Belt length causing tensioner rattle | Ford Explorer Forums - Serious Explorations

  • Register Today It's free!

Belt length causing tensioner rattle


Well-Known Member
August 30, 2008
Reaction score
City, State
Year, Model & Trim Level
03 Eddie Bauer 4WD V8
I've got an annoying rattle that appears to be coming from the serpentine belt tensioner. The tensioner (due to belt length) is right on that fine line between it's max stretch position and just short of that, so the stop keeps tapping at the tensioner body. As the AC kicks on and off, it tends to change just how close this is, and makes the rattle come and go.

So I know what the problem is. But I don't know why... I have a Goodyear Gatorback serpentine belt on it now, and it's somewhat new. So the belt should not have stretched to be too long at this point. I looked up belts on Motorcraft, Rock Auto and Amazon and found these references:

JK6-867 - 86 7/8" long, w/o High Output (apparently high output A/C)
JK6-871-A - 87 1/8" long, 4.0 SOHC and "all engines"
JK6-884-HA - 88 4/8" long, 4.0 SOHC apparently with a manual trans

Most aftermarket sites call for the JK6-867. Motorcraft's site now calls for the JK6-871-A. Im thinking that the latter number is an updated number, and the original number might have been the 867. Sometimes if two belts are really close in length to eachother, the OEMs will eliminate 2 part numbers and just replace it with one. That's probably the case here.

So here's my problem: My Goodyear belt is 86.5" long according to the description. That's shorter than the shortest Motorcraft belt listing. And yet it's too long, and causing my tensioner to bottom out and rattle. Anybody have any experience with this?

Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

OK, I'm not trying to be a smart ass, but do you have the belt routed correctly?

I had a similar issue with a belt that was the correct one, but was slightly long. Replaced it with another belt - problem solved.

I also had the same exact problem. New belt was the fix, but it did not put the wear indicator in the middle like it should. It moved it off the stop enough so that it did not bang with the A/C on. It made a heck of a racket and I thought I had the dreaded timing chain problem. I think different manufacturers belts are just not exact to their dimensions. Try a different belt.


Ahha! Ok, so what belt numbers did you guys have that didn't work right, and what belt did work right? Also, with the correct belt, about where in the tensioner range would you say you ended up?

This is sounding to me like Ford might not have picked the belt size right, or perhaps there wasn't a belt available that was exactly what they needed. I wonder if I would be better off just trying to figure out a 1" shorter belt than the one I have?

Yes, I'm sure the belt routing is correct. Plus, the belt has been on there for a year. It's only made noise for a couple months. It's like it took very little belt stretch to put the belt right out of the tensioners range.

Sorry I did not notice what belt part number I used. Just got what they looked up at the parts store. That may be part of the problem. As you found in your research. I may have the longer belt. Sometimes you can just get the parts guy to find a slightly smaller belt. If you find one that works well write down the number and post it up. Thanks.


Well, as you can see there is a pattern to the motorcraft belts, and most other brands as well. The JK6-867 appears to mean a 6 rib 86 7/8" belt, so that last digit seems to mean how many 1/8ths of an inch, and the rest is obvious. So maybe getting any belt available between JK6-860 and 867 would tighten it up by up to 7/8".

There are 2 catches. There aren't always belts in the range you're looking for, and also belt length can be deceiving about how much shorter you need to get the job done. I've found that small changes can make larger differences on the car. If I got the belt locally, I could just keep swapping until I found one that put the belt at the tighter end of the wear indicator.

Or I could just buy the shortest motorcraft belt listed above, which is the original number and not the "newer" number. This would make sense, as motorcraft may have consolidated a couple belt numbers into one "fits all" belt, and the dang thing might not be short enough.

Serpentine Belt Length?

Installed a Dayco 5060868. Supposed to be 86.75 long. Belt tensioner looks like it's way past where it should be (indicfating that the belt is too long).

What length belt ended up working for you?

2000 Explorer, 6 cyl, 4.0L, SOHC

I haven't fixed it yet. My belt is just long enough that the tensioner is right at it's max belt length. That causes the tensioner stop to chatter against the tensioner, making an annoying rattling noise. The wife thinks its an AC problem because when she turns the AC on, the noise is usually worse. That isn't true of course, its just that the AC compressor engaging is slightly changing belt drag and moving the tensioner arm a bit.

