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Discussion Starter · #1 · (Edited)
Hello Everyone, I was replacing a 2008 Serpentine Belt Pulley Tensioner with a new OEM one from Honda. (Part #: 31170-r70-a01)

The old one's bolt broke off and I was happy to extract it earlier today without any issues and was hoping everything would be smooth sailing from there on but when I went to put the new one on it would not tighten in and was loose and wobbly.

I noticed the old one (I don't believe an OEM part) had a Larger Standard 1/2-20 bolt.
The new one has a Smaller Metric 12 x 1.25 mm.

Tomorrow I will run back to the Honda Part's store (or try Lowes) to see if they have a Standard 1/2-20 bolt. I'm hoping this will fit more snug and proper.
I'm hoping the Standard 1/2-20 bolt that was in there before didn't damage/loosen the threads because I believe it is supposed to be the Smaller Metric 12 x 1.25 mm.

Has anyone run into this issue before or have any advice? I'm hoping I don't have to re-tap the threads or anything like that (I've never done that before).

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Don't waste your time with the dealer. You'll find that standard bolt at home depot or autozone. Looks like the previous owner improvised and used a different bolt. This would surely mess up the entire thread in the block. Expect to put a helicoil or similar for a more permanent fix. I did this repair on my bottom alternator bolt. I used a china made helicoil kit M-10 X 1.25 for less-$20-ebay. And used the oem size bolt. Seems to be holding up for about 6 months now. Good luck.
 

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Broken bolt isnt oem.
 

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Did you find a separate washer used as well or was it just like that? Even getting a aftermarket bolt, it should have been metric. But if you do get a similar one, hope you use a washer and figure out the root cause. This might happen again.
If you are able to re-tap with metric set and it works, then maybe you can go back to original. If not, maybe time for helicoil as the correct fix. Others might have good experience with this.

I only had one instance of the tensioner bolt break on the Corolla but luckily only metric. I had to take it in to get it out due to tools/tight space. Then used a new OEM bolt.
 

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As others have said, it looks like you're down to 2 options, and the condition of the threads in the block will determine which one is the best option.

1.) Block threads are good, just a larger size for the non-OEM bolt. Use a new bolt that matches the non-OEM standard threads of the block.

2.) Block threads are trashed or don't work with the standard bolt well (perhaps why it seized and broke in the first place), so use a helicoil kit to re-thread it to OEM spec and use the OEM bolt. If the threads are too far gone or too large, you may have to step up the helicoil and bolt to 1 size larger so it properly threads and stays tight like it's supposed to.
 

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Discussion Starter · #7 ·
Hello Everyone, thanks again for taking the time to respond!

So, I think marvinstockman is correct in that the person who worked on this Odyssey before may have tapped the hole for 1/2-20.
I found a grade 8 1/2-20 at my local mom and pop hardware store (after an unsuccessful visit to Lowes).

I just tested it out and was able to hand tighten the new 1/2-20 all the way in to test it.
The new bolt fits "fairly" snug.... there is the slightest wobble/shake when I try and move the bolt back and forth.

So the question now is:
  • is that very slight wobble normal or should it not move at all?
  • Would lock-tight work at all (even though the instructions recommend clean motor oil to lubricate).
  • Should I try re-tapping it again at 1/2-20 to see if that would tighten things up?
  • Or Maybe I will be ok using this new bolt with the slightest wobble or maybe I should definitely go the Heli coil kit route?
  • Would a lock washer on the bolt head instead of a flat washer help?
Thanks again everyone!
Ps. photo below of the new bolt... I'm assuming adding a washer wouldn't hurt.
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Discussion Starter · #8 · (Edited)
You know what, after that whole write up above, IGNORE it....
I just found out that the 1/2-20 bolt doesn't fit through the new Drive Belt Auto-tensioner!!!

So I think I will have to "use a Heli coil kit to re-thread it to OEM spec and use the OEM bolt" @ 12 x 1.25 mm

The question now is that it appears that for the Heli Coil kit I need a 31/64" drill bit... I'm just not sure if the existing hole for the 1/2-20 bolt is already larger than that?
 

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How did the old 1/2" bolt fit in the old tensioner?

Maybe you can very slightly enlarge the tensioner bolt hole to accommodate the 1/2 inch bolt? Drill it it out little bigger. If the difference is very little. Just big enough to fit the bolt but not to compromise/damage the tensioner.

