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MG MGB Technical - Where is the oil going?

My newly rebuilt 3-main MGB engine consumes about one US quart per 300 miles. My question is “Where is the oil going and/or how does it get there?”

Some facts…
1. The engine was completely rebuilt by a professional engine rebuilder (albeit a Ford, Chevy specialist). The engine was bored to .020 oversize, new County brand 5-ring pistons that came with Grant rings were installed, the head was rebuilt using oversize intake valves with hardened seats, new valve guides and new valve seals were installed and all gaskets, seals etc. were replaced. The engine has accumulated about 3500 miles since rebuilt.
2. The correct engine break-in procedure was followed for, at least, the first 500 miles. Castrol 20W-50 was used initially, replaced at 75 miles and again at 500 miles and 1000 miles. I am currently using Valvoline Racing oil 20W50.
3. The SU carbs air/fuel ratio was correct at initial startup and remains so today. The spark plugs show the proper brownish/gray color.
4. At about 75 miles, one of the freeze plugs came out and the engine overheated. The internal temp. may have reached 240 deg F. The engine did not seize nor are there any obvious signs of damage to the engine.
5. A cold engine compression test shows, 150 PSI, 155PSI, 158 PSI AND 165PSI. The hot compression is slightly higher.
6. Initially the engine had SUBSTANTIAL oil leaks, like a quart every 100 miles. I think the problem was caused by the engine rebuilder not line-boring the crankshaft journals thus not ensuring the correct clearance between the oil thrower reverse thread on the crankshaft and the block (.0030”). Additionally, the straight cork gaskets may not have been installed correctly. This problem was corrected using a “Band-Aid” solution. To fix the problem I installed a PCV valve and the correct tappet side cover from a later 5-main MGB engine. This worked miracles. The negative pressure inside of the sump stopped virtually all of the oil leaks. I know this because, before the installation of the PCV valve, I made a small sheet metal box that I mounted under the engine’s rear main and the bell housing to collect the leaking oil. That box filled up in about 250 miles. Now, after about 1000 miles the box collects approx. 2 or 3 tablespoons of oil. Additionally, there is only an occasional oil drip on the garage floor.
7. Although the spark plug color is correct, there may be (???) evidence of burning oil. For example, the sparkplug threads and seats show evidence of a black oily substance. Additionally, during normal driving a small puff of smoke exits the tailpipe during transmission shifts. The condition worsens, but is still minor, when the car is driven hard. There is no smoke during steady-state constant seed driving or during rapid acceleration or deceleration.
8. There is no evidence of oil in the radiator’s water/antifreeze mixture.

So here I am, with a rebuilt engine that consumes excess oil but with no major oil leaks and is probably (???) not burning significant amounts of oil. Did the overheating at 75 miles on the new engine cause cylinder wall scuffing, a broken ring and/or interfered with the rings seating? I am perplexed, perhaps someone out there is a lot smarter than me and can tell me where the oil may be going.

If you suspect that the problem is poor piston ring seating and you recommend new rings, please suggest a piston ring brand that is considered the best for seating and future oil consumption. Is there anything unique about cylinder boring and honing or deglazing of MG engines that I could pass on the engine rebuilder? Has anyone had a bad experience with County brand 5-ring pistons and/or Grant rings?
Frank Grimaldi

Frank - I have always been told to use straight 30W non detergent oil for break in. I was also told specifically told NOT to use 20W50 for break in as it is way too slick and can cause poor seating of the rings. I have always followed that admonition and have never had a problem with break in or subsequent excessive oil usage. Cheers - Dave
David DuBois

My old 1800 MGB engine burned an imperial pint of oil every 300 miles and yet showed no smoke at all, and it wasn't leaking more than a drip or two. Mine was caused by blowing past the piston rings.

