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My 2 cents.
I believe that IMS bearings fail when lubrication becomes inadequate due to a leaking flywheel side seal. If my belief is correct, I can only guess that dual row bearings failed less often than single row bearings because the dual row seals were better due to either design, manufacturer or assembly.
In any event, perspective is important. With single row bearings, replacement is like buying insurance against a 1 in 10 possibility of a very costly failure. The so-called insurance premium is roughly $1700 to replace the IMSB. The risk is approximately $5000 to $10,000 to replace a failed engine. Seems like a high premium to me. The probability weighted risk drops to between $500 and $1000 (10% failure probability x $5000 to $10000 engine replacement costs. So in the sterile world of probabilities, its a $1700 premium to avoid a $500 to $1000 loss.
With a double row bearing, the odds of failure improve to 1 in 100. Given the difference in odds, a IMS dual row bearing replacement is an even higher premium to pay compared to the single row bearing situation. This is because the cost to replace the IMSB is the same, but the probability weighted risk drops to 1 % of $50 to $100. Like all cases with insurance, the probability weighted numbers mean little if your engine dies due to an IMSB failure.
So this brings me to my answer to JDs question. I might replace a single row bearing as a preventative maintenance measure. I would only replace a dual row bearing at the time of clutch replacement with one exception. I would replace dual row bearings in cars with low annual miles or infrequent oil changes. These latter conditions allow increase oil contamination and degrades the seals more quickly.
Hope this helps.
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