31/64" is a hair smaller than 1/2". Thats a problem. But you can still try drilling the hole with the 31/64 bit to smooth it out. Then try using the tap for the oem bolt size helicoil. If the hole is not badly damaged-enlarged by the bigger 1/2" bolt, it still might work.
 

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You know what, after that whole write up above, IGNORE it....
I just found out that the 1/2-20 bolt doesn't fit through the new Drive Belt Auto-tensioner!!!

So I think I will have to "use a Heli coil kit to re-thread it to OEM spec and use the OEM bolt" @ 12 x 1.25 mm

The question now is that it appears that for the Heli Coil kit I need a 31/64" drill bit... I'm just not sure if the existing hole for the 1/2-20 bolt is already larger than that?
I agree with what was just suggested. It would be easier and better to enlarge the hole in your new tensioner so that your new bolt fits. Since you know the new bolt threads properly and can be torqued properly.
 

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Discussion Starter · #11 ·
Thanks @thscott and @nitely2! I'm not 100% sure if that old part was drilled to fit the 1/2" bolt but I am considering your suggestion of enlarging the hole in the new tensioner because I've never done a Heli-coil before and my work space is limited and it may be a challenge to get everything aligned properly.

My dry run of the new 1/2-20 bolt seemed ok... but did "give" a very tiny bit and had a very slight wobble if I pulled back and forth on the bolt so I am trying to consider if that is good enough to hold the pulley in place after going through all this work. (Boy, I wasn't expecting this extra challenge!)
 

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Did not know the hole was on a bracket. That would also work. Make sure you can remove all the bolts without disturbing any other parts- AC. If you need to remove the compressor to remove the bracket, that means discharging the ac system is necessary.

If the problematic bolt hole has some space behind the bracket you can reach, it would be easier to just drill out the hole through and put a longer m12 x 1.25 bolt and put a locking washer and nut behind it. If you have room in front to insert the longer bolt.
 

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Discussion Starter · #15 ·
@hpark21, I believe so.
@nitely2, I do like that idea of drilling the hole through and putting a longer bolt with lock washer and nut if there is room to do that, that is pretty creative, thx.


Thanks @thscott and @nitely2! I'm not 100% sure if that old part was drilled to fit the 1/2" bolt but I am considering your suggestion of enlarging the hole in the new tensioner because I've never done a Heli-coil before and my work space is limited and it may be a challenge to get everything aligned properly.

My dry run of the new 1/2-20 bolt seemed ok... but did "give" a very tiny bit and had a very slight wobble if I pulled back and forth on the bolt so I am trying to consider if that is good enough to hold the pulley in place after going through all this work. (Boy, I wasn't expecting this extra challenge!)
Actually, the more of I think about it, this may be the best solution at the moment?
Has anyone every had success with this type of thread sealant or threadlocker to help with a bolt that may wobble or give just a little bit?

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We use thread sealant all the time but that is not the solution to this issue though. If there is wobble, it will start over time and break again.
 

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@hpark21, I believe so.
You are absolutely correct:
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I think then, BEST method would be to evacuate the AC line, take the AC compressor out, take the compressor mount off and replace it with new one and put proper bolt in the tensioner.
While you have the compressor out, maybe consider replacing the clutch solenoid OR just install a brand new compressor all together as "preventive" measure (entirely optional).
 

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You can save on the bracket by going to a pick-a-part place(junk yard) if you decide to go that route. Should only cost around 10 bucks. Make sure the threaded hole is good before removal. But first try the longer bolt and see if you can fit a wrench to hold the nut when tightening. If you have the room. It will be a permanent fix.
 

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You can save on the bracket by going to a pick-a-part place(junk yard) if you decide to go that route. Should only cost around 10 bucks. Make sure the threaded hole is good before removal. But first try the longer bolt and see if you can fit a wrench to hold the nut when tightening. If you have the room. It will be a permanent fix.
Plus it’s good practice for how to remove and install the parts.
 

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You can save on the bracket by going to a pick-a-part place(junk yard) if you decide to go that route. Should only cost around 10 bucks. Make sure the threaded hole is good before removal. But first try the longer bolt and see if you can fit a wrench to hold the nut when tightening. If you have the room. It will be a permanent fix.
I really think this is the best choice. Probably you can move the compressor enough to get the bracket out without evacuating the system too.
 
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