Balance belt idler misinformation

[JWest]

I've noticed that there are quite a few sources around that are saying there should be a clearance both above and below the balance shaft idler pulley. It has been given as advice on this board numerous times.

I have three different versions of factory manuals that say there is a preload on the upper surface and clearance on the lower.

Here is the 1985 944 manual:

Loosen idler pulley so that pulley does not touch drive belt.

This indicates to me that the pulley WAS touching before it had to be loosened.

After adjusting drive belt tension, adjust idler pulley. Use special tool 9207 so there is a clearance of 0.5 mm between drive belt and pulley when upper portion of drive belt is PRELOADED 0 to 1 mm (see sketch).

Preload on top, clearance below.

From the 1987 update to the 944 manual:

After adjusting drive belt tension, adjust idler pulley. Use special tool 9207 so there is a clearance of 0.5 mm between drive belt and pulley at the lower balance shaft when 0 to 1 mm PRETENTION is applied to upper run of toothed belt.

Slight wording update making it even more clear that there is pretention above and clearance below.

1992 968 manual:

Remove toothed belt cover. Undo idler until the idler NO LONGER places a preload on the toothed belt.

Again, this indicates to me that is WAS touching before the idler was undone.

After the toothed belt has been adjusted, follow by adjusting the idler. Using Special Tool 9207, set idler pulley to a clearance of 0.5 mm to the toothed belt in the bottom balance shaft area and PRELOAD upper toothed belt train by 0 to 1 mm at same time.

Preload to upper surface and clearance to lower.

Another indication that there should not be clearance is the fact that no tool exists to check the upper clearance.

Here is the diagram:



Is there newer info available that I don't have that makes my sources obsolete?

[2Tight]

That 0 to 1 mm upper preload could only be measured with dial indicator as far as I know. I've always guesstimated it after tensioning the belt. I never paid attention to what people said unless it was it the FSM anyway. It doesn't take much preload at all to make the BS belt whirr. I usually get a very slight BS whirr that dissipates in about 50 miles.

[John_AZ]

On both my 924S, with a used BB, I get .5 bottom and "0" on top (just barely touching) with the slotted idler roller. There is no whine. It is also important to adjust the balance belt tensioning roller CW-clockwise to get the belt below the WP pully.



John_AZ
1988 924S + 1987 924S

[JWest]

I've got 194k on my 944 without breaking a belt (changing them around 30k). I use the factory tool and about 1 mm preload.

I've seen multiple references to 1 mm clearance rather than preload and I'm wondering if the person saying it misread and is going to cause somebody else future problems.

[John_AZ]

I have bookmarked the post and understand your "preload" comments. I have seen many posts that indicate a gap (clearance) should exsist between the belt and idler roller. This would make the belt even more "loose". I change my cam belts every other year. The balance belts when I do the WP. The BB idler roller on my cars does not let me adjust for a clearance "above" the roller. Maybe I have an updated roller when I replaced a few years back???

John_AZ

[Makis]

I replaced my belts recently and tried to preload the top side gently so that when the engine runs the roller (which is new) spins at full speed. The effect was a high pitch whine even with the engine cold (balance belt still quite loose). I reverted back to letting a small clearance and the noise just disappeared all together. So I am not sure this is the right thing but I feel more comfortable without the noise. I believe the roller is there to control the belt slack especially when the engine is cold.

[AKCJ]

You make a good point James. I for one am guilty of 0.1 mm clearance on top rather than preload. It's tough to say if it would make any meaningful difference. In operation this belt would probably move up and down what? 2 to 5 mm? (wild guess). In that light + or - 0.1mm doesn't seem significant. Just thinking out loud here.

Thanks for pointing that out. I'll probably try the preload next time.

[JonA]

Somebody tell me what purpost the idler serves by being set to .5mm from the belt at the lower bal. sprocket? I can see that at the top it would keep the long run of belt from slapping around but what does it do at the bottom?
I am doing front seals, rollers and belts now and I cannot get proper preload and that .5 mm clearance at the same time. With preload set clearance to the bottom is about 1/8"
This is after tensioning by rotating the tensioner cw as the literature specifies.
What to do?

Jon

[paper1122]

Originally Posted by Makis

I replaced my belts recently and tried to preload the top side gently so that when the engine runs the roller (which is new) spins at full speed. The effect was a high pitch whine even with the engine cold (balance belt still quite loose). I reverted back to letting a small clearance and the noise just disappeared all together. So I am not sure this is the right thing but I feel more comfortable without the noise. I believe the roller is there to control the belt slack especially when the engine is cold.

I noticed the exact same thing, I backed off roller and the noise went away.

[Rasta Monsta]

I just rechecked mine, and used the "special tool" to set the .5mm gap above the BS sprocket per FSM. With the idler adjusted for this gap, it is not possible to deflect the belt run above the idler, per West's illustration.

I can only conclude that the idler diameter, or the geometry of the adjustment slot, has been changed.

Very mysterious!

[Rasta Monsta]

Time to add more confusion. I worked on an 84 yesterday, and set the BB gap with the factory tool as above. On the 84 (unlike on my 87), the BB DID deflect about 1mm per West's drawing above. I am really wondering whether there is a difference in the geometry of the early (non power steering) BB carrier.