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MG MGB Technical - Fitting Different Heads

I believe this has been discussed, but I was wondering what heads will fit what engines.

Will a large valve head fit a 67 MGB engine? Is there any great advantage in using a large valve head? Thinking of putting a supercharger on and felt that head might be better with the SC.
Bruce-C

Also had the same question relating to crank shafts. I know other posts have said that there is one MGB crank that is better than others. Are the cranks also interchangable?
Bruce-C

Most 5 main cranks will interchange. Best one is the Flat side cast crank. Many different rod combo's as well. All heads will fit but later ones have the rear water outlet an older one does not. More a question of chamber volume than valve size, you need to watch what the compression ratio will be. Peter Burgess's book pretty much will explain all of the possible combos and the best options. Sure there is a ton of info in the archives.
Ron Smith

Bruce,

the 'large valve' heads make use of an offset oilfeed at the rear rocker shaft pedestrial. The chamber volume is 42-43mm³. If your block has allready cut outs at the top of the bore and an offset rocker shaft assy is used, it should work O.K..
Do not expect dramatic gains in power, as long as the stock camshaft is used further on.

Ralph
Ralph

This http://www.flowspeed.com/cylinder-ident.htm indicates that later heads need a cut-out in the block for the exhaust valve.
Paul Hunt

you need to speak to peter burgess , he makes a low compresion head and will supply the correct cam for the job http://www.mgcars.org.uk/peterburgess/ i purchased a fast road head and 285 cam and im very happy with the job .
da

forgot to say he as alot of time for customers and i would say he is the leading mgb specialist in the country ,he allso exports to the usa .you can run a much higher boost with this head .
da

Sounds like another addition to my library is in order. I know Peter Burgess's name comes up a lot. Time I get to know his writing a bit better.

Thanks all.
Bruce-C

I thought the big valve head was 1972 to 1974 1/2 and then the factory weren't back to the smaller valve. The off-set oil feed was later.

The flat side crank is good but very late used a full circle counter-balance cast iron which might be worth looking into.
Leland Bradley

Bruce-
The North American Market MGB engine used five different cylinder head castings over the course of its career, all of which used variations of the same 1.344” exhaust valve size. This Original Equipment exhaust valve borders on being overlarge. This was a deliberate design feature made for the benefit of the factory race team so that there would not be any problems with the homologation rules of racing associations.

The first version of the cylinder head (BMC Part # 48G 318) was used on the 18G, 18 GA, and 18GB engines, used the earlier 89.2 gr. 1.565” diameter intake valve (BMC Part # 12H 435), It can be identified by its cylinder head casting number of 12H 906.

The second version was used on the 18GD, 18GF, 18GG, 18GH, 18GJ, and 18GK engines and can be identified by its casting number of 12H 2389. Like its predecessor, it also used a 89.2 gr. 1.565” diameter intake valve (BMC Part # 12H 435), but switched to a newer version of the intake valve (BMC Part # 2115) that made use of a new valve spring cup (BMC Part # 12H 3309) and cotters (BMC Part # 2117), dispensing with the use of the earlier retaining circlip (BMC Part # 1K 372) of the earlier valve. It should be noted that both of these cylinder head castings had an identical combustion chamber height of .425” and combustion chamber volume of 43cc. Both this head casting and its predecessor are of a Weslake heart-shaped ‘Closed’ design configuration, featuring a large promontory between the valves for ducting of the incoming fuel / air charge away from the hot exhaust valve as well as for reinforcing the thin roof of the combustion chamber in order to prevent cracks from forming on the valve seats. It was given a slightly improved intake port design as well as mounting bosses on the spark plug side of the cylinder head for the mounting of air injectors. It also introduced oil drain grooves that connect the valve spring recesses to the aperture for the pushrod that operates the intake valve, thus increasing lubrication of the bore of the tappet below. This feature was deemed advantageous due to the fact that in order to simplify assembly on a mass-production basis, the same valve springs were used on both the intake and the exhaust valves. Because the intake valve was heavier than the exhaust valve, the greater side thrust loadings on their tappets that were generated by the angular deflection of the pushrods resulted in accelerated wear of the bores for these tappets. The simple expedient of channeling the oil from the valve assemblies to these tappets largely eliminated this problem.

