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MG MGB Technical - Cracked Head Cylinder Questions

Good morning. I have a 1968 mgb roadster. While changing the oil last week I noticed the head was cracked between the 2nd and 3rd spark plugs. I've searched the archives for information on alloy replacement heads, but all the threads center on performance discussions. I'm not interested in whether an alloy head is better than a ported and polished cast iron head. I am interested in anyone's experience with alloy heads, their installation, and the results. Are they bolt on, or do they require machining? Are they issues with two different metals and their different expansion rates (i.e. cast iron block versus alloy head)? Is there a preferred source for these? Proper MG has one for ~$700 US that includes valves, seats, etc. for unleaded gas. Anyone have experience with them? Thanks for the help.
Jack Caulder

Why not find a good iron head and have steel inserts installed and keep it original. You are not going to get any big power from carburetted 1800 ccs that will make a difference in comparison to today's world of 4 cylinder cars with FI, Dohc and variable valve timing. In other words the Toyota Corollas and Hundais will still blow your doors off, no matter what type of head is on it. FWIW
Alan

Jack. While it is true that much of the discussion I have read has covered tuning and modification of the aluminum cylinder head, there are a few other comments. The comment that it is no better designed than the factory head (with the exception of the cross flow head which has its own set of problems) is worth noting. If I were to purchase an aluminum cylinder head, I would want to know exactly what I was getting. Does it have the bronze-silicone valve guides and a triple angle valve job? Those items would be nice. What head gasket does it require. You are correct that the expansion rate of the cast iron block and the aluminum head will be different. There was an article on this in either B+ magazine or Sprite/midget magazine. The area of concern was the fact that, when fitting an aluminum head to a cast iron block, the auto manufacturers had to develop special head gaskets that would both allow for this movement and provide a good seal. I am not sure how the Fel-Pro/Payen type resin gasket would hold up in this application, nor am I sure how the standard gasket will hold up. But, having just gone through four cylinder heads (RB) to find a single one that was not cracked (plus the original, cracked one), the numbers I have been told, that 75%-80% of MG cylinder heads examined are cracked, may be an accurate observation. (It was made by two professional mechanics in conversation with the owner of the machine shop which does most of their work.) So, like Jack, I too am interested in getting on record an accurate picture of what the current aluminum cylinder heads are like, what problems they are displaying, what head gaskets work and are they tunable. There have also been suggestive comments that the cylinder heads available in the US (Pierce, I believe) may differ from the ones available in the UK from a different manufacturer. If this is true, it would be nice to know along with which is considered better and why. Thanks for asking the question, Jack. Les
Les Bengtson

Here's my 2 cents:
My 74 had the porosity problem of seeping coolant above #2 spark plug. I got the aluminum head from Moss (in the later version in case I ever needed to hook up the air injector rails) and had it built with no mods but hardened valves etc for unleaded gas. I used ARP studs and the copper head gasket from VB. On the studs I used an anti-seize compound where steel screws into aluminum. So far I've got 5,000 miles on it since June (it's my daily driver now) and no leaks, no problems. Right away I noticed that the wall on the spark plug side seems to be a lot thicker metal than my original head. They advertise them as running cooler but mine still runs like a typical MGB, hot in hot weather and cold in cold weather. They advertise them as not cracking or warping, but I've heard stories that they do indeed crack or warp. I can't tell any improvement in performance but this is a worn engine anyway. Overall I'm happy with it and it's been worth the money for me to have my MGB reliable to use as a daily driver. I had my work done at The Motorway, Ltd www.motorwayltd.com and all the heads they've been able to find already have cracks, warpage, or that seeping problem, so I'll try my luck with aluminum.
Hope that helps your decision-making process.
Ken T
Ken Thompson

For what it is worth, I have had two cars with Ali heads on iron blocks - the differential expansion does not seem to be an issue and both cars used steel/fibre/copper gaskets. What is a problem is that if they overheat the ali heads do warp and need to be skimmed flat before you will get a perfect seal again.

I don't know what you do to heads in the US but I have yet to see a head that was so badly cracked that cutting the valve inserts did not remove the cracks entirely. I have yet to see a head with the cracks as illustrated in Peter Burgess's book.

Chris Betson

My MGB had a cracked head I was told, when I took it to the machine shop to have new valves etc installed. They said that they may be able to weld it. I Said go ahead. The head has been on the car for 12-13 years now. Had it off once to have the hardened seats installed and new guides/valve job. reinstalled it that was 2 years ago, still going strong. I think that the cracks were in the valve pocket area and I caught them in time before serious cracking occured (just guessing)
Alan
Alan

Jack-
To my knowledge there is only one supplier of new aluminum alloy heads: Pierce Manifolds, so the one sold by Proper MG will be a Pierce head. An aluminum head shave abouts twenty pounds off of the weight of the engine and tend to run cooler under the high stresses of racing. They also require that washer-like steel shims be placed under the springs to protect the aluminum material of the head from galling by the springs. Because the cast iron of the block and the aluminum of the head have different expansion rates, the use of a high quality head gasket is mandatory. Unfortunately, the poor thermal efficiency of aluminum forces the use of increased compression of about one point to produce the same amount of power. This, coupled with the obsolete kidney-shaped combustion chambers creates in turn a problem with preignition when running on the gasoline available at a gas station unless considerable and frequent attention is paid to the maintenance of precisely correct ignition and carburetor settings. In addition, careful attention to the cooling system is a necessity as aluminum has a severe tendency to warp should it overheat badly. If this should occur, the temper of the alloy will have been ruined and thus the torque settings of the head stud nuts will not hold, the head having become just so much scrap metal. You don't get something for nothing!
Steve S.

Thanks for the discussion and experiences shared so far, it's been helpful - albeit inconclusive. Not sure which direction I'm going to go - looks like I need to talk to my trusty mechanic (was hoping to not have to). Thanks for the help.
Jack Caulder

This thread was discussed between 21/10/2002 and 22/10/2002

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