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MG MGB Technical - Question about heads
|I have just gotten a 72B. I was wondering if there were any factory heads which had hardened seats for unleaded gas and if so when they started? Thanks|
|I'm sure there are, and that the change was about the time unleaded became common or mandatory. Trouble is, I can't remember when that was. I have a 77 that has an unleaded only sticker by the filler neck.|
In the mean time, adjust the valves and then see how quickly the gap closes up. If it is stable, you aren't having a problem.
|C R Huff|
|Cars in the US built in 1975 were the first required to use unleaded.|
|72 heads had soft seats! Saying that, the soft seats, as long as they are in good shape, will last a long time on unleaded gas, many, many miles. If they become recessed to where they need re doing, or the head needs rebuilding, then have the hard seats installed by a machine shop. JMHO. PJ|
|Will the use of a lead replacement additive help, or is it just another way to spend money needlessly?|
Yes, cylinder head (BMC Part # BHM 1062), used exclusively for the North American Market, can be readily identified by its casting number CAM 1106 and is commonly found on engine numbers 18V-797-AE-L, 18V-798-AE-L, 18V-801-AE-L, 18V-802-AE-L, 18V-883-AE-L, 18V-884-AE-L, 18V-890-AE-L, and 18V-891-AE-L, all of which it was Original Equipment for. This cylinder head casting always had a special drilled port located on its top rear end for ducting hot coolant to a thermo-controlled automatic choke mechanism. It is sometimes referred to as the “lead-free” cylinder head casting as its valve seats were induction hardened in order to withstand the higher combustion temperatures of lead free fuel. This was made possible by adding 1% nickel to the molten iron prior to casting. This led to an issue wherein the iron that came into contact with the mould suffered an altered molecular structure. In order to deal with this issue, an additional depth of material was produced in specially modified molds that was machined away prior to the induction hardening process being applied to the valve seat area. This was a complex process and applied only to these cylinder head castings for the North American Market engines. However, one should be aware that once the valve seats are remachined, the valve seats will be no more tolerant of the higher combustion temperatures of lead-free fuel than those of the earlier cylinder head castings, and that lead-free fuel tolerant valve seat inserts should be installed.
|I use a lead replacement additive, a bottle lasting about 3k so it's not expensive 'insurance'.|
|The only real lead free head is one that been retro fitted with hardened seats, after the head has been machined for them. As Steve says, the '75, and later NA heads were specially modified at the factory to deal with the loss of leaded fuel. This process was marginal, but you can get away with using a lead replacement additive. If you just do around town driving, you probably don't even need that. The valve seats will eventually wear down, but regularly adjusting the valve clearance should prevent the burning of a valve head. RAY|
|I just purchased this 72B after being without one for 5 years. I put on my last B, a 64, 186K in 15 years. I drive my cars a lot, but I know nothing about the real history of this car because the person I purchased it from lied. To him new rocker arm, exhaust system, alternator ,clutch and slave cylinders were anything under 6 years old. It is a fairly rust free example but I need a couple of years to fully sort it, Thanks for all the help. Ed|
|Ah, Speaking of heads, Thought I would try my hand at some porting. The head off this GH 1800 is obviously for the American market as it has the large bosses at the plug side where the air injection into the exhaust ports goes. My question is shall I just grind away these injection nozzles when porting the exhaust side? I will leave the old guides in place while grinding, then get the engineers to place the bronze guides ala Peter burgess advice.And of course lead free seats. The head I thinks will ultimately be exchanged,I will not muck it up, rather just see how difficut it is to do this sort of work. Mike|
I think you can just knock the air injection nozzles out with a drift.
|C R Huff|
|I should add that you knock them out from the combustion chamber side towards the air rail side.|
|C R Huff|
Hot tanking will help get the Air Injectors out of their ports. Remove the Air Injectors from the cylinder head and replace them with 7/16”-20 UNF fine-threaded iron machine bolts 3/4” in length, making sure to coat heir threads with antisieze compound. These somewhat rare items can be obtained from any supplier to boiler repair shops. Do not be tempted to use steel Allen head set screws because they will have to be bottomed out into the cylinder head in order for their threads to create an effective seal. Should a casting defect be present, the resulting stress stemming from the different coefficients of expansion between that of the steel of the Allen-headed plug and that of the grey cast iron of the cylinder head can result in cracks forming between the walls of the exhaust ports and those of the coolant passages adjacent to where the plug is seated. This does not occur when the steel air injector plugs are seated in place because, being hollow, the steel of which they are fabricated can expand inwards and thus not place any stress upon the material of the cylinder head. A rare practice, in the event that the air pipes need to be replaced in future, is to put a 1/4” ball bearing under the plug in order to prevent the plug from damaging the seat. However, jamming a ball bearing between the bottom of the plug and the seat would both concentrate and increase the thrusting stress on the thin port wall. Ba-a-a-d practice. Should cracking occur, the coolant system will be pressurized by the venting exhaust gases when the engine is running, leading to leaks at the hose (flexible pipe) junctures, vapor lock inside of the coolant system, and, in some cases, a blown cylinder head gasket. When the engine is not running and the exhaust valve is closed, coolant will puddle atop the exhaust valve as well as leak into the exhaust system. If the exhaust valve is open, the coolant will enter the combustion chamber and trickle down into the crankcase, polluting the oil. One might reason, “Are we not to also fret about the same thing because of steel studs securing the manifolds to the cylinder head?” The steel cylinder head studs and their mounting threads in the cylinder head are engineered to work together, while a solid steel Allen-headed plug and the threads in the air injector ports are not, so such reasoning in such a case is fallacious. Of every ten cylinder head castings that Peter Burgess examines for their potential for rebuilding purposes, he has to reject nine due to cracks having already developed, most commonly in the vicinity of the exhaust valve seats for #2 and #3 cylinders. In our case, we are not dealing with brand new grey iron cylinder head castings, but old, tired ones. Having your cylinder head reworked by an expert such as Peter Burgess is not cheap, but is a highly worthwhile investment. However, the prerequisite removal of material from the interior of the passages and combustion chamber further weakens the casting. Putting it at risk by using Allen-headed steel plugs thrusting against steel balls when you could use something more appropriate is just plain foolish, no matter how theoretically minimal you may think the risk might be. In our reality, it is usually greater than you might think at first guess.
|Whoa! Thanks Stephen. I might just not touch the head at all, and send it to Peter. Mike|
|My original head on my originally US car cracked as Stephen described. Maybe that was because I used steel machine screws to replace the air injectors? I had never seen this warning before. It does look as if the head may have already cracked before and been repaired as I can see evidence of what looks like welding and grinding on it. I had to get a second hand head to replace the cracked one.|
|Got 3 of the air injectors out, but the last one on number one cylinder has flatted itself out with the drifting. I will have to put my thinking cap on. Mike|
This thread was discussed between 14/01/2011 and 17/01/2011
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