The third version of the cylinder head, introduced on the earliest 18V engines, used larger 94.8 gr. 1.625” diameter intake valves (BMC Part # 12H 2520) and revised ports in order to produce a bit more power at high engine speeds, although at the expense of a very small loss of torque at very low engine speeds. It can be readily identified by its casting number 12H 2923, and is commonly found on engines with the engine numbers 18V-584-Z-L, 18V-585-Z-L, 18V-672-Z-L, and 18V-673-Z-L, all of which it was Original Equipment for. Development of this cylinder head was originally started in response to a request from the factory race team for a more open-type combustion chamber that would reduce valve shrouding so that larger intake valves would become more practical on their racing engines, but it was found to offer the advantage of maintaining the power output of the North American Market cars when both the valve timing was revised and the compression ratio reduced in order to meet government-imposed air pollution standards. The 12H 2923 cylinder head castings have relatively large bowl areas in their ports, making them the most sought-after cylinder head casting for oversize valve installations. If fact, their bowls are actually larger than the 1.625” diameter intake valve for which they were intended, thus revealing the designer’s original intention that they were for racing purposes. It should be noted that this 1.625” diameter intake valve is actually larger in diameter than the 1½” SU HS4 Series and 1 ½” SU HIF4 Series carburetors that were Original Equipment on the mass production MGBs, making them a good companion for use with 1 ½” SU HS4 Series and 1½” SU HIF4 Series carburetors that have been modified for increased flow capacity, as well as for use with oversize carburetors such as the 1¾” SU HS6 and the 1¾” SU HIF6. These heads also introduced a larger-diameter port in its side for the heater valve, a feature that was continued on all later heads.

The fourth version of the cylinder head was the 12H 4736 cylinder head casting (BMC Part # 12H 4735) that was first introduced on the Austin/Morris Marina and also used on the UK/European market MGB engines 18V-846-F-H and 18V-847-F-H, all of which it was Original Equipment for. It reverted to the original smaller 89.2 gr. 1.565” diameter intake valve size in redesigned form (BMC Part # 12H 4211) that was used on the first two cylinder head castings, but had a somewhat improved intake port design that produced a 4% increase in flow at maximum valve lift, as well as offset oil feed in the rear rocker shaft pedestal (BMC Part # 12H 4737) in order to accommodate coolant passages that were redesigned in order to assist in preventing overheating of the exhaust valve of the rear cylinder. This redesign necessitated the relocation of the oil passage in the rear rocker shaft pedestal, which means that if you should choose to install it onto an earlier engine block you are going to need the later rear rocker shaft pedestal with the offset oil port (BMC Part # 12H 4737). It also reverted to the original smaller 89.2 gr. 1.565” diameter intake valve (BMC Part # 12H 4211) that was used with the previous 12H 4736 cylinder head casting. It should be noted that for the North American Market this particular cylinder head was used solely on the 18V-836-Z-L and 18V-837-Z-L engines that were produced from September of 1974 through December of 1974 for the so-called “1974 ½” models.

The fifth version of the cylinder head (BMC Part # BHM 1062) was 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-836-Z-L, 18V-837-Z-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 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 where 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 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.

All three of the later type cylinder head castings have a lower combustion chamber height of .375” and a volume of 39cc. Being of kidney-shaped ‘Open’ design and featuring a larger squish (quench) area, as well as a reduced promontory between the valves, they are a considerable refinement of the earlier heart-shaped design. These cylinder heads can be identified by their casting numbers that are to be found on the top deck of the cylinder head, underneath the rocker arm cover. With the exception of the 12H 2923 head casting, these new cylinder head castings had larger coolant passages at the rear of the cylinder head, but they also introduced redesigned coolant passages with greater surface areas to assist in dealing with the higher combustion chamber temperatures that resulted from efforts to reduce exhaust emissions. In addition, the extra material provided created both the indentations behind the spark plug holes as well as the mounting bosses provided for the installation of air injectors for the exhaust ports on these cylinder head castings, along with a shelf on the edge of the casting on the same side, which had the additional benefit of making them more resistant to cracking between combustion chambers #2 & #3, and the blowing of cylinder head gaskets due to warpage.


