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MG Midget and Sprite Technical - What's happened to my pistons!!

Ah, my cup runneth over!! No sooner have I diagnosed the source of my badly running engine following HG failure than I find something else to worry about!

I've just taken the head off prior replacing the HG and the broken rockers. After cleaning the piston tops I find that #1 and #4 piston tops have dozens of little nicks in them. Ay first I thought they were just little bits of carbon stuck on the piston but on closer inspection found them to be little nicks. They look just like somebody had gently stabbed the piston with a knife point. They are only small - slightly bigger than a pinhead and not very deep but can be felt with a finger nail.

As these were nice and new 900 miles ago, what is causing this. Is this a sign of detonation? I figure it is nothing to do with my HG failure (there is a nice little scar on #2 piston where the HG metal reinforcement hit it).

I assume these nicks are not serious enough to do anything about provided whatever is causing this is elimiated. Otherwise it could get expensive. I'd post a picture but it is too dark in the garage and they are too small to really see in a photo. Any advice.

Chris
Chris H (1970 Midget 1275)

No it is unlikely to be detonation as this normally softens the edges of the pistons and you can see where the things have almost melted. The damage you appear to be describing sounds like some sort of foreign body?? and mechanical damage

Are the spots/cuts uniform over the surface of the 2 pistons or is it localised in one area?
Robert (Bob) Midget Turbo

Roughly all over but just those two pistons.
Chris H (1970 Midget 1275)

How very strange??

What was the story with the broken rocker any connection??
Robert (Bob) Midget Turbo

Bob - there is a thread concerning the rocker called 'Compression: should I worry - follow up" that gives the story. The exhaust valve on #3 remained permantly shut so that the excess pressure blew the HG and continued to give grief (would not run) after HG was replaced until the problem was spotted a few days ago. I can't believe the two are connected.

I'm puzzled by this because it looks like impact damage (even if very minor) rather than excess heat. I can't think of anything that could have got inside the cylinders to cause this. I'm not missing any bits and the obvious things like air filter bolts are wired for security.

I'm wondering whether to just leave this as is or whether to polished the marks out.

Chris

Chris H (1970 Midget 1275)

Here is a picture of #1 piston. Each little bright spot on the piston is one of the little 'nicks' I mentioned. There is no obvious pattern - they are scattered all over. They are not deep and I managed to gently polish most out with well lubricated P2500 wet and dry (the grade for finishing paint).

I can't think what might cause this. Anything substantial (like a nut or washer boucing around would surely do more damage. Contact with the valves would be restricted to particular places. Grit or other foriegn bodies might mark the bores but would they cut into the pistons - I think not. Another mystery.


Chris H (1970 Midget 1275)

Here is a picture of piston #4. It's terribly out of focus but I think the large number of nicks is evident even here (all the bright speckles). I can't retake the photo as I have polished out these marks.

Could foreign bodies such as grit or metal particles be driven into the top of a piston when the charge ignites? I don't know where such bodies would come from as I only ever run the engine without an air filter when adjusting the carb and never out on the road.


Chris H (1970 Midget 1275)

How did the plugs look when tou took them out...
Onno K

Wouldn't you have noticed the damage when you pulled the head 3 weeks ago? Because it only affects 2 pistons it could be the aftermath of grinding paste left under the valves.
F Pollock

I know this isn't very likely but I wonder if this is a sign of impurities in the metal of the pistons burning out of the faces of them.

The "nicks" seem to be randomly scattered round the piston and whether up on the flat or down in the recess they look alike.

I expect something more prosaic will be found to be causing it, but honestly it doesn't look like FOD to me.




or have you been attempting the old hillbilly grit blasting method pouring sand down t'ole to grind out the inlet and exhaust ports? :(
Bill1

metal-rash ;-)
seriously, no idea - never seen damage like that before, usually much bigger/deeper marks, and not pinpoints either. Are the plug electrodes still all there and in good nick?
David Smith

I did not look too closely at the pistons when replacing the HG so it was only when I cleaned them this time that I spotted this. No valve grinding was involved currently (head off, head on) so unless this relates to start up 900 miles ago then grinding paste is not likely to be the cause.
Chris H (1970 Midget 1275)

Looks like something like swarf has been sucked in and done the damage.
Cheers John
HALL JOHN

>>> Looks like something like swarf has been sucked in and done the damage. <<<

That thought occurred to me as well, but you'd think that if this was going to happen in two cylinders, they'd be the pair served by one of the carbs. In this case, the affected pistons were 1 and 4. I.e., only one each piston for each carb. Oh well.

Are your throttle plate screws all present and accounted for?

-:G:-
Gryf Ketcherside

Chris, you seem to have the same marks on the cylinder head in the first picture, bottom right hand side, is the damage possibly some some sort of chemical corrosion ? leaking oil or antifreeze as a result of the head gasket problem ?

Alan.
Alan cotterill

Alan

i think the 'marks' on the block are a trick of the light. I can't find them in reality.
Chris H (1970 Midget 1275)

Hi
Bit hard to tell from the pics but
Diesels go a bit like that if they get water in the fuel leaving little marks in pistons and combustion chambers --Having said that, diesel comp. ratio is a lot higher though Are there any marks in your chambers in the head ???????
Willy
William Revit

If you put it like that William, sounds like cavitation
Alex G Matla

Given the rocker failure which resulted in a major HG failure which in turn had serious blow back through the carb, I wonder if the marks could be the result of debris from the HG. If #3 blows debris out through the carb, could that debris them be sucked back into #4 and #1? The bits of HG had to go somewhere.
Chris H (1970 Midget 1275)

You should have some evidence of indentation twixt valve and seat. Do not be fooled by carbon indentation on ex valves though. You should be able to check progress of debris along the head and maybe track down the starting point. It can get confusing as debris can travel through the ex manifold from chamber to chamber and via the siamesing of inlet ports and centre ex ports. Debris often traps in ports around guides. The back pressure from the broken rocker will have blasted anything loose from the inlet manifold/ carbs/ maybe even filters. You may find marks in the carbon on the firefaces of the valves.

Peter
P Burgess

Peter
Thanks for your thoughts on this. My feeling is I shall never know for certain but HG debris seems a possibility. I'll be needing a rolling road session when everything is put back together so may be in touch.
Chris H (1970 Midget 1275)

This thread was discussed between 21/11/2011 and 24/11/2011

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