Idler Pulley bolt & Tensioner Pulley

[JaredH]

88,000 miles:



During Generator Swap (Burnt the B+ stud off the back, I had never removed before, you should check yours for tightness!) I discovered that my idler pulley and tensioner pulley had play in them.



I ordered the replacement pulley for the tensioner (Dayco #89094) and I was surprised to see that cast into the tensioner assy "Dayco!" Swapped out the tensioner pulley, no problems and reinstalled. I also found some play in the A/C clutch pulley which is not available separate from the compressor itself. So I figured since no chirping, jumping, or other belt issues the tensioner still had enough force on the belt and by not adding a new one may prolong the A/C clutch pulley life... we'll see.



Went to swap out the idler pulley with the new Gates idler (#36315) and spacer and found an item of concern. When the factory mounting bolt is insterted through the new gates pulley and spacer there are only about 7 threads (~3/8") showing due to wider bearings vs. 11 threads (~5/8") showing on the factory setup. Maybe this is why people have posted issues with idler pulley bolts shearing?



I have attached some pictures:

1st showing difference between Gates (Left) and Factory (Right) Spacers

2nd threads protruding from Gates Spacer/pulley

3rd threads protruding from Factory Spacer/pulley



So my plan is to get a longer bolt of the same grade.

Factory METRIC bolt measures:

2 3/4" long

13mm head

Thread width ~3/8" which should be M10

Thread pitch of 1. 50

8. 8 Grade



Anyone else have this issue? Thoughts? Advice?



Thanks,

Jared

 

[SierraRam]

I agree that the longer bolt would be a good decision, you would think the would provide one with the new kit.

 

[GCroyle]

Interesting about the B+ connection, I'll check that one out.



Wonder what the rest of the story is from the supplier? If they offer, no make that ask for it from your supplier if not readily available, for the 800 Tech Support number and ask them for the rest of the story.

 

[mwilson]

I would assume that Gates is using the same idler for more than one application and are boxing the same idler with vehicle specific spacers to create several different part numbers.



I would be a lot more concerned about the difference in length of the two spacers and making sure that the new spacer places the idler in the correct position relative to the engine block, belt and tensioner arm rather than how many threads are showing.



The full holding power of a bolt is achieved with a depth of threads equal to the diameter of the bolt. Any more length is a feel good factor, it does not help hold the fastener in place.



Providing that the spacer is squarely tightened against a flat surface and the rule above is followed it will take the same amount of force to shear that M10 bolt that is threaded in 3/8" as an M10 bolt that goes all the way thru and sticks out the backside an 1" or so . It will provide an equal clamping force.



Does that make sense to you? Sometimes I don't explain things very well.



Mike.

 

[S.Morris]

+1 on what mike said. Thanks, saved me some blackberry typing.

Steve

 

[JaredH]

Yes Mike I confronted the difference in length of the spacers... When both were off the truck, spacers/pulleys were put in place and sat on the same flat surface. All pulley grooves matched up perfectly. I should have mentioned in my original post that the reason for the difference in spacer length is due to increased bearing width on the new gates idler pulley.



I do understand you on the bolt "holding power. " I didn't check the formula but sounds right, haha. Of course that is assuming that the bracket and bolt suppliers cut the thread grade same as used in the theoretical calculation and that my $89 craftsman torque wrench is giving me the correct preload on the bolt (which may or may not be made of the exact steel specified on the dwg and heat treated/case hardened exactly to specification, not to mention the torque spec was probably written for a new bolt with a specified amount of thread sealant added at time of manufacture). If thats the case then yes, I believe that by a rule of thumb the first five threads are responsible for the majority of the holding power. Thats leaves me an extra thread... . I like a larger factor of safety though...



I had to throw it back together last night with the old factory pulley/bolt. Hope to get the new one on by the end of the weekend.

 

[mwilson]

I don't want people mis-informed about the length of the provided bolt having a darn thing to do with any shearing issues. If the bolt shears off at the face of the tensioner it is from too much side load, over torquing of the bolt or the idler bearing freezing up.

You could run the bolt out the back side 2", double lock nut it and then for good measure bend the remainder over with a hammer and it still could shear off. :-laf:-laf

 

[JaredH]

Neither do I want people to be mis-informed! OK so I agree with what was previously stated, "The full holding power of a bolt is achieved with a depth of threads equal to the diameter of the bolt. " This is a correct statement if both materials are new and as for a bolt/nut combination graded as the same. That is a rule of thumb in that fastener/engineering world. Not that I always trust Google but you can find many sources that say this very fact.



However, we are not dealing with a bolt and bracket made of the same material. The bolt is hardened in some fashion and the bracket is made of cast iron (which is brittle, not ductile). In instances like low carbon steel and, as mentioned, cast iron the rule of thumb for threaded fastener length of thread engagement is 1. 5Xdiameter of the bolt. Please feel free to google, look up in the machinists handbook, or any engineering book you may have lying around.



Based on the diameter of the bolt in question and the factory length of thread engagement even cummins exceeds this rule of thumb (1. 5xdia). Perhaps they know a thing or two about durability... and that cast iron since brittle normally holds the most preload in the first couple threads. Which can hold the preload force on initial toque down, however in nature the sharp edges from thread cutting/forming can cause the first thread to crack and transfer the preload one more thread back... . this CAN continue until the bolt preload does not exist and the spacer is no longer held "squarely tightened" against the engine block. And as we all know... in this situation the bolt is prone to failure.



Gcroyle, I took your advice and contacted the manufacturer:



I asked:

"Due to the increased bearing width of the Gates aftermarket idler pulley the factory OE idler pulley bolt does not extend into the factory bracket as deep. With the 36315 Gates idler pulley and spacer the OE Bolt threads into the bracket 6. 4mm less (~6 threads engaged). The OE bolt threads in a total of 15. 9mm (~11 threads engaged) Has Gates researched this? Is this amount of thread engagement OK for durability? Is there reason for concern? Should the OE bolt be replaced with a longer bolt?"



Gates tech responded:

"Your are correct it is not enough threads - don't use it - and we'll have to re evaluate. "



So I asked what this was based on, what gates' recommended.



Gates tech responded:

"Well we normally try to follow the OE however in this case we did not catch the bolt issues, the OE pulley that we have shows to have 11/16 of exposed threads so if you going to try and find another bolt make sure a hardened bolt and not swingset grade. "



Haha, I love the "swingset grade" statement.



This is what I have found. As for me I will be mounting my idler with a longer bolt!!



Please correct me if I am wrong!

Thanks guys

 

[JaredH]

As a follow-up I used an M10 Flange Bolt (M10 - . 5 X 80mm). Length should be 75mm, I could only find a 80mm long bolt so I had to grind 5mm off the end (keeping it cool with water as to limit mechanical property changes) to eliminate the bolt from bottoming out.



The flange is a bit larger than the factory but does not affect the retainer function which the bolt torques against. If anyone needs a bolt I have 9 more so let me know!





Thanks,

Jared