StratfordShark

Recently completed TB replacement. Idle is rock steady but I can feel a sort of shake at idle that wasn't there before — very conscious of this as replaced motor mounts recently so driveline was very smooth. It feels as if something isn't balanced. I did change cam gears but timed the new ones accurately with 32Vr tool. There is no vibration, hesitation or problems on acceleration

Looking down at the pulley I think I can see a slight wobble. Am I imagining it or is it possible to see on this video?


I am almost certain I replaced the crank thrust washer correctly (recess towards engine) as well as the washer over crank gear. The AC pulley is oriented as it was when removed.

Searching I found some cases where people had not pushed the harmonic balancer fully home, so the pulley was off centre when crank bolt torqued down. I wonder if I made same error but by eye as much of the harmonic balancer appears to show in front of centre cover as before I removed and replaced it. What I do remember thinking is that when I replaced the crank pulley, there was very little "meat" on the crank nose for it to be seated.

If I want to strip things down again to check my work, what can I get away with not having to repeat? I was thinking I could just pull fan shroud for some extra room, remove accessory belts, lock crank (doesn't matter where) and undo the crank bolt. Would this be ok or do the cam covers and centre cover need to come off again?

Thanks

Leon Speed

Yes and no.

There seems to be a slight radial run out but I'm not sure it isn't normal.

Bilal928S4

After the motor mount change, you will have a slight vibration or shake of the engine at idle. All my cars have it.
I think Sean said it is due to the old wiring from the distributor to the spark plugs.

Mrmerlin

if you put either thrust washer in back wards it would be tearing up the edges of the belt ,
and there would be lots of rubber dust, does this look like its happening?

The other thing that happens besides putting the damper on backwards is ,

Installing the thick washer under the crank bolt on backwards,
so the head of the bolt rides inside the counterbore of the washer.

The result of this is that the AC pulley, damper ,the black belt pulley, and crank drive will all be loose.

Since the belts are turning and the whine noise you hear is the alternator charging ( the heaviest load is at idle just after starting)
The washer must be on correctly.
All looks normal to me

StratfordShark

Thanks all for your advice.

May just be post -TB paranoia!

I'll keep an eye - she's done a few hundred miles since the belt change with no issues, so I'll just wait till re-tension and check things then. I set the timing using 32Vr without spinning belt on starter, just turning crank by hand, so it may be the timing will need slight adjustment then as I think Porken favours settling timing after moving cam with starter motor.

FredR

Adrian,

I rather suspect you would have to have something seriously out of whack cam timing wise to cause something noticeable to happen and I very much doubt that is the case.

There is a general concern about the age [28 years in your case!] degraded state of these balancers- whether removing them and thus taking the compressive pressure off the assembly causes them to delaminate a little may be a possibility- of course absolutely no idea whether something like this could be in play or indeed what the consequences might be- the science of these things is beyond my current knowledge base.

I believe some of our more experienced engine builders are into this stuff.

Rgds

Fred

StratfordShark

That's very interesting about the damper Fred. I'd never thought of it as being separate parts, just an interesting heavy lump! I'm sure mine is ok though, and it just slid off so didn't need a puller which I understand from reading other posts (prompted by your's) is one cause of damage separating the centre from rubber part.

Mrmerlin

NOTE you can verify the damper is still in its correct position by placing the crank keyway at 4 O clock place the damper on the crank, the TDC mark will line up.

OR put the crank key at 3 O clock then the damper will be at 45 BTDC

StratfordShark

Are those rough or precise positions? If 4 o'clock = TDC then isn't 3 o'clock 30 degrees BTDC?

FredR

Adrian,

On my motor the balancer came off quite easily-it felt like a nice working fit- but then we do not have the salty crud flying around in winter time and very little corrosion issues. This thing is a bit more than a heavy lump but that is in fact just what it is- a mass to absorb some energy transients as the engine accelerates/decellerates- a bit like a mini flywheel on the other end of the crank- something I can appreciate but know squat about. As I understand the unit is a composite of two metal plates and the rubber in the sandwich.

In using the term "delamination" we are not talking in terms of inches of movement here just a notional relaxation that does not reverse itself due to loss of elasticity- something not readily discernible to the eye when viewed off the motor but nonetheless measurable in-situ.

What caught my eye was your "perception" of a slight wobble. You could try clocking the external face of the damper- others on the list may know what is normal and what is not in terms of run out measured.

If nothing else it should tell you whether your eyes are deceiving you.

Rgds

Fred

StratfordShark

That's an interesting idea Fred, thanks.

I suppose it would require stripping off belts and crank pulley, since I think the AC pulley covers almost all the front surface of balancer. Must admit I've not used a dial gauge (use micrometer to check crank end play), so would need help on where/how to mount gauge for this measurement.

Mrmerlin

Are those rough or precise positions? If 4 o'clock = TDC then isn't 3 o'clock 30 degrees BTDC?

Please just try it, you will see for yourself.

FredR

Adrian,

The upper front surface of the balancer is exposed and independent of all belt travel. Ideally you would mount the dial gauge on the front of the engine possibly using the frame that holds the spark leads. If you have a magnetic base it might just hold firm on that frame work.

Rgds


Fred

Mrmerlin

if you try to tighten the crank bolt without the AC pulley on,
the bolts washer will bottom out and the pulleys will all be loose.

GregBBRD

The harmonic damper design was....ill advised, at best.

For harmonics to be efficiently transferred from the crankshaft to a "harmonic absorber" the absorber needs to be in direct contact with the crankshaft....not a slip fit, like on the 928. Nice first attempt for a harmonic absorber, for Porsche, but they completely missed the first week of "Harmonics 101".

The rubber that bonds the inner ring of the harmonic absorber with the outer ring is now, 28 years later, hard as a rock. Harmonic absorption stopped long ago.

The result is that the main bearings and the cradles on almost every 928 engine we take apart, these days, are significantly pounded.

Install one of my custom machined press fit (heat to install) inner hubs, along with an ATI outer shell....and vibrations are required by a huge percentage. Timing belt changes are still possible without removing the inner hub, since the ATI outer shell bolts on.