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Alternator belt concerns

Old 10-31-2020, 07:24 PM
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Default Alternator belt concerns

Had my starboard alternator pulley nut come loose while cruising to our hangout cove. Appears the pulley and fan stayed on the shaft long enough to ruin the alternator shaft threads. Gathered up the parts I could see, cut the belt and continued on with just the port alternator. Replaced alternator with a slightly more modern version. Got the belt in the mail today and went out to install it.

When I compare the old belt to the new, the old belt is visibly wider then the new one. When I ordered this belt I originally looked up the part number printed on the old belt (57-48122 DF). Found it had been superseded by 57-48122Q so I ordered that. At first it appeared I got the belt I ordered looking at the cardboard sleeve the new belt came in, but it seems too narrow. The really weird part is that both belts have the same part# printed on them except for the "DF" and the "1920" values after the part #. Not sure what that denotes.

The old belt seems to fit the pulley groove a lot better than the new. belt. Also notice the new belt possibly has some wear near at the top of the "V"??? I can see reinforcing thread for a bit then it's hidden under rubber, then appears again a little further down. Don't know if that's normal or a defect, or signs of being improperly installed and run at some point.

Has anyone run across this before?? Seems to be a pretty common Merc alternator belt.

Old belt.




New belt






Package for new belt.





Same part numbers printed on belt.






Old belt fit on old alternator pulley.






New belt fit on old alternator pulley





Signs of wear or manuf flaw?

Last edited by zz28zz; 10-31-2020 at 07:27 PM.
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Old 11-01-2020, 01:19 AM
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The threads on the sides of the new belt are normal. Nothing to worry about.
As far as the width difference, that is VERY minimal and I would not worry about it.
Personally, I've never used a Merc or QS belt. I just go to the auto parts and have them match the size.
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Old 11-02-2020, 10:01 PM
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I used to have a locally owned auto parts store not too far away where you could go there with a boat, car, truck or tractor part and walk out with a new part in just a few mins. Unfortunately they got bought out by a chain and now they're just like all the rest of the chain auto parts stores. Tell them it's for a boat and they instantly go into a tailspin. Actually had a clerk try to talk me out of buying a Holley 6.5" power valve since it wasn't a "marine type".

Dug into this a bit more. I had sent Merc an email asking abt the tolerance they spec for belt width. To my surprise, they quickly responded with exactly the info I was seeking.
"The belt width is within the minimum specifications of the eng drawings for 48112Q which has a width of 0.38" +/- 0.03 inches." The measurement value I sent them was .350", so it passed but just barely.

Picked up another belt from my local marine shop (this belt appears to be "old stock"). Packaging is slightly different. Characters printed on this belt are exactly like my orig belt (57-48112 DF). Decided to take multiple measurements on each belt to check for deviation. Belt is 48" long so measurements were taken every 4 inches for all 3 belts (orig. online, local). Found the belt I had bought online was consistently thinner and deviation in width was definitely higher causing some measurements to be OOS.. I had to experiment with my measurement technique until I was able to get consistent readings when measuring the same spot but I now believe I'm within ~.005 deviation due to technique.

As Griff said, the belt would probably work just fine. However, I wasn't happy paying big bucks for a belt and then discovering the process deviation.. Seems I either got a oddball belt or Merc manuf control is slipping. I'll probably send the online belt back for a refund, especially since the online belt costs ~$15 more than local purchase (I was being lazy). See attached..






New belt bought locally on old alt pulley.





Different packaging. Note date.





Same exact identification printed on locally purchased belt.

Last edited by zz28zz; 11-02-2020 at 10:09 PM.
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Old 11-02-2020, 11:13 PM
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Standard automotive V belt width is 3/8" =.375"
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Old 11-03-2020, 12:20 AM
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I was a bit surprised to see Merc quote 0.38" as the target. Assumed they would have stated 0.375". Maybe they were rounding? However, why round if you're quoting a +/- of 0.030??

I'm still trying to figure out exactly what "Made in USA" VS "Origin United States" really means. Originally thought maybe the portion of the manuf process that determines belt width had been farmed-out overseas and they got sloppy. Spent an hour researching but all I got out of it was a headache.
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Old 11-03-2020, 02:21 PM
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Just spoke to a customer rep at Merc. He confirmed target width of 0.38" but then says the tolerance is +0.030" and -0.020". If that's true, my online purchase is even further out of spec. I offered to send the belt to them so they could measure it themselves, but they declined.

Rep also stated they have sold 1200 of the belts this year and that they are manufactured by Goodyear.

This belt has a manuf date of 8/21/20 so heads-up if you're buying a new Quicksilver alternator belt.

I know "it's just a v-belt" but its still my hard earned money and I expect to receive a product that meets it's design spec. Within my profession (semiconductor manuf), if a customer reported receiving an out of spec part, a flurry of meetings, OCAPs, audits and follow-up meetings would automatically be triggered. Guess i live in a different world.

I climb down from my soapbox and get back to work now.

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Old 11-03-2020, 02:29 PM
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Sorry to say, but Merc doesn't give a flyin ‘ **** anymore. Especially the older stuff. Yeh, price says their proud though. Lol. They are ‘nice enough’ on the phone and email though.

See this:
Mercruiser. Yup, beware. Manufacturing issues.
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Old 11-03-2020, 05:51 PM
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Yep, I remember that post well. Hard to believe their plan is to sell all the bad ones first.
Guess this is what you get when there's no serious competition..
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Old 11-05-2020, 09:33 AM
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I run nothing but the Gates high performance belts and have had good luck so far. Got over 11 years on the last set and I run pretty hard. Best part is pretty much any auto parts can get them.
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Old 11-05-2020, 06:35 PM
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You`re waaayyy over thinking this . it`s fine.
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