308 Cam Belt Drift

It can only be caused by misalignment of the pulleys. The cam pulleys can only be off if the heads were resurfaced off of straight or the cam bearings were align bored wrong. It is usually caused by a tensioner bearing or mounting issue.

 

Brian,

Can you elaborate and exactly what to look for?

 

Something, anything that prevents the pulley axis from being exactly parallel to the lower drive pulleys or the cam pulleys.

I forgot another possibility. It will do that if the bearings on the lower drive gears are bad but in that case it should drive to belts forward, not back.

 

"My OEM pulleys did not experience drift."

Verify that the pulleys are correctly installed/seated correctly. If so, put the OEM pulleys back on. If they don't drift, leave the OEM's in place. Running it as it is is losing a fair bit of the belt's "tooth strength." You definitely don't want that!

 

I agree completely. Also I have seen the fences tear up the belts.

 

Mike said something that makes me think of another possibility. Never seen it with factory pulleys so did not think of it before. If for some reason in the manufacturing either of the pulleys is tapered it would do that too. It doesn't take much.

Why did you replace the original pulleys?

 

If the lower drive gears are worn the 'drift' should have occurred with them too.

Yes?

 

Absolutely. But after a few revolutions no matter the source of the problem it will end up at the edge of every pulley.

 

It should be easy to measure the new pulleys for taper. This looks like it must be the problem.

 

Yeah, as long as the full width of the belt is on the pulley, it's fine, no need to be centered. As noted, I'd agree that the aftermarket replacement pulleys are defective.

 

Interesting. This thread got me thinking about fences. I have never seen belt walk but my datapoints are low. I have seen the aftermath of walking where there is failure ripped up timing gears or fence chewing belt or fence falling off. There are many who like to tack weld the fence to the cog with some pretty stout TIG welds. Ferrari makes cogs that seem to last the life of the motor yet poorly peens on the fences. I have seen many fences rattle on the cog. This is either sloppy lazy cheap or fences are somewhat to maybe useless to Ferrari. On the 308 the head has inboard and outboard fences. I guess that is some kind of belt security desire. On the 550 there are no fences at all preventing the belt from riding off. It all depends on parallelism of the system. I guess the OP's pics make a case for fences! It was smart to crank the motor and check your work. I'm pretty sure 90% of people would have closed everything up and unknowingly run those pullys to perhaps a failure.

Right or wrong I think of the fences as not real important. Are you pros welding fences? Helms is a big fence welder. I have TIG, MIG, and gas in a state of readiness within 20ft of me yet I have never bothered to weld a fence that wasn't loose.

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Wayne, it’s worth giving the lower pulleys a wiggle while in there as the bearings are prone to wear in the 2V and early QVs with 40mm bearings. I agree with others than there may be a slight taper on the new pulleys. If you have the steel OEM pulleys, stick with them. I’m sorry I dropped the ball a bit on sending you a set— a move across the pond got in the way :-(

 

The only cars I have ever seen loose fences on were 3.4s and I almost never work on them. I have never welded one. Lots of cars dont have fences on any but the drive pulley. More than that should never be needed.

 

Looks like you have a couple pulleys on backwards. The fences should alternate.

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The picture you are looking at is a 550, not a 308.

 

This is the diagram for a 308, https://www.ricambiamerica.com/car-diagrams/ferrari/v6-v8/308-group/308gtb-gts/timing-system-tappets.html
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The drawing I posted in #16 is for a 550. FBB posted a picture from his 550.
If everything is running true, fences shouldn't really be necessary, except for one on the drive.

 

No fence on 550 cam pulleys. All 4 are the same.

 

If I'm not mistaken on the 550 and those without alternating cam pulley fences, the idler pulley on the tensioner has a crown, we're talking in the thousandths of an inch. The lower drive is an outboard fence uppers are inboard, properly aligned pulleys won't need much to keep the belt in place, the fence is more of a "bump" stop, poorly aligned pulleys the belt will 'walk' right off, even with a fence.
If you've watched the belts hum along at near redline the fence makes sense, harmonics and frequency cause the belt to move similar to a plucked guitar string, the belt will move about a bit when doing this, very different from walking off due to poor alignment.

 

No fences on the cam pulleys. Drive pulley has inner and outer fences. Don't know about the bearing.

 

That is the thing I appreciate about Superformance, they really do care about their products and customer service. I have experienced that first hand.