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MG MGB Technical - Broken Piston

I have a 1972 MGB that was fully restored with only 3,000 miles on it. The restoration was done a little over a year ago by a restoration specialist. The car was running rough and had white smoke coming out of the tailpipe. My mechanic says I have a broken piston.

What would cause a brand new rebuilt engine to have a broken piston?
Rob Butterworth

Rob. Did the mechanic remove the cylinder head and find a broken piston or did he just make a "best guess"? White smoke normally indicates coolant getting into the combustion chamber and being expelled with the exhaust gasses. This is not caused by a broken piston unless it has cracked the cylinder block. They, you should be hearing a clang-clang-clang.

I think I would get a second opinion. Sounds like the head gasket has gone or a cracked head--both of which will produce white smoke in the exhaust. Les
Les Bengtson

Sounds like a classic blown head gasket. Swap it out and it oght to be fine. A lot cheaper and simpler than going ahead and tearing down the motor. If it still smokes--then you know you've got some other problem. "Your mechanic" may not be as astute as you'd like or maybe he's looking for some easy money.
R. L Carleen

Sounds like a classic blown head gasket. Swap it out and it oght to be fine. A lot cheaper and simpler than going ahead and tearing down the motor. If it still smokes--then you know you've got some other problem. "Your mechanic" may not be as astute as you'd like or maybe he's looking for some easy money. I'd be talking to the restoration specialist.
R. L Carleen

Was the head gasket ever retorqued in the
3000 miles since (assumed) engine rebuild?
Failure to retorque could have caused this.

Make sure to get the Payen gasket set (Moss or

I have a different approach. I just started up after setting for at least three years a 1979 midget 1500. Ran smooth but then the room filled with white smoke. extreme white smoke. I thought the worst and went away from it bemoaning a rebuild coming up. Next morning I started it up ,again and it ran perfect with zero smoke.Two days later I ran it again only to have the room fill completely up with white smoke and then the engine could not rev over 1200 rpm in neutral.Much gnashing of teeth was followed up by a phone call to a good aquaintance. His idea was that the little beasties had filled the exhaust up with seeds and nuts etc. He was right, I had to cut off the muffler and weld in a replacement. Works fine now.
Sandy Sanders
conrad sanders

Nobody really said much about the coolant level or what was going on there. Is there a steady stream of bubbles coming from the top tank or is it pressurising and overflowing both of which are a sure sign of blown head gasket. On the other hand if the coolant level remains constant I doubt if it is the head gasket. Have you tried a compression test. I'm not saying it isn't the gasket just don't remove the head until you have pretty well worked out what the problem is or you risk putting it all back together with the fault still there.
Iain MacKintosh

I have some experience with broken pistons:

As the others have pointed out, you'll not get white smoke with *just* a broken piston -- it would have to have done damage to the head or block. What you would expect with a bad piston is banging noise and total loss of power on one cylinder. Before doing anything else, take Iain's advise and do a compression test. You could also have your mechanic (or better yet a different mechanic) pressure test the cooling system. If it leaks down, then you know you've got a head gasket gone or a crack somewhere.

(FWIW, the piston in the picture did zero damage to the head or the block. None.)

Rob Edwards

all good points that have been raised and head gasket would be obvious...however another reason for white smoke is a damaged brake servo (if fitted!). Noticed any change in fluid levels?, also see if the inside of the oil filler cap has a mayonnaise coating on it, water and oil getting mixed up. Best bet is as Iain says..check compressions dry and wet, in case its a ring gone and work out from there.
good luck

I'm working from memory on this, but when my motor was rebuilt I had the same clouds of white smoke. My mechanic mentioned he had used *chromed* piston rings as an experiment - someone had recommended them. I took it back and he swapped in classic *cast iron* rings and everything's been fine since.

Yes, I know that white smoke is usually coolant, but I was assured that this was, in fact, blow by.

So - I don't know if I was given a crock, but all I know is that he stood by his work and the car has run like a champ for 12 years or so afterwards.
John Z

What on earth is a broken piston ! - to the range of possible causes add water in the fuel and the failure to hone the bores when the engines was refurbished - honing is essential and can get overlooked in a busy shop - in terms of probability the head gasket is close to the top of the list 'tho - If you need to go deeper, you can get the pistons out with the engine in the car.

Oh - and yes - change mechanic.


As language can be very poor tool for communication - Rob, do you think your mechanic may have said (or meant) a broken piston RING?

In my case, my mechanic did mention something about the chrome rings not sealing/setting well with the honing he had done, hence needing replacement with rings that would seal.
John Z

Broken piston means you'd be lucky to get the engine to run at all...and even then...LOTS of smoke and noise. A broken ring would give burning oil or other mechanical noises. $10 says it's the heaad gasket.

Do a compression test wet and dry as per several threads that have been on here .
S Best

Hang on a minute. Did this "specialist" overhaul the engine and if so what did he do to it? Was it smoking immediately you took repossession of the car. If so I also think that this fellow has more than a little responsibility to put the thing right. Hope though that this "specialist" is not also your "mechanic"

Don't however discount the comment about the brake servo problem because the engine could well be drinking the brake fluid.
Iain MacKintosh

For all of you that say a broken piston is accompanied by a lot of noise, NOT so! While running a supercharger on my 74 MGB, I broke a piston. It was accompanied by bluish white smoke and NO rattling. What broke on the piston was the area in between the rings. It did not make noise and luckily did not score the cylinder wall. I'm not saying that Rob does not have a bad head gasket, which is what I would guess, only that a broken piston does not always make noise or lose gobs of power.

This thread was discussed between 25/10/2004 and 01/11/2004

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