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MG MGB Technical - Rebuilt Head Crack Probablility

What is the probability a cast iron head reworked for lead free will develop cracks post-rebuild? This of course assumes no cracks or those around the seats will be cut away for inserts. There seems to be little discussion of this in the archives.

I have an unknown vintage 18V in a 65 with a big valve head. No external leaks or cracks. The head was given minimal work when I replaced the short block 30k or more miles ago. I ran out of money and just lapped the seats (they were integral to the head with pitting but they cleaned up somewhat). Positive seals, new rocker assembly and new springs were installed. The cast iron valve guides were marginal.

I hate to go to the expense of rebuilding a 30 year old head just to have it crack in a short period of time. On the other hand the alloy heads are costly and won't add any performance boost.
Robert McCoy

Its my understanding that there are two kinds of MGB cast iron heads. Cracked ones and ones that are going to crack. That said, I have a 74B w/122000 miles on it which I've had for 23years. I have for a long time thought about a port and polish job on the head but have not done it because the head has never been removed... and If it ain't broke... I think my head is OK as I've always treated the car very well. About ten years ago I replaced the tranny with an O/D tranny and at the time the radiator had a problem or two. At that time I had the radiator re-cord and the car has always run much cooler ever since. You might consider it as high temp seems to be what causes the cracks in the head. Good Luck I'd like to find out later ifyou did it an how it went. Bob
Bob Ekstrand

Robert - is there any indication that a valve job is required at this point? If not, why even consider installing inserts at this point. To pursue this point even further, ask the machine shop at the time a valve job is necessary if the valve seats can be cleaned up and continue operating properly without installing inserts. Unleaded fuel doesn't cause recession of the valves nearly as fast as was once feared and the plain seats will usually last a long period of time before the need for inserts unless you are racing the car. Since the inserts are an interference (read, they are pressed into an undersized hole machined into the head, they are going to put a bit more strain on the surrounding material), they can't help but make the head a bit more prone to cracking. We have used our MGB on a daily basis on the freeways around here and between here and California for the past 10 years and have not had any valve problems in spite of not having had installed hardened valve inserts. I will have then installed when and only when the need arises. Cheers - Dave
David DuBois

If your cylinder head has never been rebuilt, the valve guides are probably hardened by now anyway. Just leave them be until replacement becomes necessary.

As for likelihood of the head cracking, there is no way to tell. If it's been magnafluxed then you're good to go. I suppose you could go a step further and have it x-rayed but how much money do you want to put into it?

For the record, my original 1965 cylinder head finally developed several cracks after an abusive 300,000 miles. I'm replacing it with a head of similar vintage that has been reworked by Flowspeed.
Steve Simmons

Hi everyone

With regard to valve seat recession, if you recut the seats and run leadfree recession can occur fairly rapidly. An uncut seat that has run leaded for millions of miles retains a protective layer and will more or less run leadfree unless severely abused.

With regard to length of life of a crack free head...we haven't seen many of ours cracked and returned over 24 years that is why we can give such a good guarantee.

Peter
peter burgess

Before you have cracks repaired, 8yrs & 20k+ I had my motor rebuilt by a Very Good builder and car is just fine. However, a week into the break-in period, I noticed and he admitted that he had missed the 1 2<> 3 4 crack in the head (Between 2&3). He said not to worry and put in an additive that he guaranteed to repair forever. Now we all know he was full of Horse $#*& but I said what the Hell, and guess what? I can get the name of the stuff but he said it had been around for many years but was very hard to get and if interested, I will call the fist of the week and post as to what it was. He was so sure of himself, that he said no matter where I was and if that crack ever caused me to put a white rag on my antenna, he would send and pay for the entire wrecker charge and pay for any repairs, FOREVER...
Steve C.
Steve C

Peter Burgess, I appreciate the input as you have such extensive successful cylinder head experience.

I haven't cut the old seats just a lapped them prior to reassembly. Who knows how long they've been off lead.

I haven't had any problem with valve clearances tightening up so maybe things are holding steady. I also don't push it too much and I've never overheated it.
Robert McCoy

Hi Robert, sounds ok, give it a go, the worst that can happen is the seats recess and you have to put seats in.
Steve C, we have the same iron sealant I think. It repairs freeze cracks in blocks etc. I have even see it fill the gap of a paper gasket failure on a water pump. We sell it mainly as a get you home for serious head gasket failure or cracked heads on the modern ally headed cars.

Peter
peter burgess

Seems like I heard new car makers put a puck of something in new blocks as a sealer backup. Maybe only some makers do this, don't remember the details. Anybody else heard of this, or am I just imagining things again?
Tom

When I used to help work on a short track race car we always had some Bars stop leak in the tool box. It's quick and very effective, don't know if it lasts forever though. Forever on a short oval track may be only 30 minutes or less.

Clifton
Clifton Gordon

I know GM had a lot of leaks years ago with their aluminum block engines...porosity in the castings and they sealed it with some sort of cellulose based additive. I understand it worked pretty well but does seem like a cheesy fix instead of properly redoing their manufacturing processes. (Wonder why they lost market share?)

For small leaks I've had good luck with Barrs but it didn't do much for a cracked heater tube in my old Taurus.

Guy's thanks for you input. I feel rather confident that if I rebuild an uncracked head or one with cracks that can be machined out during seat installation it is a good bet.

Peter, the big valve head don't have much room between seats. Can you install inserts on both exhaust and inlets? Just a former engineer asking technical questions so no need to rush reply.
Robert McCoy

Robert

We only fit exhaust seats if we can.If the inlet seat has to be repaired you can fit inserts up to 1.69" if you keep to small inserts for the exhausts.

When I get to work tomorrow I will check the name of the sealant.

Peter
peter burgess

Peter, Iron Sealent rings a bell, I will likewise check with the builder tomorrow. It was not the common things you get off the shelf like Barrs.
Steve C.
Steve C.

This thread was discussed between 09/01/2008 and 13/01/2008

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