Darklands

I had last week a timing belt failure on my 86.5 5 speed.The bolt for the timing belt pulley broke at fast driving (140 mph) on autobahn.
The laso pump was 2 years old and ok and the belt was 25000 miles and also 2 years old.
In the last time a few pulley bolts broke on the old design of the s pump.

A few members of the German 928 community think we should replace this 8 mm bolt every timing belt job.
I don´t know if the porkensioner has this critical piece?
After disassembling the engine I know the collateral damage of this failure.Luckily I have a set of spare heads.
I think after 235000 km I should rebuild piston rings,rod -and crank bearings.
A new complete gasket set is also needed.

SeanR

Are you talking about the pivot arm bolt? The one that holds the tensioning arm in place?

Glen McCartney

Sorry to hear of the failure. I always wondered about that bolt, when I bought my car and did my first timing belt the PO replaced that shoulder bolt with a bolt and sleeve that I found bent. I switched to the Porkensioner with my last TB job, shoulder pivot bolt is not used, much better system IMHO.

Good luck with the rebuild.

dprantl

Do you have pictures? I am having a hard time visualizing how the bolt holding the timing belt pulley could break... Or are you talking about the shoulder bolt that holds the arm?

Dan
'91 928GT S/C 475hp/460lb.ft

PorKen

Sorry to hear that, Karsten.

Let me know if you need anything.

145K miles is not that much. I think you could just put the new heads on?

(BTW: how did the V3 chips work on the autobahn before the engine went quiet? )

Darklands

My car is at the shop and my friend tells me the diagnosis after pulling the engine.He says the S 4 design hasn´t this weak point any more.
I think he means the shoulder bolt.

Ken, I drove at 7000 rpm in 5th gear..... ;-)
Joke aside, I received the chips the day after.
Loosing the belt at high speed isn´t spectacular.Quietness,a lot of lights in the instrument cluster and no more the feeling of sitting on the tip of a saturn 5.
A little cloud of oil in the back mirror and than waiting for the yellow angels( the german car club )

PorKen

No shoulder bolt with a PKT.

(You don't even need the pulley{s} under the crank gear as in this pic.)

Darklands

Loosing the belt at high rpms isn´t good for valves,isn´t it?
I take pictures at the shop from the disaster.

Bill Ball

Most likely. The infamous shoulder bolt that seems to get stretched and bent. The water pump and bolt design changed in 87 and I can't recall any problems with the updated one that has bigger thread diameter and is installed as a stud on the pump so it is not removed and reinstalled when serviced.

A thread with pictures of the offending 8mm bolt gone bad:
https://rennlist.com/forums/928-foru...apart-omg.html

Here's some damage from one that broke:

Schocki

Sorry guys, but what is a shoulder bolt exactly?

Darklands

A 0,50 € bolt makes 5000 € damage.
Ken,I think I´m a believer from now in your design!

Schocki

I was a PorKensioner believer when Roger showed me an example two years ago.
I don't change TB's any more without a conversion, it is so much better!

rexpontius

+1 the Porkensioner convinced me immediately, it was actually the first part I changed on my 928 since I have it :-)

Great work Ken! (I replaced all my fuel lines under the bonnet recently, so it's about time I get the s300s chips + fpr)

Major bummer to have the belt go at high speed/rpms.
Hope the engine is repairable with your new heads and some €€€ :-(

James Bailey

Shoulder bolt is a bolt with large nut with a large barrel where the bushings ride but a smaller threaded stud which attaches it to the pump. So it looks quite robust and a large wrench fits BUT easy to over torque. The S-4 uses a stronger stud with a support brace added.

jwillman

This is my old one. Roger sells a beefier one but to other posters points even that one can be easily snapped when over torqued