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Join Date: Oct 2003
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What is the purpose of the balance belt idler?

While laying on the cold garage floor (well cardboard) this weekend readjusting my balance shaft belt, I couldn't help but wonder....what is the balance belt idler for?

It seems to me that just provides a path for the belt on it's long travel. It has no bearing on tension setting - Clark's said to keep it out of the way. It's so close to the lower balance shaft cog, it requires that .5mm setting, but it looks to me like that's just to keep it out of the way.

Is the idler, just that "idle"? And really doesn't do much of anything? Why not a metal ramp like the timing belt?


Also, I changed all my rollers, cogs. The cogs looked pristine. Do you need to change them every time?
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Old 01-09-2006, 05:01 AM
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I've wondered that too. My best guess is that it's there in case there is any "belt slap", in which case it would actually come in contact with the belt and keep it in line. As far as why they didn't use metal ramps like the timing belt - my guess is that the balance shaft belt has cogs on both sides, and they probably only use the ramps on the "flat side" of a belt, so they couldn't use the ramps with the BS belt.
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Old 01-09-2006, 06:57 AM
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I think EA hit it. Just in case of belt slap. Possibly, as a secondary purpose, to eliminate the possibility of the belt jumping out of the lower balance puley (the .5mm clearance)

The cogs should be changed. Even though they looked pristine, it's not the cogs that you're really changing, it's the bearings that are part of the cogs. That's what really needs to be changed.

You can probably get away with not changing them, but most people here think "better safe than sorry" The thought of a $25 bearing & cog assembly, causing $1500-2500 worth of damage is more than most people will want to gamble. I think the general consensus is, well, i'm changing the belts, so i might as well change everything in there that can fail at the same time.
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Old 01-09-2006, 08:16 AM
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Good to see that the old Porsche "while you're in there" holds true for both the 911 and the 944. It must be murder on the 928 guys.
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Old 01-09-2006, 08:29 AM
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I know what the balance idler is for - to create a wicked whining sound when I get the clearance wrong :-)
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Old 01-09-2006, 09:58 AM
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I know what the balance idler is for - to create a wicked whining sound when I get the clearance wrong :-)
LOL I think of the balance shaft belt whine as my poor man's supercharger.
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Old 01-09-2006, 10:23 AM
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