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SWIVEL PINS, Fracture Analysis - FS-220

broken swivel pin broken swivel pin broken swivel pin
This doesn't happen very often. My MGA has accrued over 400,000 miles driving in all conditions including lots of rough gravel roads and lots of years of regular autocross competition on sticky race tires. It has never had any broken suspension parts (except one broken sway bar). One lower swivel link was replaced due to worm threads. I attribute this to lack of proper maintenance by multiple previous owners (during the first 20 years and 150,000 miles). That said, this does happen often enough to merit precaution and occasional crack check inspection of these parts.
broken swivel pin
The fractured swivel pin shown here is an aftermarket part with improper machining. Click for larger pictures. Notice where the lower thread ends just before the larger shank diameter. Here there is an incomplete shallow thread cutting into the tapered shoulder where there should be a clean relief radius cut just below the root diameter of the thread. This extended thread cut makes a stress riser in the part right where it has highest stress in service. Looking at the photo of the fractured ends, it appears that a stress crack started at the lower side (in the picture), propagating upward until the crack was 3/4 the way across before final fracture. Since the crack is in a near perfect plane I suspect it started at and followed along a line where the thread was over run and errantly cut into the shank where it should have been a smooth stress suppressing radius.
clean swivel pin
This picture shows a new swivel pin with a clean relief radius with no marks from an extended thread. This is an important feature for replacement parts.

clean swivel pin
I'm not sure if this one is a crack or an improper continuation of the thread, but I wouldn't put this part back in the car.

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