This business of what oil to use for breaking in a rebuilt engine is a minefield. I have recently rebuilt my Rover V8 using parts from the reputable engine specialist Real Steel over here in the UK with new rings in re-honed bores. I specifically asked them what oil to use, and they said Valvoline Turbo 15-50, both for breaking in and for daily use afterwards. Apparently this is a fairly old fashioned mixture containing whatever is needed by old fashioned engines. So far, during the first 1000 miles, it has used almost no oil at all. They did not recommend the Valvoline synthetic racing oil.
Mike Howlett

Frank,

My 3-main engine was rebuilt 4 years and 11.000 miles ago, and regularly consumes about 1 deciliter per 100 km. I believe that is about half the consumption you have, if I understand your quart correctly. It was run in on straight 30 oil for the first hours, then run on 10w40 for the first season, and now on 20w50 - all mineral. It is bored to .060 oversize.

Although the consumption I have is not very different from what was quoted in the first test of the MGB in 1962, I suspect I have some crankcase pressure. I also believe some oil escapes through the rear crankshaft bearing, where there is no proper oil seal.

But I can live with it, and don´t plan to take any action!

Tore
Tore

If you added a PCV valve to an engine that pre-dated it (i.e. the 18G engine, 18GA had one as standard) where did you connect the PCV valve, and what did you do with the 'other' vent into the crankcase? There were always two, in engines with PCV or carb ventilation the fresh air inlet is filtered and *restricted* to prevent too much airflow through the system, which a) can pull too much oil through and b) weaken the mixture. Adding one to an 18G engine you should have restricted the inlet. If you did this by blocking it off altogether then the flow should be negligible unless it is pulling it (and dirt) past the crank seals, but then you won't be getting any crankcase ventilation at all which can caise internal corrosion. The best thing to do is fit a catch bottle between the PCV and the crankcase. If you plumb both inlet and outlet in at the top, but separate the ends of the pipes (outlet higher than inlet) then you will see by oil building up in the bottle if that is the route of your oil burning.
Paul Hunt

I would vote for the PCV valve it's surprising how much oil can go that way. A road draft system via a catch tank may show you what is going on, the oil won't neccesarily appear in the catch tank but it may stay in the sump. It is also possible to over baby your bores and glaze them. This means the small machining marks get burred over rather than bedded in to the rings this results in oil being trapped on the bore surface and not being scraped off by the rings. Re-honing is the answer here.
Stan Best

Hi Frank:

I had my 18V engine (1973 B) rebuilt 3 years ago. It used oil immediately at an alarming rate (high to low on the dipstick in 125 miles). There was no smoke from the tailpipe, and only a little dripping evident. Also upon the initial startup and cam break-in, a coolant leak above spark plug 3 was noted.
The rebuilder took a look when I removed the head, and we noted that there were pools of oil on the tops of the pistons. He said that the rings have not seated. I believe that they were an AE piston/ring set. We ordered Hastings rings and installed them. However, to my eyes, the bores did not appear to have been honed. The swirl marks were exactly horizontal which looked more like the boring tool marks. I had the block honed on a special machine that created the hash marks at the correct angle. Also, the original tappets had been refaced, and they showed noticeable wear on the tear-down, so new ones were installed.

After the engine was reassembled and installed, the engine ran great, and has used very little oil so far. It has been about 3000 miles since the second rebuild.

So, if it comes to it, I would strongly recommend Hastings rings, and the correct honing procedure is essential.

I had suspected improper crankcase ventilation as a possible reason for my problems. My system was plumbed exactly as it was from the factory. I used a clear piece of tubing instead of the rubber hose from the front tappet cover to the Y that leads to the carbs. It stayed clean and oil-free for the time it took to see the oil level drop from Hi to Low on the dipstick, which suggested to me that the ventilation was not the source of the problem. But certainly crankcase ventilation is a critical factor in oil consumption problems.

Hope that helps,

Erick
Erick Vesterback

This thread was discussed between 12/05/2009 and 14/05/2009

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