I have no idea why we're having this problem, except to say that I think Ford may have messed up with belt length. Maybe there is more than one set of accessories they put on the truck from the factory, and we got the ones that required a shorter belt, yet Ford thought that one belt would work for both versions or something. I have no idea.

I'm tempted to just go to the parts store and start trying belts that are roughly 1/2 to 1" shorter.

Please report back what you find.

Any update on this issue? I have that same chatter as well and was hoping a belt replacement would fix it. What brand belt did you guys end up using to fix this?

Haven't fixed mine yet, but guys please post up what belt you have that does NOT work correctly also. My Goodyear Gatorback 4060865 (6PK2195) is too long.

hi this buck, i ave just gone thru this with 98 w/40 engine sohc, i wish i knew i had belt stretch problem i have gont thru 2 weeks of frettin knock, i changed ac pump and locked tension iddler,,, i asked for help then took all belsts of and ran engine, no knock, all pulleys were smooth turning, no grumble from alt,, i had solid knock in alternator, almost bought new alternator,,idler, alt, water pump all smooth,, waht the heck,,,went back put it all back together and looked at tension,,, the dogs were right together, i was sick,, i thot i had that dreadedkiller dowel pin, killer cam belt tensioner fail,,,my belt used is ac delco 6K864 88932768,, its dimension are 13/16X 86.4,, im going to get new original belt and if that doesnt work then ill try thhe next sohoter at 85 7/8,, ford taurus um 2005???sory for typo, strokee and arrest 10 yerars ago,, the harder i try ti fix, tje worse it s gests, buck

Buck, glad to hear it was just your tensioner. I'm not sure if the situation is caused by motorcraft revising the belt part number (using a longer belt now), or if they just got it wrong in the first place, or if it's the aftermarket belt I put on my truck that is the source of problems.

The belt tensioner stop hits the moving part of the tensioner, causing a racket. It often gets worse when you turn the A/C on, presumably because of the slight extra tension the belt sees when the compressor is engaged. Apparently, I am not alone in having this issue.

Could you please post back what belts you find work and do not work, and what the magic length is? Thanks!

Looking through RockAuto at fitment gains this:

2000 4.0L V6 SOHC shows 4060868 (86 3/4" effective length), K060866 (87 1/4" length), 4060865 (86 1/2" effective, 87 1/4" outside), and JK6867 (86 7/8" length),

So... there is 3/8" difference in lengths between longest and shortest the belts that are supposed to be the right fit.

The '02 belts are same width and # ribs as the '00.

2002 4.0L V6 SOHC explorer shows 4060855 (85 1/2" effective length), K060854 (85 7/8" effective, 86 1/4" outside length), or 6K854 (85 7/8" effective length)

Once again has 3/8" difference here. Not saying Ford and aftermarket suppliers would have some particular reason for the slop, but I've come across some parts year mixing Ford made when putting together the engines. Like say when I found the "correct" tensioner did not fit my 2000 SOHC (bar on block instead of peg), or seeing slightly different sized idlers from one truck to the next walking through a junk yard. Makes me suspect the belt length would be different too.

I would suggest you try on one of the longer ones for the 2002 if the 2000 suggestions just aren't working right. 1/2" to 1" shorter might be a lot in some cases, but is very likely to be within the working range of your tensioner given belts also stretch over time.

One wouldn't want the range of the tensioner to be taken up all the way, causing the belt tension to be at it's maximum potential, for fear of premature accessory bearing wear.

You wouldn't want it too late, or the belt will stretch and it will quickly start rattling against the "loose" edge. This is my problem.

Somehow, you would want it near the middle when the belt is new, and near the end, but not at the end, when the belt is ready to be thrown away. So which belt would do the trick? Perhaps each model your would vary and require experimentation.

this is exacetlywhat i just went thru,, made me think it was that killer cam problem, then alternator knock,,,my belt looked good but after reading oof the differences in belts i went to get really close to thr parts guy,,, my old belt was in good shape appearance wise but,, oh well,,the number ac delco 88932768 6K 864= 6k, 6 ribs,, 864,, 86.4 inches=on the box 13/16 X 86 3/8, i bought carquest gates 86 3/8 and 85 7/8 thinking i needed shorrter belt, guess hwat, those numbers printed on the slip are not the dimensions of the actual belt ,, they are vage suggestions vague,, i measred the belts against one another, te 85 7/8 belt was longer than my origina;6k864, but the replacement K060858, 86 3/8 was significant shorter than my original and right on, buck

With the K060858 installed, was the tensioner right in the middle of the two stops?

