Technical Information (MGB 25)

MGB Cylinder Heads

Last Modified - 2/29/00

There were 4 types of cylinder head fitted to the USA marketed MGB's. These can be identified as follows:

We have seen other non smog cylinder heads, or non smog cylinder heads modified for smog purposes, installed, having different identification marks, I presume these were from other various vehicles that had the "B" series engine fitted and, apart from the Austin Marina, (affectionately known as "Son of Austin America"), were marketed in the UK and various other countries.

I will list the non smog cylinder head identification numbers that I have come across over the years, also matched alphabetically with its' smog counterpart.

Cylinder heads AB&D had 1.565" intake valves.
Cylinder head C had 1.625" intake valves with larger port sizes.
Cylinder heads AB&C had in line oil feed rear rocker pedestal.
Cylinder head D had offset oil feed rear rocker pedestal.
All cylinder heads shared the same size 1.344" exhaust valves.

Cylinder heads A and B had a combustion chamber depth of .425" with a capacity of 42.5cc to 43.5cc. These cylinder heads were matched with 6.2cc shallow dish pistons for a CR of 8.8:1.

Cylinder Heads C and D had a combustion chamber depth of .375" with a capacity of 39cc and were matched with large dish 16.2cc pistons for a CR of 8:1. Due to the shallower combustion chamber all 18V engine blocks had exhaust valve relief notches machined on the periphery of the bore to allow for exhaust valve clearance. I would estimate these notches to each have a volume of approx. .2cc.

When calculating your CR be sure to include the volume (3.21cc) of the compressed cylinder head gasket thickness (.023"). You will also need to calculate the cc volume of the piston below deck area (D2 X .7854 X .0?? X 16.39), this information can be found by reviewing the relative MGB and MGA technical articles. To arrive at these CRs the piston was left approximately .040" below the engine block deck. With a standard bore of 3.160" this will equate to approx. 5.12cc. We can verify this by doing a little math as follows:

To arrive at an 8.8:1 CR on a standard bore size 3.160" we would require at clearance volume of approx. 57.5cc. Formula: Clearance volume + Cylinder volume divided by Clearance volume. 57.5 + 450/57.5 = 8.8:1. Likewise to arrive at an 8:1 CR would require a clearance volume of approx. 63.5cc. Therefore 63.5 + 450/63.5 = 8:1.

Based upon this original bore size, we can then calculate the original engine block deck to piston crown distance, which equates to approx. .040".

Using this measurement as a guide, 16.2cc pistons assembled with cylinder heads A or B would result in a CR of approx. 7.7:1. Assembling 6.2cc pistons with cylinder heads C or D will result in a CR of approx. 9.4:1. Piston interchanges for the 5 main MGB will allow the 6.2cc pistons to be mounted to the 18V series connecting rods (2 styles). If the 16.2cc pistons are to be mounted on the earlier style connecting rods you will need to machine piston pin retaining grooves in the piston to retain the piston pin.

Machining the engine block deck so as to leave the OEM piston, (when fitted in combination with the correct cylinder head), flush with the deck would result in the following approx. CRs:

Since many any engines have been rebuilt in their lifetime, the above is is only a guideline for the potential engine rebuilder. Reboring to oversize, redecking the engine block, resizing connecting rods or machining the cylinder head will all increase the CR. If you wish to be extremely accurate you could have the connecting rods, as well as the crankshaft indexed. This would mean that center to center on the connecting rod and the crankshaft throw would be equal from cylinder to cylinder.

British Automotive custom made flat top forged pistons come with a new deck height of 1.7" rather than the factory measurement of 1.660". This allows enough piston crown material to be removed, instead of redecking the engine block, for the purpose of running the piston flush with the engine block deck.

Cylinder heads A,B,C&D have same exhaust port throat diameters, measured at the base of the valve seat insert, of 1.146" to 1.166".

Cylinder heads AB&D have the same intake port throat diameters, measured in the same manner as above, of 1.302" to 1.322".

Cylinder head C has a larger intake port throat diameter of 1.460" to 1.480"

A number of years ago we did a flow bench test on brand new stock OEM cylinder heads C&D. These results are as follows:

Cylinder head C with 1.625" intake valve and 1.345" exhaust valve.

Cylinder head D with 1.565" intake valve and 1.345" exhaust valve.

* Cylinder head C as above with simple modified exhaust port

* We installed the new exhaust valve guide .250" above the recommended installed height, then using the guide ID for centering purposes, we simply ran an appropriate sized cutter down and removed .250" from the boss and the protruding portion of the exhaust valve. The valve guide was then pressed back in to the correct installed height specifications. We were lucky on this particular cylinder head, for the next cylinder head we modified we broke into the water jacket.

