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MG MGA - Broken crankshaft

Well I verified last night that my 1600 engine has the dreaded broken crankshaft. It occurred on the #4 throw, right where the back throw arm intersects the journal. A straight line crack though the full width of the throw arm. This failure actually occurred after I returned from GT33 last year. About a week after I returned I was tooling down the highway about 60 MPH when I felt this shudder and the engine started clattering. I initally thought I'd spun a rod bearing, but it didn't sound like a typical rod knock. The engine would idle, but if acceration was attempted there was a distinct clattering sound. I shut it right down and called for a flat bed. When I got it home I started it back up. It started fine and would idle, but the awful sound returned with the slighest rev increase. Still, it's pretty amazing that it would still run like that. I removed the engine and installed the spare. As you can tell, I've just now got around to tearing it down. The good news is I have a serviceable spare crank that I acquired about 1972. There is no other apparent damage. Guess I have another winter project. Thank goodness for that heated garage!

G T Foster

On a point on the different types of crankshaft: I've read somewhere that the MGA, especially the mk2 had a superior crankshaft compared with the stock b series shafts in that it is forged and not cast and it also has better machining. I've got a spare 1622 shaft from an A60 or similar that I keep as a spare for just such an eventuality as happened to Gerry but I do wonder whether the A60 shaft would stand up to heavy use and high revs compared with its intended more sedate use in a farina saloon.
J H Cole

Considering where the crank broke, the flywheel and clutch assembly is being carried on the single rear main bearing, detached from the rest of the crankshaft, no longer guided by the front and center main bearings. This allows the flywheel to wobble a little out of plane, not a lot perhaps, but plenty enough for the mechanical scroll seal surfaces to come into physical contact and chew up the precision surfaces.

No matter that it screws up the scroll seal surface on the crankshaft, as that piece is trash and will be replaced regardless. Big problem is that it screws up the seal surfaces on the engine block and rear main bearing cap. It will then leak oil profusely when a new crankshaft is installed.

Been there, done that, which is why I developed and installed my version of the rubber rear main seal (after I already had the engine back together).

The proper repair is to shave the main bearing caps and have the block line bored to restore proper dimensions to the rear seal surface in the block and main cap. See here:

Diameter of main bearing cradle will be 2.146".
Diameter of the scroll seal bore will be 2.138".

Notice the scroll seal is slightly smaller diameter than the main bearing cradles. If the seal surface is line bored to the same diameter as the main bearing cradles it will leak profusely. This difference in diameter must be maintained during line boring, which will add a little to the cost of the boring process.

Also see Confidential Service Memorandum MG/212 on the rear main seal dimensions here:
Barney Gaylord

Thanks for the thoughts on the scroll seal. No substitue for experience. I do have a complete spare engine. Maybe I should consider pressing it into service. I haven't pulled the crank out yet to examine the mains. Will probably do that tonight or tomorrow. I called the machine shop about reconditioning my spare crank. Guess I'll also ask about reconditioning the seal surface as well.

G T Foster

J H Cole,

Is the engine in your car MK11 1622?
A spare 1622 crankshaft will not fit in a 1500 or 1600 MK1 engine.

M F Anderson

No, it's not a MK II. The spare crank is from a 1600 MK1.
Thanks for your input.

G T Foster

This thread was discussed between 22/01/2009 and 23/01/2009

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