MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 MGB Technical - cracked head

water around #2 spark plug what is best cousre of action
ray

Ray. First, make sure the cylinder head is actually cracked. "water around #2 spark plug" is not a definitive statement--the "water" could be coming from a head gasket leak or a cooling system leak depending on where the water is. So, the first step is to remove the cylinder head and have it crack inspected. That is the best "cousre of action" right now.

If the cylinder head is found to be cracked, the best course of action will be something that you will have to decide. Possible choices range from "sell the car" through replacing the existing engine with a different engine, a V-8 or V-6 conversion perhaps. The middle road is to find a good, crack inspected cylinder head, of the correct type for your engine (they vary) and install it after having it rebuilt to unleaded specifications. Much of what is possible will depend on the year and the condition of your car, neither of which we know anything about.

Les
Les Bengtson

RAy,

If head is cracked it is still possible to find a good used one or have it welded. Or buy an econotune or an Al crossflow. But you must have in mind that if you improve compression from a new head, very often it is engine bottom which could not sustains the stress.

Cheers,

Jean G.
Jean Guy Catford

If you believe it is cracked, pull it and have it magnafluxed by a good automotive machine shop. Almost all of them can repair it by either conventional welding or a process called plug welding. It's a pretty rare one that cannot be repaired. If it cracks through a previously repaired crack you're pretty much out.
tom (member)

Ray-
I suspect that people are over-reacting to your problem. The head might be cracked, but the problem may be nothing more than a blown cylinder head gasket. I agree with Les that the best course of action would be to have the head checked for cracking by a competent shop. At the same time, have it checked for warpage. If it proves to be uncracked, but simply warped, the head can be saved by skimming its mating surface. In that case, reinstall it with either a Fel-Pro or Payen resin-type head gasket. If testing reveals cracking, do not bother trying to save the head by welding, as such a repair is usually nothing more than a temporary fix to get you down the road.

Welding cast iron is a very tricky thing, requiring the right tools. Contrary to what some welders might tell you, as a former Tool & Diemaker I can explain why it cannot be done on a bench in the garage. The problem lies in the fact that a casting is essentially just a bunch of bubbles held together by metal. There is always the risk, even though the alloy of the engine block and the alloy of the welding rod may be the same, that the density of the weld will be different from that of the density of the casting. This results in different rates of expansion and contraction when the casting heats and cools. If the density of the weld is not the same as that of the cylinder head, the casting will crack where it adjoins the weld and you will find yourself right back where you started.

However, because creating a weld is nothing more than a matter of heating the metal alloy of the rod to the point that it flows into and heats the metal of the casting to the point that it liquefies and blends with the molten alloy of the welding rod, it is possible to achieve the same density if certain conditions are met: First, the temperature of the molten metal of the welding rod should be no higher than that necessary to attain a molten state. Second, the casting should be heated in a heat-treating furnace until it almost melts (about 1600o). The white-hot iron casting then is removed and the weld applied with a cast iron welding rod only, then the casting is quickly placed back in the furnace and very slowly brought down to room temperature in controlled stages. Although this controlled cooling process will help to allow stresses to even themselves out, the casting may become warped and require machining.

Why is it so necessary to heat the casting to a predetermined temperature in a furnace instead of just heating it with a torch on a welding bench? Simply so that the temperature of the weld will be as close as possible as that of the casting. Why is that so important? First, because of the density issue already described above. That requires a degree of precision control that a welder cannot attain with a blowtorch on a workbench, even though he may sincerely believe that he can. Face it, the man is a welder, not a trained Metallurgist or a trained Tool & Diemaker. He simply does not know any better. Secondly, due to the fact that the heat differences are not as localized, the localized thermal stresses created by the extreme heat of welding will be minimized and not be isolated to the area immediately around the weld. Cast iron conducts heat very slowly, so the closer the temperature of the iron of the casting to that of the weld when the welding process begins, the less thermal stress will be generated in the areas adjacent to the weld. This elaborate procedure is necessary in order to eliminate the possibility of cracking due to induced thermal stress, which is a separate issue from that of weld density. The whole idea behind the process is often called “stress relieving”, a process that I am sure that you have heard of. Now you understand just what it is.

Needless to say, this process is expensive, but justified when working with a rare and irreplaceable cylinder head, such as would be the case with one from a 1910 Rolls-Royce. If the problem is with a crack in the cylinder head, I would just scrap it. There are many used heads available in good condition for far less money than what the above-described process costs. You would have to pay for the additional machining costs on the cylinder head either way that you choose to go, so why bother with the expense and risks of doing a proper welding job when there are good used heads available?
Steve S.

Ray,

I have yet to meet water that will flow upwards from the head gasket to No 2 plug!

Look carefully at the casting on a diagonal line from the plug to the centre stud - clean the paint and any rust off with a sanding drum on a dremel.

You will probably find a thin brown line that is a crack - look also in the mirror image location over No 3 plug.

Cracks in these locations are not successfully repairable - the casting is just too thin in these areas for stitching or welding/brazing. Look for a new head!
Chris at Octarine Services

Ray-
Chris has just pointed out a significant matter of interpreting what you posted. When you say that water is "around #2 spark plug", do you mean that it is in the general area beneath it, or puddled in the recess into which it is screwed?
Steve S.

can fill block with water ex tank half full go for short drive around 45 come home ex tank full are over flowing water at base of sparkplug
ray

Ray. From the fact you mention an "ex tank", I am going to assume you are referring to the expansion tank used on the rubber bumper cars having the last model radiator. It sounds like the cooling system is being pressurized by either a leaking head gasket or a crack in the cylinder head which is allowing the coolant to be pressurized. Again, the only way to make such a determination is to do a physical inspection of the cylinder head and gasket.

Les
Les Bengtson

This thread was discussed between 19/04/2009 and 20/04/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now