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MG MGA - Weak 1600 crankshafts?

I read in a book recently that 1600 crankshafts tend to break, and for this reason many 1600 engines now have a 1500 crankshaft.

Now I have been under the impression that these crankshafts are identical! Can it be that they are not strong enough to take the extra torque in the 1600 engine, or is this not really true?

And how is the quality of the new crankshafts that are made?

Tore
Tore

Not sure, but I think that the 1500 and 1600 cranks (which are identical) have breakage problems. I have a 1500 engine awaiting teardown, but only to see what caused massive holes in the block. The hole on the right hand side reveals a bearing cap sitting in the sump. When I extract the engine this winter, I will post pictures here to show the failed parts. Don't know at this point if the crank failed, but my guess is that it did and caused the subsequent damage to the block.
James Johanski

My first A a 58 1500 coupe had the crank break. It broke between the first crank journal and the first rod journal. Engine still ran though with what sounded like a heavy rod knock. Replaced only the crank and all bearings, no additional damage.
R J Brown

1489 and 1588 cranks are the same and are definitely weak.

The 1622 crank used thicker cheeks (and slightly narrower main bearings) and stands up to hard use quite a bit better. The 3 main MGB crank was best of all and was basically the 1622 crank with larger diameter main bearing journals.
Bill Spohn

Scary. How common is this? What are the chances it will happen to the 1588 engine in my coupe project when it is back on the road in a (very?) long time?

I suppose it has to do both with a weak design and fatigue from high mileage and high revs? Of course I do not know what previous owners have done to the poor engine, but it still has the original factory pistons, so I can always hope for a low mileage and a little life left in the crank?

Tore

Tore

My dad broke the original crankshaft of my 1500 back in the mid 60s, but he said that was due to not having gotten the block align-honed when it needed to be. He had that work done, the cylinders re-sleeved, and installed a re-ground crank when he put it back together around '66. That engine was still in running condition after a lot of use when I removed it from the car in '91. So, I guess the moral of the story is that if these engines are rebuilt properly, there is probably nothing wrong with the crank.
Del Rawlins

My 1600 broke the crank too. This was before the motor was rebuilt. The oil pressure was quite low at speed and if I remember was just barely 40 lbs. At idle the pressure dropped to maybe 5 lbs. I replaced the pressure relief valve and spring and replaced the oil pump looking for more pressure. I tightened all of the rod cap ends and all the journals (?) and it wasn't long after that that I broke the crank. It knocked like crazy but I was able to limp home. The main contributor was slop in the crank. A proper align bore after building the journals up by welding hopefully has solved the problem.
David Holmes

Knock on wood,I had trouble with my 1600,with rod side play.I tried several cranks and rod combos and got readings all over the board.Highest was .040,lowest was.010. Turns out it seems,every rod out there has been cleaned up,removing several thousands each time.Cranks also have been removed from, no two measured the same.Ended up having crank welded up and reground to match the best rods I could find.Highest now .010 lowest .008. Had engine line bored and balanced,and at 65 mph I'm turning 4600,so far for 10yrs no trouble
gary starr

Some of you mention line boring of the block. That means is not true from the factory, or is it distorted during many years of use? I believe if the main bearings are seriously out of line, you should be able to feel it when you rotate the crank by hand during assembly?

Tore
Tore

Cast iron blocks do not usually distort in use, not even when overheated, unlike alloy blocks which do have problems.
The usual reason for a line bore is that a main bearing has seized on the crankshaft, broken the locating tabs, and the bearing shell has been turning in the block.
The main bearing cap surface, the surface where it joins to the other surface in the block, is machined to remove some metal. The bore for the main bearing is now undersize and not circular, and can now be line bored back to the correct dimensions.
This is simple for the pushrod MGA, but difficult for the Twin Cam.
When line bored, the crankshaft is now slightly higher in the block. No problem for a chain drive camshaft, but creates a problem of gear mesh for the Twin Cam half-speed shaft, which is gear driven.

Mick
Mick Anderson

This thread was discussed between 24/09/2007 and 25/09/2007

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