On June 25, 2009, Randy Kegg in Maryland wrote:
"I have a rebuilt MGA head, into which someone has installed MGB intake and exhaust valves. MGB valves are a little larger, so my first thought was that this is a good thing and would add to the performance of the engine. However, I have been told that installing the bigger valves is a bad idea, because of the increased shrouding of the valves by the walls of the combustion chamber, resulting in a decrease in air flow. What do you think? Will this head develop more, the same, or less horsepower on a 1500 cc engine than a stock head"?

Don't snicker at the larger valves. They can be okay goods.

It is very common to install one size larger valves when doing a standard valve job without installing new seats. If the seat is worn down so the standard size valve sits a bit too deep, the next larger valve can restore the proper seating height. Then you need to cut another angle (or straight bore) on the inside of the seat to reduce the land width to normal specifications (0.040-0.060-inch wide land when new). Land too wide will reduce contact pressure which could result in carbon buildup on the contact surfaces. The carbon might subsequently crack leaving a leak path that can then erode the valve seat or valve head in a serious way. End result then looks like this:
http://chicagolandmgclub.com/photos/a_head/1043.jpg
http://chicagolandmgclub.com/photos/a_head/1070.jpg

"15" head on a 1500 engine should be an improvement with larger valves, much the same as the "16" or "18" head, but having the correct compression ratio without needing to be shaved. To be completely up to performance of the "18" head it may need a little porting. This might be fairly easy if you had both heads side by side, just open up the "15" ports to match the "18" style. Be careful not to run into the pushrod holes or the oil feed hole(s) near the back.

When larger valves are installed it helps to open up the port diameter around the ID of the valve seat and the head casting immediately beyond. Do not leave a sharp inside corner where it turns between vertical and horizontal ports.

The last couple of "18" heads I had done were massaged a bit more around the valve seats and immediate port area. When new hard steel seats were installed the seat chamfers (all of them) were cut to slightly larger diameter so the seat contacts the valve very near the outer edge of the valve head. The ID of the seat and port were also enlarged a bit. This has a few beneficial effects.

When the valve sits closed on the seat, heat will transfer from valve to seat to cool the valve head. Contact nearer to the outer edge of the head reduces the likelihood of a hot edge that could cause pre-ignition or run-on (I hate hot spots). In combination with the larger port bore just inside the seat it can act almost like one size larger valve, giving better flow without running into the valve head shrouding problem caused by a larger valve. This seems to work very well in the real world, although I don't have any flow bench or dyno testing to quantify it. It is however a fairly cheap way of enlarging the ports when you are having new seats installed.

Any time larger valves are installed you should be conscious of side shrouding. I seem to recall the magic number is 3/16-inch minimum space between the valve head and adjacent wall of the combustion chamber (as long as it does not run into the head gasket). To be neat about it you can use a milling cutter and dress off any sharp edges with a hand grinder afterward. For less particular home work you can do it all with a hand grinder, using an old valve to protect the valve seat while you're working in close. If your de-shrouding results in larger chamber volume the head can be shaved a bit to get back to original volume.

For most stock Austin B-series engines the valve head motion is entirely within the height of the combustion chamber with room to spare, so no chance of hitting the block. If you install a high lift cam or high ratio rocker arms or shave the head appreciably, then you need to pay attention to how far the valves move toward the block. The smaller bore 1500 engine might need clearance eyebrows cut for both intake and exhaust valves. 1600 or larger engines generally only need eyebrows for the exhaust valves, as the intake valves (standard size) move within the cylinder bore.

The 1972-1974 MGB "L" head had one size larger intake valves and shallower combustion chamber for higher compression. For those engines the factory installed eyebrows for the exhaust side, and I think all subsequent 18V engines had the eyebrows (as the "L" head was continued later for European market cars).