worf928

This is the 'serious' thread. For sarcasm thread see: https://rennlist.com/forums/928-foru...-balancer.html

Subject of discussion:

worf928

I've never pondered the vicissitudes of the OE balancer. This is the first I have seen where it came apart. I don't recall seeing a similar failure reported on Rennlist. However, I could be wrong and there have been failures like this that I just didn't commit to memory.

It seems that the rubber 'balancer' part is just glued to the hub?

It's from a 50k-mile '87. Garaged most of its life except for the first 3-ish years where it was a four-season daily driver in Boston.

Why did this fail the way it did?

Manufacturing defect that took this long to metastasize?

Glue let go due to age?

Or due to mishandling in the past?

Thoughts?

SwayBar

Can you post more pictures of it?

Red Flash

Well, the first 2 thoughts that come to my mind are (1) glue application procedure and (2) material incompatibilities. Or, maybe a mix of both?

I also missed exactly how this happened. It separated while being dismounted or when it was dismounted it had already separated?

Fascinating find. Are you digging deeper into the engine? Or, is this just TB/WP job? The condition of the bearings would really interest a lot of people here.

Thanks for posting.

GregBBRD

I've been talking about this problem, for years.

I've seen several like this. I think I threw them all away, when I moved, but I'll take a look.

I've also seen many wirh the rubber cracked and separated...ready to break. I'll see if I have any of those left, sitting around.

Old age. Rock hard rubber.

It's an inevitable failure. And once the rubber gets hard, it doesn't transfer the harmonics from the crankshaft to the dampener. The "dampening" the dampener is designed to do no longer occurs.
Beats the main bearings/the crankcase to death.

BTW....not only a 928 problem. So many American engine dampeners ended up as heavy chunks of metal flying around, the NHRA made rules about them.

Red Flash

Or, here's an idea. Old rubber gets hard AND shrinks, but the metal plate to which it is afixed does not shrink. Sonner or later stresses at the interface between the two materials break the glue bond.

the flyin' scotsman

I truly hope the engine wasnt running when it let go!

our cars are now getting very old; 40-25..........most, if not all rubber is at EOL.

SwayBar

I'm glad I have one of Greg's balancers, I have to decide which car to put it in, the GT racecar, or the S4 - I'm leaning towards the GT.

GregBBRD

GT racecar.

We changed the hardware from regular allen to torx. If you were sent a damper with regular allens, call Mary and she will send you the hardware de jour. (They still can be problematic to remove, since all hardware takes more torque to remove than it takes to install. Surface friction increases with time.)

I ignore ATI's suggestion to use Locktite....the last thing we need there is increased thread resistance.

worf928

So, it's probably a good thing I put down the tools Saturday afternoon: I wasn't on my game, or I would have realized that ...

... *of course* most folks are going to want to see more pictures if for no other reason than for the fascination of a 'balancer autopsy.' And ...

... that folks would want more context.

Yup. Exactly as I wrote in the non-sarcastic part of the sarcasm thread.

So, here's the context in addition to the very small bit provided in the original post.

This 928 was 'in' for a TB system check along with fluids and a few other odds and ends. Note that it was running great. The belt was given a full service two years and about 250 miles ago (yeah, I know it needs to be driven more) so it was due for the post-install tension check/adjustment.

Upon rotating the engine to TDC I note that the right-side cam gear is off by about 60°. That's far enough off that - at best - the engine shouldn't be running well. I did a sanity check with a TDC indicator in #1 that indicated that the crank was, in fact, at TDC when the cam gear marks were aligned. Obviously at this point the balancer marks were suspect.

Upon attempting to remove the balancer it was plainly obvious that the rubber part had detached from the hub because it was spinning more-or-less with finger pressure.

Here are some more pictures. More text follows in another post.

Back side of hub. I think the black stuff is where rubber separated from rubber but the glue didn't let go and the silver is where the glue let go. (This assumes even full-surface application of glue originally.)



Closer:

worf928

Here's how the pieces go together:


Close-up of the leading side of the rubber:

worf928

One leading indicator. When I was under this S4 draining the coolant I noticed some 'yellow-ish' dust on the underside of the tensioner and lower crank roller assembly. I asked the owner - busy doing some of the odds and ends - if he'd traveled through some area with yellow dust.

(Ignore Doc's custom ATI hub. That's, obviously, how this is going to be fixed. It's just where I left off on Saturday as the hub needs to cool for a long time before installing the balancer.)

worf928

Without knowing the materials we can only speculate.

Yup. And you often need to heat the hub to remove the balancer. So, you are stressing the rubber/metal interface.

ATI balancer will go on. Post-install belt tension and tensioner oil level will be checked.

If folks want the main bearings examined you'll have to convince the owner that his 50k-mile 928 needs bearings.

I don't know if that will be necessary.

Yup. And old glue.

Add that, as you well know, the effort required to remove the balancers is highly variable. Sometimes they 'fall' off. Sometimes you have to heat the hub and then they will 'wiggle' off. Every once in a while, you have to heat the hub and very, very gently use a puller with wiggling and heat to get them off.

This balancer has always(*) required the latter method to remove.

(*) And this is why I'm so interested in this particular harmonic balancer.

No one except me has ever touched it. I did the very, very first belt on this one in 2008 and again in 2017. (Yes, 20 year-old belt the first time I saw it - full records, original owner was contacted to confirm - and believe it or not, it's the best looking belt in my 'collection of old belts that tell useful stories.')

I'm caught wondering if *I* caused this failure due to 'over-heating' the balancer to remove it, or if this failure on this 928 was inevitable simply due to the original hub/crank tolerances. And if I did cause the failure, how do I change my removal procedure for a balancer that won't wiggle off when it's just 'warm.'

I'm leaning towards the latter: This balancer was 'destined' to come apart because of the tolerances.

Why? I don't know how you get a 'seized' balancer off of a crank if you can't get it to expand enough to wiggle off. You can't do it with a puller 'cold' because that will destroy the rubber. And if you can't remove the balancer you can't change the belt. So, it seems like it's a rock and a crusty-old hard place if a warm balancer won't come off: If you have to get it truly hot to come off, you may have stressed the glue to the point where the rubber bit will separate.

Folks: Thoughts?

The very obvious response in my mind is:
- all OE 928 harmonic balancers need to be chucked into the garbage.
- the two-piece design of the ATI balancer is the way to go, because you can heat the hub 'all day long' to get it on/off without stressing the 'balancer' part.
- ATI recommends throwing them away every ten years of heavy use.

dr bob

Clean with rubbing alcohol, then a spray of deoxit, then hondabond or maybe some of that black 3M super weatherstrip adhesive.


Serious: I wonder how many of the balancer hubs have been heated or puller'd as part of a TB project. Or sprayed with brake cleaner.

Daniel5691

Interesting.
Yet another part which i need to order from Good Doctor Brown...

Thanks for the information,