Welcome to our resource for MG Car Information.



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 MGA - Crap....just crap

Everything is just falling "f"ing apart on this car.

OK here's the story from the compression test.

#1- 145

#2- 135

#3- 30




#2-85 !!!!! (started to increase back to 95 as it dried up)

#3-30 (again)


I knew #3 was bad as soon as I pulled the spark plug. 1,2 & 4 plugs were nice and tan #3 had a black crusty coating with pitting and erosion on the electrode arm.

So what's next?

Based on the reaction of #2 cylinder during the wet test I'm going to guess that I should start at the head.

Any words of wisdom before I just pull the head on this thing.
T McCarthy

sounds like a minium rings , valves and bearing job to me. Pull the motor and check the crankshaft , timing chain and gears. If it is sad inside maybe a good second motor would be less painfull on the pocket
S Sherry

Rings for sure. Since the reading in number three didn't change wet or dry, you likely have a small hole in the piston. This would also explain the oil you had dripping from the valve cover hose and the hihg oil consumption. Holes in pistons are normally caused by lean mixture, over heating, or a dropped valve, we would have heard the valve.
J Heisenfeldt

OK I'm confused.

Why rings? Cylinder #3 has a decent Dry compression and a bad wet compression?

I thought a bad dry compression and a good wet compression where signs of bad rings????
T McCarthy

According to your first post #3 is at 30 wet or dry. This would indicate either a hole in a the piston or a stuck valve. A stuck valve will not effect crankcase pressure which makes me lean toward a piston with a hole as the blow by it would create would cause your oil issues. A stuck valve would also cause a miss and popping out of the exhaust or intake. Since #2 went down during the wet test you may also have a blown head gasket between #2 & #3. #1 & #4 got better wet which would indicate bad rings.

Pull the head to start, wet or dry the compression readings are bad, indicating some type of internal problem.
J Heisenfeldt

More clues/information.

Whoops, J you are right. #3 stayed at 30 regardless

#2 is the one that dropped when wet. It was 135 dry and 85 wet. This morning it was back up to 125.

Other clues is that there is a definite "burnt" odor to the oil.

I had hoped that perhaps I'd have simply a blown head gasket between 2 and 3 but I'm worried that my slipping distributor may have caused a burn out in cylinder #3. Here's hoping its only a burnt valve and not a burnt piston.
T McCarthy

This is likely one of those good news/bad news deals. The bad news,it sure sounds like you holed a piston. The good news is the hole is likely near the center, meaning the cylinder walls will be fine. A center hole is normally due to heat caused by lean mixture or retarded timing. Damage from spark knock tends to damage the ring area.
J Heisenfeldt


Here's the proposed course of action.

Pull head inspect damage.

If there is noticable damage to piston/cylinder wall pull block.

Any other suggestions?
T McCarthy

Thats about all you can do unless you can get your hands on a bore scope and put it thru the sparkplug hole to look around inside each cylinder. From the discription of your problems you are going to need to pull the head at a minimum.
J Heisenfeldt

First, check all valve clearances. ITs free! Then run another dry compression test. If number 3 is still at 30, you most likely have a burnt #3 exhaust valve. Don;t go overboard. You may not need to tear the entire engine apart--just a valve job.

An increase in wet compression is the sign of worn rings. Its the oil that fill the excess gaps and raises the compression. Hole in the valve, it machs nict how much oil you put in the cylinder--the hole is still there!

Since this is a BMC B series engine, its most vulnerable item is exhaust valves #2 & #3, I vote for bad valve. jm2c

Paul Hanley

Too bad.
mike parker

Discoveries so far.

I started to take the head off however the washers have thrown me a road block. See other thread.

Thus far I found that the head stud on the rocker bridge at cylinder #1 has enough play to wiggle in place. I assume that means its broken somewhere.

There is some preliminary evidence of trouble with the #3 exhaust valve: at almost complete let up on the rocker screw you can't get a feeler through so either a degraded seat or valve.

T McCarthy

Modify that last comment to say the rocker has "significantly more" let up to get feeler through and strike "almost complete"
T McCarthy

Tysen, did you rotate the engine when trying to back off the rocker screw? If the valve happened to be open you may have pressure on the rocker even with the adjuster backed all the way off as the tappet is on the peak of the cam lobe.
J Heisenfeldt

Ok here's the story. Pictures tomorrow.

Definitely a blown head gasket between #2 & #3 potentially burned exhaust valves in #2 and #3.

Lots of other stuff going on that I will look forward to hearing your in put on.
T McCarthy

I had the exact same compression problem with my 61 A.
I pulled the head and all of the valves had recessed into the valve seats. The #3 exhust valve was bad. Replaced all of the valves and seats, replaced the oil seals, cleaned the carbon build up off of the pistons. Put it back together and the engine purrs.
JP Jim

This thread was discussed between 06/08/2008 and 08/08/2008

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.