Victory at last!

I tried Dayco belt# 5060860. It's an 86" belt. I bought it at Advance Auto Parts online for $20, and picked it up locally. Using Promo Code P20 saved me $5 off retail (20% off!). If you guys aren't on the bandwagon to buying things online and using promo or coupon codes, you're missing the boat! Buying more expensive stuff, there's some much bigger coupons available. Try googling "coupon codes" and the store name.

Anyhow... here is the belt tensioner before, with my old Goodyear 4060865 belt (87 1/4" long according to specs):


Notice how the tensioner arm is at about the 10% left position. That's way too close to zero, and caused my tensioner to "rattle" as it went between say 15% and 0%, hitting the tensioner stop. Now here it is again with the new Dayco 5060860 belt:


It's now at more like the 60 - 70% tensioned position. Any tighter of a belt, and the belt probably would have hit the tensioner body. One thing I learned is that you can't try to go to 100% tension. The tensioner might be able to get tighter yes, but the belt won't fit on the engine anymore. It's just too tight of a fit, unless you want to pry it on with a 3' pry bar or something. This is as tight of a belt as I would dare run, and I'm glad it will break in, and loosen up a bit eventually. Probably will end up at around the 50% mark once broken in, so I think that's just fine.

Note: you could probably also go even longer and still have success. Dayco makes a 5060865 belt that would be 1/2" longer. My original belt was 87.25, and was too long.

I've noticed something along the way, and you guys did too. THe "865" belt length was 87.25 according to the specs. Apparently, there is an actual belt length, and an effective belt length. Or maybe that could be read "inside diameter" vs "outside diameter", I don't know. But whatever the belt part number suggests the length is, the actual length is longer by about 3/8 to 5/8 of an inch. That explains a lot. So compare apples to apples when shopping.

Now that my belt tensioner rattle is gone, I figured I would clean the MAF sensor's hotwires while I was in there, using CRC Mass Airflow Sensor cleaner. This should be done on regular intervals, trust me. About every time you clean or replace the air filter, just shoot some MAF cleaner onto the little resistor looking wires inside the small round tube that's inside the MAF. The MAF is the sensor that's bolted to the air cleaner box. You can remove it to clean it, or just reach the can up inside the air filter lid. Whatever you do, do NOT get aggressive with touching the wires. They break easily. Just spray them clean. This trick helps fuel mileage stay strong.


FYI, this job would have been tough without the right tools. The tensioner requires a 3/8 ratchet type drive to loosen it. I used my OTC serpentine belt tool, which has been well worth the $25 - 40 I paid for it. It gets in where bulky tools can't.

I also watched the thing run, and learned 2 things: 1) when the motor revs up, the belt tensioner heads towards loose by about 20%, and the gap between the belt and the tensioner body grows by about 3/8". 2) When you engage the A/C compressor, the belt tensioner heads towards loose by about 5%, and the gap between the belt and the tenssioner body grows by just a little bit. So the belt will never get tighter or closer than it is right now. So this belt proves to be perfect! An 86" belt worked great, 86.5 would probably work fine too, but 87.25 was too long.

Fixed tensioner rattle on 4.0 l Ranger

So I finally figured out that my rattle came from too long of a belt. Advance Auto and Rock Auto both call for a Dayco 5060883 (an 88.3 inch belt, as I understand it.) I put on a Dayco 5060880 (88.0 inches), and that took care of it.

Thanks to all who posted regarding this annoying---and needless---problem; your experieces helped me greatly. Am I alone in thinking this kind of thing shouldn't have happened in the first place? I really have better things to do!

Join the Elite Explorers for $20 each year.
Elite Explorer members see no advertisements, no banner ads, no double underlined links,.
Add an avatar, upload photo attachments, and more!

Wrong post.