it is now very acceptable. As the 3 main engine does not have a proper oil seal at the rear crank bearing, some oil is always leaking out. I donīt now what caused the sudden change in consumption, but the same may perhaps happen to your engine?

Tore
Tore

Everyone here would benefit from what the top motorcycle guru says to do. Go to www.mototuneusa.com and look for breakin secrets. I followed them (right after a rebuild) and have no smoke or leakage issues.
JM Morris

This thread was discussed between 08/07/2011 and 10/07/2011

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