Concerning connecting rods-

The obliquely-split connecting rods (BMC Part # 12H 998) for cylinders #1 and #3, (BMC Part # 12H 997) for cylinders #2 and #4 first used in the three main bearing 18G and 18GA engines used a smaller-diameter.750” (19.05mm) wrist (gudgeon) pin secured by a pinch-bolt small end, thus making them noninterchangable into later engines without using their wrist (gudgeon) pins as well as their accompanying obsolete four-ring pistons. Both they and the obliquely-split connecting rod (BMC Part # 12H 1019) of the five main bearing engine (18GB, 18GD, 18GF, 18GG, and through early 18GH Series) weighed in at a ponderous 980 grams. Not only are they heavy, they are notoriously weak for use in highly stressed engines.

The horizontally-split connecting rods with balance pads used in the late 18GH, 18GJ, 18GK, and through early 18V engines were a notably lighter 845 grams. There were two variants of this connecting rod. The first variant (BMC Part # 12H 2444) was introduced in June of 1970 and is found in the 18GH, 18J, and 18K engines. It has a bushed small end for use with a floating wrist (gudgeon) pin that is secured within the piston by circlips. The later variant that was introduced in March of 1971 is found in both the late 18GK and the early 18V engines. It has an unbushed small end for use with a press-fitted wrist (gudgeon) pin (BMC Part # 12H 3596). The final connecting rod design (BMC Part # CAM1588) used in the late 18V engines had no balance pads and were the lightest, weighing 760 grams, slightly in excess of 1/3 more than the Arrow Precision connecting rod. The most desirable connecting rods for any normally-aspirated street engine are also the lightest ones as, due to their reduced inertia, their reciprocation will not only produce less Primary Vibration and power loss, but also less stress is placed upon their big end bearings as well. These can commonly be found on engines whose identification numbers start with 18V-883-AE-L, 18V-884-AE-L, 18V-890-AE-L, 18V-891-AE-L, 18V-892-AE-L, or 18V-893-AE-L.

Be aware that the connecting rods used on the 18GB, 18GD, 18GF, 18GG, 18GH, 18GJ, and through early 18GK engines use the larger 13/16” (.8125” / 20.6375mm) wrist (gudgeon) pins that floated inside of a press-fitted bushing in the small end of the connecting rod. This bushing was later eliminated in the connecting rods of the late 18GK through 18V engines. These later connecting rods also used the larger 13/16” (.08125” / 22.6375mm) diameter wrist (gudgeon) pins, but in their case, they were press-fitted into the connecting rods, so your pistons must be chosen accordingly. However, the small end of these later connecting rods can be machined in order to permit them to accept the earlier bushing if floating wrist (gudgeon) pins are desired so that a floating wrist (gudgeon) pin can be used. Due to the fewer parts involved with a press-fitted wrist (gudgeon) pin / piston assembly, more precise control is attainable than with a floating assembly. However, wear of the wrist (gudgeon) pin bores of the piston is greatly accelerated. Actual stresses, depending on cylinder gas pressures and engine speeds, are influenced by wrist (gudgeon) pin ovalization and bending, as well as by the hydrodynamic oil film pressure distribution in the pin bore. A floating wrist (gudgeon) pin configuration allows the wrist (gudgeon) pin to rotate during engine operation. As compared to a fixed wrist (gudgeon) pin, this prevents the floating wrist (gudgeon) pin from repeatedly flexing into a fatigue-inducing bow shape. Reduced wrist (gudgeon) pin clearances always have a positive effect. Wrist (gudgeon) pin to pin bore installation clearances can be reduced approximately 50 percent and configurations can thus be designed for higher specific pin bore surface pressures at peak cylinder gas pressure. Once the lower limits are established (example: scuff issues), upper installation clearances can be minimized. In the cases of both types, an assembly lubricant such as CRC Sta-Lub Extreme Pressure Lubricant must be applied to prevent galling during initial fire-up of the engine.

