When in need of a replacement crankshaft for the MGA Twin Cam engine, your options may at first seem to be somewhat limited. Good used parts or New Old Stock parts are obviously very hard to come by. A billet crank can be made, but the cost is relatively high.

Bob Bayliss in South Africa has put a 5 main bearing MGB crank into his 3 main bearing Twin Cam block without difficulty (relatively speaking). The two intermediate main bearings (#2 and #4 of the 5-main journals) run in open air in the 3 main bearing block. The intermediate main journals are not drilled for lubrication, so they are not a pressure loss problem. With this set up you can use the MGB rear main seal and diaphragm clutch. You also need the flywheel from a 1965-1967 MGB with the GB engine. This is the first of the 5-main engines, still attached to the 3-syncro gearbox with the small flywheel.

You need to graft a Twin Cam style nose piece onto the standard type crankshaft for use in the Twin Cam engine. You can do the same with a 1622 engine 3 main bearing crank or the early MGB 3 main bearing crank, and use the original Twin Cam flywheel, or flywheel from a late production MGA MK-II or early MGB 3 main engine. The main bearing journals are 1/8 inch larger diameter on the MGB crankshaft. If you use an MGB crankshaft you might seriously consider line boring the journals in the block for the larger bearings as opposed to turning down the MGB main journals to match the Twin Cam block. The larger main journals will give you a stronger crankshaft.

The new fabricated nose piece must duplicate the front dimensions of the Twin Cam crankshaft. It should be a light press fit on the nose of the standard crankshaft with key(s). It should use a long through bolt for the crank nut to reach completely through the new nose piece and screw into the nose of the standard crankshaft. If you do not do this, there may be a problem securing the nose piece to keep it running concentric and in balance.

Do not even think about using the MGA 1500/1600 crankshaft in the Twin Cam engine. This is not as strong as the later cranks, and definitely not up to the specs of the Twin Cam crank. The 1500/1600 crank has wider center main journal, so the thrust washers would have to be shimmed or otherwise made thicker. Also some recent new replacement 1500/1600 cranks are made from castings rather than forgings, having even less strength, and also known to exhibit small blow holes in the casting. These may be suitable for a mild street engine (1500/1600 stock configuration), but definitely not recommended for any up rated engine.

Aside from the nose end configuration, the 1622 crankshaft is similar configuration to the Twin Cam crank, and should be a very good option. John Wright has built a Twin Cam crank from the 1622 crankshaft in this manner with only minor questions about concentricity of the front extension. All original MGA cranks were forgings, as well as MGB cranks (at least until the 18V engines, and maybe all through 1980). Cranks used in some non-MGB 1800 engines could have been castings rather than forgings (Austin Marina comes to mind). The original Twin Cam crank is reputed to be an especially strong item - a so-called 'red crank'. The 1622 crank may or may not be up to the same spec, but should be a reasonable substitute in the absence of the real McCoy.

On Feb 18, 2024, Bill Spohn in W. Vancouver, BC, Canada wrote:
"I came across this pic of my customized Twin Cam crank. It is based on a factory MGB 5 main cross drilled nitrided race crank with a special nose extension I grafted onto it to carry the Twin Cam sprockets etc. The stock v=crank nose of the B crank is intact. Just created keyways in the add on extension and made it a press on".