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Recently, I bought a brand new idler pulley for serpentine belt system. When I opened the package, it included the idler and its bolt (picture 3) + a plastic dust cap (picture 2) + a piece of cardboard (picture 1) which I'm not sure what should I do with it! Is the cardboard thing also a kind of dust cap? Should I install it on the back of the pulley? Guide me please! I have seen plastic dust caps before but I have some concern regarding the use of them. The plastic cap can prevent dust and water from entering the bearing but I think it can make it harder for the bearing to dissipate heat because it covers the whole surface area. Correct me if I'm wrong and tell me whether you think the dust cap should be installed or not.

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  • Did any instructions come with it? What does your old one have? Is there a reason you think they would send you a dust cap if it wasn't for use? Where did your concern re heat dissipation come from - have you had a heat related failure in the past?
    – Rory Alsop
    Oct 1 at 12:20
  • @Rory Alsop - No instructions inside the package. I haven't had any heat related issue in the past because my previous bearing haven't had any dust caps. It is just what I think about it and as I said, I may be wrong. This is the first time I buy a bearing with caps and I need to know more about them before using them. Oct 1 at 12:31
  • In general, the folks manufacturing these know about them. If you haven't had an issue, you shouldn't have a reason to try and second guess them.
    – Rory Alsop
    Oct 1 at 12:33
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The "cardboard thing" is just there to keep the bolt in place while in packaging. Discard it as it's not needed. Leave the cap off during installation, then place after everything's together. It's basically a dust cap. It isn't truly needed, but will help with longevity.

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  • @Paulster2 - Do you think the plastic dust cap may come off due to engine or chassis vibrations? What will happen if it suddenly comes off during driving? Oct 1 at 12:53

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