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MG MGA - Oh No.....

Yesterday was the hottest day this year, so I was really looking forward to taking the car out to our first cricket match for over 7 weeks. I'd gone about 4 miles when I heard a loud "pop". About 50 yards up the road, the cockpit filled with blue smoke, smoke from out of the rear and the vents on the front shroud. Fearing the worst, I opened the bonnet and saw smoke from the front airfilter, the rocker box vent and the dipstick hole. I managed to get the car back home on 3 cylinders. Went to the cricket match, batted at No. 8 and was out for a golden duck, bowled not too bad but took no wickets. I'm going to take the head off later this week to find out how bad the damage is. I'm hoping it's just a head gasket but with the smoke from the interior, I think it's either a valve or piston ring.
Can it get any worse? The wife couldn't care less so I just needed to rant off to someone.
Hope you all enjoy your summer.
Ray.
Ray Allden

You sure it wasn't just a backfire from running far too rich? I'd change the plugs first and start it up again and see how it sounds.
dominic clancy

Hi Dominic,
Thanks. The amount of smoke was quite heavy so, as a natural pessimist, it couldn't be that simple. I've just took the head off and found No.3 piston cracked. I've uploaded a picture as I've never seen the like before. I rebulit the engine 1485 miles ago (4 years) and never driven it hard. I would be grateful if anyone could throw any light as to why this should fail like this. The bore is also scored. How far can these engines be bored? The rebuild was bored to 20thou with new pistons, crank regrind etc. etc.


Ray Allden

Ray,
What brand of piston is it?

Ray
Ray Ammeter

Interesting failure. Doesn't look like it was hit with anything on top. Wild guess, defective piston cracked from thermal stress. I may tell better tales once you get it out and see the bottom side.

If the bore is not badly scored, hone it to clean up and drop in a new piston. If it is too badly scored to clean up with honing, then strip it down for machining and have the one bore sleeved to match the others.

Barney Gaylord

My 1600 is bored .060 over but I'd listen to Barney.That way you'll only have to get 1 piston and work on 1 cylinder.
gary starr

Ray, did you clean up the piston?
Is it possible that this cylinder was running lean and gradually went? The piston looks as if it lost some metal in the middle of the crack, and projected outward.
Mike Parker

Hi Mike,
I did clean the piston, there was a slight build up of carbon on it, it definitely wasn't running lean.
Hi Ray,
I think they are Hepolite, I'll check when removed.
Hi Barney,
I haven't taken the piston out yet, as you know, it's a major job so I'll try to start it the weekend. My thoughts were the same that it's probably a faulty piston. As I rebuilt the engine over 3 years ago, even though it's only covered less than 1500 miles, I doubt if I can go back to the supplier for any Warranty but I will try. I like the idea of re sleeving just the 1 bore, I've attached a picture of the scoring, it's pretty deep. It looks like the gudgeon pin has moved when the piston cracked. I'll let you all know when it's stripped out and post a picture of the underside.
Ray.



Ray Allden

Ray

Chicken or the egg? ...or did the gudgeon pin cause the piston to crack. I was scouring some old motor engineering books last night for any reference to causes of split pistons. There was one mention (1930s) of a bent con rod putting a twisting moment on a gudgeon pin and causing a piston to split.

Steve
Steve Gyles

Hi Steve,
Of this I'm not sure. I hope to find out when it's stripped. If the last report was in 1930, it seems a bit unlikely , but there again !!
Thanks,
Ray.
Ray Allden

Ray,
Steve's mention of the connecting rod made me look again at your pictures, and the piston is split fore to aft, slightly off center, which would be slightly off center-line with the wrist pin (gudgeon pin). Check and see whether you have installed the connecting rod to the piston correctly, as well as the wrist pin to the piston. The manual states that "It is essential that the connecting rod and piston assemblies should be replaced in their own bore and fitted the same way round, i.e. with the gudgeon pin clam screw on the camshaft side of the engine. The piston crowns are marked 'Front' to assist correct assembly to the connecting rods."
Mike Parker

Hi Mike,
Thanks for your comments. I'm hoping to start the removal this weekend but I may not be able to complete it. I'm fairly certain that I did connect correctly but I'll let you know when it's stripped.
Regards,
Ray.
Ray Allden

This thread was discussed between 06/08/2007 and 08/08/2007

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