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 Midget and Sprite Technical - 1275 headgasket & piston woes

Had a nice trip out to Malvern at the weekend and a not so happy trip home.

On the way the car started cutting out every time I stopped, and was a bit of a nightmare to get started again, but other then that nothing too bad. On the way home it was definitely missing, and not at all happy. Still I limped home - probably not a wise choice.

Compression test showed probable gasket blow between 2&3. Gasket was replaced for same reason a few hundred miles ago, and a different head fitted at the time.

Head off and indeed the gasket has gone. More worrying is the damage to no3 piston, which I'm assuming was the cause of the gasket going too, rather than a result of it ???

Pics
http://www.theclubman.co.uk/temp/head%20002.jpg
http://www.theclubman.co.uk/temp/head%20004.jpg
http://www.theclubman.co.uk/temp/head%20005.jpg
http://www.theclubman.co.uk/temp/head%20008.jpg

Questions really are what can I do to get this running again cheaply (if thats at all possible), as I have zero funds to plough into it.
As far as I can tell the block will need a skim as there is pitting where the gasket went so its an engine out job.
I have no knowledge of rebuilding engines (headgaskets are about my limit). Can I swap just the one piston? Will that mean a rebore or just a hone on that cylinder ??

The engine never over heated. Oil pressure was great. Oil is still oily, water was clear.

Then there is the issue of why that piston decided to eat itself? Given the timing is fixed across all cylinders, I presume a lean mixture on that cylinder. Car is running standard twin SUs, so how could one cylinder end up lean??

Running Lumenition Optronic ignition. All plugs were much the same colour and not erroded or damaged.

Sorry for the rambling and all the questions.

Thanks in advance for any help & advice.
G Hawkins

I think that might be carbon as opposed to piston damage....

Your now faced with a dilemma - clean it to check and send lumps of carbon into the gap between the cyl and piston or strip the engine and find out that a simple scrape and clean up would fix it...

If you want to be ultra cautious then drop the sump and take just no3 out, give it a clean and then see....
Toby Anscombe

Its definitely damage

Easier to see in this pic maybe?

http://www.theclubman.co.uk/temp/head%20006.jpg
G Hawkins

Odd thing is that it looks like old damage - i.e. there is significant carbon deposit over the damaged area so it may not be related to your current head gasket problem. Just because you have found a fault doesn't mean that it is the only fault!

Guy

Guy Weller

I cant get the photos to pull up...getting a 404error

Prop
Prop

Not that it has anything to do with anything, but are pistons no 1 and no 4 completely different to no 2 and no 3?
Garyfrog

Prop

Links work if you cut and paste them, but not if you click .... no idea why.

G Hawkins

if there realy is some metal gone there might hav been some pinking or detonation.

is the engine std of modified?
are the carbs tuned properly (no worn jets etc)

yes you can replace only one piston if you are on a realy tight butget.
a light hand hone might be needed since the rings will be replaced as well

i would not use this piston because piston top damage can mean grove damage resulting in oilconsumtion and loss of compression.

my advice pul the sump pul all of the pistons and check them out
then decide what to do next
Onno Könemann

I think it is possibly early signs of detonation. The gasket has burnt through between 2 and 3. Very often this then leads to a burnt area of land between the 2 cylinders like someone has attacked it with a burning gun.

The only saviour is as Guy has identified that the damage has now been "carboned up"


Have you ever heard pinking?

If it were me I would clean the tops of the pistons with a bit of solvent and scotchbright and then I would be looking to see if the damage was caused by heat or by mechanical effect.

Let us know what you think then.

Robert (Bob) Midget Turbo

Have I ever heard pinking....

I have to say no, based on the descriptions on the internet. I get the impression it's not something I could fail to notice?
G Hawkins

Clearly the head gasket has blown but this may have nothing at all to do with the piston damage. My point was that the apparent piston damage may be long-standing, and the original cause, whatever it was, may no longer apply.

How long have you owned the car?
Guy Weller

Because.....
If you have had it for a while and the damage was caused before you took it over, it may be that the piston is holding together well enough - as evidenced by the further deposition of carbon.

I would clean up the piston top to inspect it more closely. Put some grease into the piston / cylinder gap to stop the chunks of carbon getting down the bores. Slightly risky strategy perhaps but if you are a "glass half full" person with a tight budget you may be OK with it.

Maybe if you are a real optimist you could also skip the engine out for block skimming bit and try an MG Metro Turbo gasket next time?

Guy

Guy Weller

I have had it since 1990ish. It was driven (without issues other than points wearing out) for a few years then laid up for 10+ years (marriage, what can you do) then put back on the road for Spridget 50. The engine had stuck/siezed in storage. I squirted redex and wd40 down the plug holes and left it to soak for weeks. Oaccasionally rocking the car in gear.
Once it freed up it became apparent it would not run. Compression lacking on 2&3. Head off revealed a bit of a messy head and a gap in the gasket between 2 & 3. No 3 cylinder exhaust valve was also very recessed into the head, but none of that could have happened whilst sat for 10 years and it was running okay beforehand. To my shame I cannot recall any damage to the piston (just turned 40 the memory is going) dont think I overly looked as I was in a rush to get it going.
Put on a new gasket and a metro head went to Spridget 50 everything hunkydory. That was 500 miles or so ago....

I suppose the damage could have been there last time and so maybe from when I bought the car. If so, as you say maybe apart from not being ideal, the piston is holding up and the headgasket going again is not connected.

Looking through old pics I took last time, 2&3 are at the bottom of their strokes so maybe thats why it was not obvious to me then like it is this time ???

Hmmmm food for thought and lessons to learn.

Well. I am on a tight budget - if I go without food I might be able to afford a new head gasket so I might just suck it and see. After all if it lasts another 500 miles then at least I get to use it this summer.

On a related note, if I was able to source a good 2nd hand piston to replace it with, would any old 1275 A series piston do (Mini etc) or are the midget ones different??

Thanks for all the input. It's much appreciated.

Graham
G Hawkins

you would need one that is exactly the same as the ones in there and neer a set of rings as well

Onno Könemann

If the budget is that tight, you could take Guy's approach and assume it's old damage on the piston. Two HGF's in quick succession suggests the head should be skimmed, and from the the photo I suggest so too does the top of the block. You could do the block with a large flat file. The new gasket should then last until you have funds to fix it all proper like.
Mike (I might be cheap, but at least I'm rough) Allen
Mike Allen

My strategy is perhaps somewhat risky, but if you are not racing, only plan on travelling modest distances and have a "recovery to home" break-down policy then it might be worth trying it. It maybe depends how anxious you are as an individual as much as the condition of the car. It does look from the carbon on top of the damage that the piston top is not currently getting any worse.

The ideal is to get a replacement piston (Check the existing for any oversize stamped numbers). Someone her might have some matching spares? If it were not for the issue of skimming the block then you could drop the sump and replace the piston in situ without much problem. Maybe the Metro turbo gasket would be sufficient to hold between #2 and #3 as it is.

Guy
Guy Weller

This thread was discussed on 22/04/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.