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MG MGB Technical - #2 is dead
|My 79 B started to loose RPM's at idle today. When I got into work I noticed that there was a signifigant amount of oil in the air cleaner on the Weber DGV and some splattered about the engine compartment. When I got home, the compression test made me sick to my stomach. One week prior, the test read: 1-150, 2-147, 3-150 and 4-160. Now #2 is at 50 pounds and the others are similar to the previous test. The drop in # 2 and flow of gasses from the tapet cover and oil filler cap are telling me that I am experiencing blow by caused by failed rings. Please tell me there is another possibility. Am I looking at a complete rebuild? If so what is the best case and worse case in dollars?|
Your assistance is greatly appreciated.
|Tom. When my daughter's car experienced a similar drop (on number three cylinder) the cause was a cracked exhaust valve. Her car had a documented 96K miles on it. Thus, a leak down test (pressurize the cylinder and see where the pressure seems to be escaping from) would be in order. If you hear a hissing sound from the exhaust manifold, it is from an exhaust valve. If you hear a hissing sound from the carbs, it is an intake valve problem. If you hear a hissing sound from the crankcase, it is a piston/ring problem.|
There is, always, a little oil in the air filter on the Weber DGV carb.
Splatters around the engine compartment, most commonly, indicate the front tappet cover needs to be tightened. They seem to loosen up in use.
My experience is that a sudden loss of compression is most likely related to valves rahther than a sudden collapse of piston rings. In my usage pattern, daily drivers, the sticking of the piston rings is rare. Cracked valves are not common, but, I have had two happen in my life. That is two times more than sticking pistons.
|I'd agree with Les that a valve is the most likely problem. Personally I wouldn't bother with a leak down test as it will cost you cash (unless you have your own facilities). Whatever the problem the head is going to have to come off to sort it (valve or piston rings), so just pull the head and see what you find. If there are no obvious visual problems with the valves of #2 then hit both valves with a hammer (from above and with space underneath the head!) and see if one is slow in closing compared to the other valves. If it is a valve then replace it and clean the area. Often a valve breaks because of a build up of carbon, say from burning oil or running too rich. If there are signs that there is carbon built up in the other compression chambers then reove those valves and clean the area as well when you have the head off. If the valve is sticking you might need to have the valve stem guide replaced, which needs a press, so could be a shop job. I didn't when i had a sticking valve. The replacement moved freely. A new valve in the UK is £5 and a payen cylinder head gasket kit is about £18. So maybee $35 and an afternoon or a couple of evenings to repair. That is best case.|
When you have the head off you should also be able if there are broken rings or piston. If there are then it is an engine out job, and chances are that the cylinder will need bored as there will be scores if the rings have gone that badly. By the time you take the engine out and get at the offending piston you'll be mopre than half way to a rebuil, so might as well go all the way! That will be quite a bit more than $35!
PS Look at http://www.mymgb.co.uk/Engine_Pages/Gasflowedhead.htm to see what my cylinder head looked like before and after cleaning.
|I D Cameron|
|Les / Lian,|
I pulled the head off and the valves seem to be in good shape. I can't see any obvious damage to the rings. No gouges on the cylinder wall. However, all of the bores are very smooth and I did feel irregularities (bulging?) towards the top of the cylinder wall on all of the cylinders. Does this sound like the scenario for a rebuild?
|Did you get a conclusive leakdown test on the compression? I hope it wasn't just a false reading on #2, but at least you'll have a new head gasket. I would take the head to the machine shop and get their opinion before reinstalling it. Did the car make wierd sounds while it was running poorly? Maybe one of the valves was sticking open, or had a little carbon stuck on the seat. I don't know how to check the rings now without pulling #2 piston.|
After the head job, you could just put it back together and pray that it isn't rings. But the machine shop will have good suggestions.
| Thomas;; Try laying the cylinder head on it's side with the ports up and pour diesel or kerosene into the ports one at a time. If you have a bad valve you will see the leaks around the valve seat. The bumps you refer to are they the ridge at the top of the cylinders? This is caused by the rings wearing the cylindes below that line. |
please keep all of us informed.
I did the gas in the ports as suggested and they held the fuel. I rolled the head around looking for leaks and nothing.
I remembered a tip I heard somewhere about leveling the cylinders and putting oil in to see if the rings are holding...#2 is not holding the oil and leaked it out in about 45 minutes. The other 3 are still holding the oil at almost 2 hours. I think it's the rings.
Can I use regular compression pistons with the late model head? What are the benefits / drawbacks.
Any feedback is greatly appreciated.
This thread was discussed between 18/05/2006 and 20/05/2006
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