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Broken crank

Posted by John Hamilton

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Broken crank #1
John Hamilton Austin, TX, USA 1965 MG MGB "The Great Pumpkin" 1968 MG MGB GT "Buck" 1970 MG MGB "Dad's Car"	Posted: May 31, 2012 10:08PM Registered: 5 years ago Posts: 1,698

Bad news, what I'd hoped was a broken flywheel turned out to be a broken crankshaft. It broke between the filet of the #4 main journal and the #4 rod journal. All of the bearings looked great, no signs of overheating or oil starvation, and no other signs of mechanical damage. Does this sound like metal fatigue? Unfortunately, the #4 rod has a small crack and the #4 piston kissed the head. Do you think I can salvage the pistons and just replace the crank and rods? The block is completely intact, so I guess it could have been worse. I'll add pictures if I can figure out how to downscale them.

If I can't be fast, I'll just be obnoxious!

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Re: Broken crank #2
Basil Adams About 12 miles from Sears Point, USA	Posted: June 01, 2012 12:55AM Registered: 6 years ago Posts: 5,979

JH- Many possibilities but if that crank had been turned at all, if the radius left was not very generous (or a sharp angle) that can easily become a failure point. I have a friend that just bought 4 street-prepared 10/10 cranks from a supplier that went out of business. PM or call or email if you need one. Thanks. Basil 707.762.0974 basiladams@yahoo.com

Basil C. Adams
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MKIII Elva Courier (E1056)
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A coupla late MGBs
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Re: Broken crank #3
Speedracer Hap Waldrop Greenville, SC, USA 1967 MG MGB "The Biscuit"	Posted: June 01, 2012 05:50AM Registered: 7 years ago Posts: 20,317

Ditto what Basil said, of all the cracks I found cracs on, they are all at the filet radius and most were a result of some machinist grinding a crank undersize and not leaving a good radius, I have seen a few cracks on otherwise standard factory crank, or nicely redone radius on ground cranks, but for the most part it is result of poorly turned cranks.

John when you get a replacment crank, if it is standard, look at the radius on it , then compare it to the one that broke, that may give you a answer. I've seen machinist do every idiotic thing in the world here, some would step the wheel a thou, aseries of steps if you will, to give the illusion of radius, but if you looked closely, you see the steps, each one now becoming a stress riser, I also just seen where they didn't dress the grinding wheel and left a sharp angle.

Hap Waldrop
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Re: Broken crank #4
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA	Posted: June 01, 2012 08:40AM Registered: 4 years ago Posts: 3,226

I certainly agree with poor grinding being a contributor to crank failure. In addition, the location of John's fracture, #4 rod journal is typical of a fracture caused by high torsional crank vibration amplitudes as well as loss of support due to a broken #4 main cap.

John, is this an 18V block with bolted caps or the earlier and weaker studded caps?

Regarding saving the piston that hit the head, if the rings remain free, not trapped by distorted piston top land, it may be saveable. Other wise NO. confused smiley

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Re: Broken crank #5
John Hamilton Austin, TX, USA 1965 MG MGB "The Great Pumpkin" 1968 MG MGB GT "Buck" 1970 MG MGB "Dad's Car"	Posted: June 01, 2012 09:30AM Registered: 5 years ago Posts: 1,698

Thanks for the info. The crank had been turned .010/.010 by a reputable race shop, now out of business, but I will pull out another uncut crank and compare the filet radius. The rings are still free on the piston that smacked the head, so I think it will be okay. I'll get another racer/engine builder to look at it this weekend to be sure. If my budget allows, I'm looking for a wedged and lightened crank and a better set of rods, mine had the very large balancing pad on the bottom. The Maxspeeding rods aren't that much more than prepped stock rods and look like they'd be a whole lot lighter.

Oh yeah, this is the later 18V block with main bolts, not studs.

If I can't be fast, I'll just be obnoxious!

Edited 1 time(s). Last edit at 06/01/2012 09:55AM by John Hamilton.

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Re: Broken crank #6
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA	Posted: June 01, 2012 10:02AM Registered: 4 years ago Posts: 3,226

John, I can provide lightened, balanced OE rods with ARP bolts for $450/set. They'll weigh around 750gms.

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Re: Broken crank #7
malcox Malcolm Cox Napa, California, USA	Posted: June 01, 2012 10:57AM Registered: 2 years ago Posts: 68

All the ( 3 main MGA ) cranks I have broken ( 4 of them )fatigue broke at the same fillet/journal interface.
Although the desirable fillet radius is 0.125", I have yet to see one larger than 0.080"
malcolm

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