MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 - Piston Broke

Hi all,
I've just removed the engine to establish what happened. I've attached a picture of the gudgeon pin, it has snapped by the pinchbolt flat. I have posted again to show the inside of the piston, large chunks have come away. I just cannot tell if the piston or gudgeon pin was the cause. To be fair, the pin is 40 years old and the shock of the car running again after standing unused for the last 15 years told it's toll. One other thing, the main bearing shells located in the main bearing caps were worn. The engine has only covered less than 1500 miles since the rebuild and these are the only worn bearings. I am getting the cankshaft micrometered to see if there is a problem with the regrind as the big ends and all others are ok. I am really annoyed at this but as I said before, I rebuilt the engine some 4 years ago before the car hit the road 3 years ago, so the machine shop are washing their hands of it as it is "out of a reasonable warranty period". I know it's perverse but if the piston had not failed, it could have led to a much more serious problem if the crank had failed due to a bad regrind. I'm going to see them again when they open on Tuesday (after the bank holiday) to present them with the damaged parts to see if I can prick their conciense


Ray Allden

Ray. You are very lucky. I had a similar problem many years ago. Freshly rebuilt A-H 100 engine where the wrist pin broke, the upper part of the connecting rod broke and the rod went through the side of the block. The problem turned out to be that the machine shop had over tightened the pinch bolt on the rods. You might want to check what yours are torqued to to see if that could be part of the problem.

I would recommend that you have all of the connecting rods and wrist pins crack inspected to see if there are any cracks developing. Not very expensive compared to the cost of an engine rebuild or replacement.

Good luck with the project and thanks for posting the photos. I have never seen such a failure before, so it is of considerable interest.

As to the main bearings, I would also suggest testing the crankshaft for straightness. The use of Plasti-Gage when fitting up bearings is highly recommended. It will help identify undersized journals or oversized bearings. If the line of plastic, after it has been compressed, is not of uniform width, look for a possible bent crankshaft. Inexpensive, easy to do, and it can provide much useful information.

Les
Les Bengtson

What all was replaced during the rebuild? Old pins with new pistons? (I don't know if that is a potential problem or not, just throwing out ideas.)
Steve
Steven B

I am reading this with interest also. I broke #4 piston coming back from the Scottish Games in Mesa, AZ. I can see a crack across the top of the piston, but have not had time to do more than remove the head and spot the damage. The bore looks clean, but I will have to wait until the piston is out see if there is any damage. I need to get on that because our driving season is approaching out here.

Dennis, Az
Dennis

Hi Dennis,
It would appear you have been luckier than I. I have just stripped the engine down and cleaned. By the way, I read earlier threads saying that they had removed the engine/gearbox as a unit, as you have the head off like I did, it is MUCH easier now to remove just the engine.
Hi Steven,
I did replace just the pistons with the original gudgeon pins, I'm going to replace all now when I do rebuild.
Hi Les,
Thanks for your comments. I have spoken to the machine shop, they will measure the crankshaft when they re sleeve and rebore. As to the torque settings, I rebuilt the engine myself and checked and re checked settings etc. so I am confident they were ok.
I'll post again when I get the engine back to see what they have found out.
Regards all,
Ray.
Ray Allden

I hate being pissed n' broke.
That is the typical result when you have engine trouble like that!
Good luck with the re-rebuild!

Cheers,
Rich
Rich McKIe

Thanks Rich,
I very nearly gave that addendum when I posted but the wife forbade me !!!
Regards,
Ray.
Ray Allden

Ray,
Well, I guess that th good news is that it doesn't look like your piston was in backwards. The bad news is it looks like the wrist pin failed. I guess we should look at replacement of the wrist pins when rebuilding, eh?
Mike
mike parker

I've just called in to Leacy MG to get the new gasket sets etc. and asked for a new gudgeon pin. It appears they are only sold complete with the piston. As the people who carried out the rebore and regrind supplied the pistons and shells etc. I now know that new pins were not supplied so I'm trying to find out where I can get a full set of 4. (I don't want this to happen again!)
Ray.
Ray Allden

Ray.
With that pin failing like that I would certainly replace that connecting rod and destroy the old rod. It must have a severe strain on the upper end. Could it have been a piston boss failure that caused the pin to fail?
Sandy
Sandy Sanders

If in doubt, throw away the con rod and gudgeon pin and piston. If the con rod fails in service, you will be extemely lucky not to have it punch a hole in the block, then the whole engine is scrap. I may have a conrod or two that you can have for the cost of the postage, and they may well have pistons and gudgeon pins attached if I still have them.
dominic clancy

Hi Sandy and Dominic,
I must be honest, I never really gave it a thought to not use the original con rod. I've just checked the cost of a new one and it's only £35.00 They have one in stock so I will get a new one.
Sandy, I still don't really know why it failed. As I posted previously, the rods are original. The engine had covered 81,000 miles before I rebuilt it so the extra pressure must have come since the rebuild, so I would suspect the piston rather than the gudgeon pin as the cause.
Dominic, The piston is scrap so I wouldn't have been using that anyway. Thank you for the offer of the rods but the cost of a new one is minimal so I will buy new. I'm still awaiting a reply on a set of pins.
Many thanks for your help and comments.
Regards,
Ray.
Ray Allden

This thread was discussed between 26/08/2007 and 04/09/2007

MG MGA index

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