Obviously, the danger with this modification lies in the inconsistencies of the casting material thickness present in the area behind the valve guide. Premature break through into the water passage can occur. Therefore, we do not recommend that this be carried out, however it does show that opening up the exhaust port in this general area will be beneficial to exhaust CFM flow rates. A good source of information on cylinder head modifications can be found in "How To Power Tune MGB 4 Cylinder Engines" by Peter Burgess. Moss part # 213-175.

Current replacement cylinder heads have a raised PMI identification mark on the front of the cylinder head. This stands for "Pierce Manifolds Incorporated", and are available in non smog or smog configurations i.e. air injection threaded ports.

Let me give you some dimensions and my observations on a recently acquired (Feb. 2000) new cylinder head fitted with 1.625" intake valves and 1.345" exhaust valves. Cylinder head thickness 3.105" Combustion chamber depth .425" approx. Combustion chamber 42cc. Intake port diameter 1.400". Exhaust port diameter 1.200".

First, this new cylinder head was warped, (required the removal of .007" material). This warp extended from between 1&2, 2&3 and 3&4 cylinders. Also at around the same time, we had the opportunity to check out a used aluminum cross flow cylinder head. This very low mileage unit had been recently purchased by one of our customers. This cylinder head was also warped in the same manner. My conclusion is: should either of these two styles of cylinder head warp, they will warp to a much greater extent than the OEM cast iron cylinder head, under the same circumstances i.e. overheating. This could lead to greater engine damage.

Second, on the OEM style cylinder head, we could not duplicate the stated combustion chamber capacity cc (37cc). With the spark plugs (WR7DP4 Bosch platinum) fitted and the valves installed, we recorded 42cc and 41.5cc after resurfacing .007"). Usually the rule of thumb is by removing .010" we have a 1cc loss in clearance volume.

Cylinder head thickness was recorded at 3.105"

Intake port diameter 1.400" approx.

Exhaust port diameter 1.203"

This stock aluminum cylinder head had 1.625" intake valves fitted. When we get the opportunity we will have another aluminum cylinder head flow tested with the stock 1.565" intake valves fitted. In the opinion of the machine shop who flow tested the head (Crowther Racing Engines phone 707 778 6050) was that; the fitting of a 1.565" intake valve would have very little effect on the intake CFM flow rate ?.

If you compare the above intake and exhaust CFM flow rates with that of cylinder head D there is in fact an all round improvement. Percentage exhaust flow to intake flow, beyond .100" valve lift, was good at 75% plus flow, although the 80% reading does look a little out of place.

If you compare the above intake CFM flow rates with that of cylinder head C you can see, above .100" valve lifts, that cylinder head C outperforms the aluminum cylinder head intake CFM flow rates.

If you compare the above exhaust CFM flow rates with that of cylinder head C you can see that the flow rates are very much the same. To bring the exhaust flow rate into the 75% plus range, cylinder head C definitely requires flow improvements.

There is definitely improvements to made on the aluminum cylinder head intake flow. Priority is in reducing the sharp right angle transition area as mentioned previously.

Let us presume that we have done this improvement and increased the intake flow to that of cylinder head C.

From these projected intake CFM flow improvements we are now going to have to improve the exhaust CFM flow rate, above .200" valve lift, to bring into the 75% plus range. I am sure this could be done without to much machining.

NOTE:

Originally, the MGA & the MGB 3 main bearing engine shared the same head gasket (OEM part # GEG366 Moss part # 296-400), However, this was factory superseded to GEG377 (Moss part # 296-405) for all MGB engines only. Part # GEG366 will work with a standard bore size of 3.160" when fitted with the original cylinder head (A) or type (B). However, should you either rebore oversize or install cylinder head type C or D you must use part # GEG377 only. The reason for this is;

Reboring, from the original size of 3.160", will result in the head gasket overhanging into the cylinder bore. Obviously, the greater the overbore size the greater the overhang. This could result in the gasket bore periphery raised edge not sealing as was the intended design.

Using this gasket with cylinder heads C or D will definitely create a more serious problem. The gasket overhang, well into the combustion chamber area, almost ensures that almost all of the raised sealing edge is exposed to the combustion process.

What is outlined above also applies to the current OEM style replacement aluminum cylinder heads. Should a head gasket failure occur with this type of cylinder head, you may be looking at more than just a cylinder head resurface.

Also, if you install this type of cylinder head with dual valve springs, valve adjusting shims (.015") must be placed between the outer valve spring and the cylinder head valve spring seat base. This will prevent damage to the aluminum seat surface. Also, be sure to retain the original inner valve spring locating collar Part # 460-220.

For single valve spring setup, retain the lower valve spring locating collar Part # 460-225.

Also, I might as well point out, that if you are using the aluminum style upper valve spring retainers Part # 460-305 use an additional 015" shim in the same manner.