Shot peening and electropolishing of the connecting rods are necessary only if you are going racing. However, smoothing of all of the edges on a connecting rod can greatly aid in decreasing the possibility of pressure risers from forming fissures that will develop into fractures. Note that exotic lightweight connecting rods such as those marketed by Carrillo (588 grams) and Arrow Precision (570 grams) are primarily intended for racing use and are unnecessary for use in all but the most radical of street engines, although their lower reciprocating mass will reduce both horsepower loss and the amplitude of vibration caused by primary imbalance, although not its frequency. They also both make use of cap bolts in order to secure their main bearing caps instead of through-bolts or studs, thus endowing them with the rigidity that is necessary for reliability at unusually high sustained engine speeds. It should be noted that both of these designs use floating wrist (gudgeon) pins.

Perhaps the connecting rods made by Arrow Precision deserve special mention at this point. These are made from vacuum de-gassed nickel chrome alloy steel (Double-remelt), which has a very low sulphur content of .025%, which means fewer inclusions and faults. Simply put, double remelt is a method of producing steels that are very low in non-metallic inclusions. Solid inclusions such as silicates and gaseous inclusions such as nitrogen, oxygen, and hydrogen form microscopic stress-raisers within the steel. Trace oxygen and nitrogen form oxides and nitrides. For highly stressed components, these inclusions need to be reduced to a minimum level, lower than that which can be achieved in the normal steelmaking process. The high-quality steels are therefore produced in an arc furnace in which they are degassed by way of bubbling argon through the melt. They are subsequently cast into electrodes. These electrodes are then remelted under a vacuum, further reducing impurities. The production of double remelt steel is thus simply repeating this process for even higher quality. As these processes are obviously very expensive ways of producing steel, they are only used for critical applications where maximum strength is needed. When forged, this means that imperfections are reduced to an absolute minimum, ensuring excellent consistency. In the case of connecting rods, this permits a high level of strength to be achieved while using a minimum of material, thus reducing weight. This steel is referred to as EN24 (817M40 British Standard). EN24, as used by Arrow Precision, is comparable to 4340 AISI / SAE steel, both of which are available as both air melt and as vacuum remelted steel. This being the case, checks should be made when comparing with another manufacturer’s product as to just what type of 4340 is being used. 4340 is higher in hardenability than any other standard AISI grade. Other names for 4340 include Werkstoff 1.6565 and Kurzname 40 CrNiMo 6.

Steve S.

just pass the cash and get the head for the job ,you can wrire down as much tech stuff as you want but its not getting the jod done ,the £ is weak so spoil your self and get a purpose low comp head from

http://www.mgcars.org.uk/peterburgess/
daz

Bruce-
Peter Burgess’ books are: “How to Power Tune MGB 4-Cylinder Engines”, and “How To Build, Modify, And Power Tune Cylinder Heads”. The latter should be considered to be a companion volume. Both are essential reading for any neophyte tuner.
Steve S.

Steve: I read your post with great interest. My 80 has the 12H4736 casting head (PO replacement) since worked by Dave Headley who was impressed with its modification potential. It now has 1.65" intakes but, as I recall, had 1.625's in it when I sent it to him. At the time, we weren't sure of its origin. Now that we know more, I am curious if it was ever used in production with the big valves or if that must have been from a previous mod.


Randy Voss

Randy-
I believe that the 12H 4736 head casting was produced only with the 1.565" intake valve. 1.625" intake valves are a common modification since it uses the same "open" combustion chamber design as the 12H 2923 head casting that used 1.625" intake valves.
Steve S.

This thread was discussed between 08/12/2008 and 18/12/2008

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