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MG MGB Technical - There's a hole in my piston !!!

MGB 1800 Engine- No.3 piston badly pitted around edge on spark plug side for around 3 inches, and burned through. The top ring was already broken when the piston was removed, but other 3 rings are OK, although all the splits in the rings were aligned at the point of burn through.
100 miles earlier I did a compression test and No. 3 was down 5% dry test, compared to the rest, and equal when wet tested. This was carried out because although there was a good spark, the plug kept oiling up, so I changed the plug several times, but it made no difference. I had assumed a faulty O ring on the inlet valve to No. 3 was letting oil into the chamber, and so was on my way to remove the head for some remedial work and a decoke. The head gasket doesn�t appear to have blown. When the head was removed all the oil and water-ways were clear and the thermostat was functioning normally. When the piston gave up the car was doing 60mph on the motorway and had been for about 30 minutes.
Can anyone suggest why the piston gave up, as I don�t want to put in new pistons and refurbish the head for the same thing to happen again.
Paul Johnson

Paul,

i do not know whether it is the same fault that lent to the problem with your engine, but i had a simular problem on my B when leaded fuel was withdrawn and my faulty dizzy burned away parts of the piston as exlaned by you.

The vacuum capsule on the dizzy did not work right and there was wear in its bearing that was discovered as one of the reasons as the wrong timing did the rest on my car.
I would have the dizzy checked first by a competent service workshop.

Hope this helps

Ralph

Ralph

Paul. Three things can cause this problem.

First is something hitting the top of the piston and damaging it sufficiently that it will, immediately or over time, break.

The second thing is a too lean mixture. This is most commonly found on cylinders two and three because of the siamesed exhaust port causes them to run leaner than cylinders one and four.

The third thing, as Ralph notes, is timing. I have found that some of the points cams on the Lucas 25D4 distributors (the only ones I have tested) can be very inconsistent as to when the points open and close. This can cause the timing to vary by cylinder. Over advanced timing will damage a piston due to pinking/pinging (pre-ignition).

A combination of all these factors can also come into play. Excessive build up of carbon deposits within the combustion chamber and on top of the piston (often called "coke") effectively raise the compression ratio to some indeterminate degree. This alters the ignition timing requirements. It also can cause the fuel, which would otherwise be fine, to be, in effect, of too low an octane rating.

An accurate, reliable distributor is a good starting point. Cleaning the piston crowns and combustion chambers while the head is off is an excellent idea. Setting the mixture so that it is within the proper range for cylinders two and three rather than only using cylinder one and four as your indicators is a good idea. (Cylinders one and four should be slightly rich to ensure that two and three are correct.)

Beyond that, a physical inspection of the part by someone competent to determine the actual cause of failure is not a bad idea. Peter Burgess or Chris Betson are in the business and might be a possible source of such information.

Les
Les Bengtson

Hello,

Paul usually it is a typical situation of pinking( read pre-ignition). If your car under load you car hear a noise like a ket set where keys were shocking together. You have pinking. And what Les wrote is applicable...

Cheers,

JGC
J.G.CATFORD

Paul-
It is possible that your ignition system is the culprit. If the diaphragm in your vacuum advance capsule is ruptured, then your ignition timing would be running on pure centifugal advance. If you were unaware of the problem and set the ignition timing according to standard setting, then preignition could be a very real danger. To test the vacuum advance mechanism, remove the distributor cap and disconnect the vacuum line from the induction system. Suck hard on the line while observing the points plate for movement. If it doesn't move, the diaphragm is ruptured.

Les mentioned the phenomenon of timing wander that results from an inconsistent opening point for the contact breaker points. This can often be improved by installing a new bushing for the distributor action shaft in order to reduce or eliminate wobble of the distributor action shaft. Because the bushing is a sintered type, it should be soaked overnight in a bowl of oil before installation. However, if the load bearing section of the shank of the distributor action shaft is worn (usually as a result of inadequate maintence, then a new bushing may not be enough.
Steve S.

Oil lowers Octane which can lead to detonation (pinking) (+ coke increasing cr), shockwaves distube the protective layer on piston and build heat possible leading to pre ignition (piston on up), most destructive but not audible.
Paul Wiley

"If the diaphragm in your vacuum advance capsule is ruptured etc."

This is more likely to cause *retarded* running than over-advanced, and over-advanced would be required to cause pinking/pinging/pre-ignition. When setting timing no vacuum advance should be present, which requires disconenction and plugging of the port at the inlet manifold. It's not usually mentioned if you have carb vacuum since that gives no vacuum and hence no vacuum advance at idle.

However a ruptured vacuum advance diaphragm *will* result in a weakened mixture on the rear carb, i.e. Nos. 3 and 4. If you have manifold vacuum and this leak is compensated for by richening the rear carb to give 'correct'mixture at idle, then you will have a progressively over-*rich* mixture further up the throttle range.

Also retarded running results in higher cylinder temperatures as the fuel is used less efficiently.
Paul Hunt 2

I agree with Paul, no vacuum advance would mean exactly that. You'd get retarded timing when there was a manifold vacuum, timing spot-on with the throttle wide open.

If there was oil getting into the chamber then it may well be coked-up which could lead to pinking (as opposed to detonation). But I would think the pinking would have to be pretty bad to burn the piston. Certainly enough to hear it!

Plug fouling either means oil getting in, or mixture too rich, or wrong plug.

BTW Steve, what maintenance can you do to a distributor to reduce wear in the shaft?

Neil
Neil22

So far, I have never had to rebuild an MGB engine but I have rebuilt a few of Detroit's finest. Paul says in his original post that all of the ring splits were aligned at the point of burn-through. Is that normal for an MGB engine? I have always staggered the splits so that no two splits were aligned. Correct me if I am wrong but if all the splits are aligned, would you not have an opening from the combustion chamber straight into the crankcase which would result in a loss of compression, blow-by, excessive oil comsumption, foulled plugs, etc?

Mack

A Sneed

Paul-
As we know, the fuel/air mixture combusts at a fixed rate, which the reason for advancing the ignition timing as engine speed increases. However, the heat energy has to actually go somewhere. If ignition is initiated at a less than optimum time for this to take place, the pressure wave inside the combustion chamber ends up chasing the piston crown down the cylinder instead of exerting maximum force against it. The result is decreased power output. If the ignition timing is correctly advanced, much of the heat energy is absorbed by the piston crown because that's where the greatest effect of the pressure wave is. However, if the timing is retarded, even though power output is lessened partly by having less pressure on the piston during the downstroke (and more on the upstroke), a larger proportion of the heat produced has nowhere to go except into the roof of the combustion chamber. Hence, hotter running is the natural result.
Steve S.

This thread was discussed between 25/11/2007 and 01/12/2007

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