Fel-Pro is my go to for gaskets. I 100% agree with you on that one. I prefer Fel-Pro over OEM for gaskets and I have yet to see a Fel-Pro gasket fail.
Fel-Pro is my preferred gasket supplier, although I'm not allergic to other brands. Just bought a Mahle gasket set for the intake on the '97 Plow Truck with L29.
OTOH, here's the Fel-Pro gaskets that failed on my '88 K1500 5.7L. Kinda suspecting the intake manifold is warped. In fact, I'd better go out and check the intake gaskets again, tomorrow.
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IIRC the belt needs to be replaced, but the tensioner doesn't? That's at least what my shop teacher told me.
Tensioners do wear out. Mostly noticeable when the tensioner "jiggles" as the engine runs. There's a damper inside, the tensioner arm should be steady. When it jiggles, the damper is "done". It's also possible that the spring that sets the tension can relax, leading to belt slippage.
The pulley on the tensioner arm also wears out. Bearings go bad, and the (generally plastic) pulley itself can become worn. The pulley/bearing is often replaceable separate from the tensioner assembly.
Why is the (lower) tensioner pulley covered in melted rubber from the belt? (Upper pulley is new replacement, for comparison.)
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Because this bearing...
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...didn't have balls enough to do the job.
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Not scientific or anything, but I'd say that--in general, and with exceptions especially for catastrophic failure like pictured above--you'd wear out two belts to a plastic tensioner pulley, and two pulleys to a tensioner. So a tensioner is certainly worn-out after four belts, maybe fewer. YMMV.