Timing belt replacement incident

[aseem]

Hi guys. I have been working on replacing the timing belt on my 1989 GT, rebuilding the belt tensioner and replacing the coolant pump at the same time. Also some serious cleaning.

Everything has moved smoothly, until it was time to put the new belt in. The engine is positioned at 45 degrees before TDC, and the fly wheel lock is in place. I was threading the belt according to the WSM, and was checking the tightness between the oil pump and driver side cam wheel when the belt suddenly slipped. I thought the teeth slipped, but I soon found out that the cam shaft had jumped forward.

The driver side cam rotated clockwise seen from the front past the TDC mark on the wheel.
This picture describes the situation:

Black arrow is the timing mark on the back plate. The blue arrow is the TDC mark on the cam wheel, and the red arrow is my 45 degree before TDC mark.



I can't easily move the cam back to my 45 degree mark. I have read that at 45 degree before TDC, the cams should be able to move without damaging the valves. But this sentence from the WSM scares me: "Damage may be caused to the valves if the camshafts move suddenly".

So, what kind of situation am I in here, and what can be done to get out of it.

Appreciate any input on the subject.

[harborman]

When I did my 86S a few years ago, I think my drivers side cam also moved, but only a notch or two. I think I rotated it counter clockwise back to position. But I am not absolutely sure now.

[harborman]

I remember threading the new belt starting around the crank, oil pump and to the driver side cam, keeping tension on the belt as I placed it around.

[stepson]

I am always leery of pushing a cam gear backwards back onto a cam lobe. I have never heard or read anything definitively about it one way or the other. Maybe it would be okay, but I believe it would be much safer to turn the cam that far removed clockwise 2 turns back into correct position.

[aseem]

After searching a bit, it seems the cams can be moved both ways with the engine at 45 degrees BTDC, but I agree that it would be safest to rotate the cam the way it always moves. Why risk anything when the movement is this large.

Thanks for the input guys.

[harborman]

Originally Posted by aseem

After searching a bit, it seems the cams can be moved both ways with the engine at 45 degrees BTDC, but I agree that it would be safest to rotate the cam the way it always moves. Why risk anything when the movement is this large.

Thanks for the input guys.

Be careful! Before doing anything make sure you know the correct way. There was a big write up on doing the timing belt either here or Rennlist.

[aseem]

Yes. I'm mainly following John Pirtle's write up from Rennlist + all other info I have come across + the WSM. Everything was going fine until the cam suddenly decided to jump forward.

I have replaced the coolant pump, rebuilt the belt tensioner, and fitted new bearings to the lower bracket under the crank, and replaced both rollers.

Thanks

[harborman]

Originally Posted by aseem

Yes. I'm mainly following John Pirtle's write up from Rennlist + all other info I have come across + the WSM. Everything was going fine until the cam suddenly decided to jump forward.

I have replaced the coolant pump, rebuilt the belt tensioner, and fitted new bearings to the lower bracket under the crank, and replaced both rollers.

Thanks

It looks like it only moved maybe 2 or 3 notches? That is not much. Have you tried moving it back counter clockwise? I think mine was off a notch and I did rotate it back and get the belt to go on. It is important to keep the belt taught from the crank up and around that first cam gear. So thread the belt starting on your right as seen from the front of the engine working it around to the passenger side cam. Starting from the crank.

I meant Putting the belt onto the drivers side cam first, then over to the passenger side cam gear down around the tensioner and back to the crank.

[aseem]

no, unfortunately it moved more. The white dot at the red arrow was lined up with the timing mark on the back plate (black arrow) before the jump. So the cam that was at the 45 BTDC position is now past TDC (blue arrow is the TDC mark).

This all happened when i was lightly pushing the belt between the oil pump and the left cam gear to check how thought the belt was.

[harborman]

Originally Posted by aseem

no, unfortunately it moved more. The white dot at the red arrow was lined up with the timing mark on the back plate (black arrow) before the jump. So the cam that was at the 45 BTDC position is now past TDC (blue arrow is the TDC mark).

This all happened when i was lightly pushing the belt between the oil pump and the left cam gear to check how thought the belt was.

OK I see, maybe 7 notches, still not a whole lot. Try moving it counter clockwise back but don't force it. The crank didn't move so it is still at the 45 degree mark and locked. If it feels like it is binding stop.

[harborman]

Looks like it moved 6 notches.

[harborman]

Any success with the timing belt????

[aseem]

After reading up on the subject, I found out that the timing cam gears was worn, and needed replacement. The gears was worn through the outer hard coating, and into the aluminium. Since I wanted to make the timing belt system right, i decided to replace them. So while waiting for parts, I'm currently working on the low side of the engine, replacing the motor mounts and oil pan gasket. I had the cork gasket lying in my stock, but during the work i decided to go for a silicone gasket. So had to order that as well. Replacing all the oil pan screws with stud and lock nuts from 928Motorsports.

Also doing a lot of cleaning, blasting parts like brackets and bolts and sending to re-plating. It is annoying that when you order new bolts etc from Porsche, they are blue/blank plated instead of the yellow chromate that has been banned in EU. (i think Porsche used Cadmium plating, and that is banned, but zink and yellow chromate is still available from plating shops)

Parts arrive on monday.

btw. porsche has a new part number for the cam timing gear with improved coating.

Worn timing gear:


new timing gears:


Lifting jig:


Large oilpan:


Large bottom end. Funny how you can see all the way up into the underside of the pistons. Not in this pic....

[aseem]

Oh, yeah. I managed to turn the cam back into position. Just took some leverage using the big nut, not the 17mm bolt

[harborman]

Originally Posted by aseem

Oh, yeah. I managed to turn the cam back into position. Just took some leverage using the big nut, not the 17mm bolt

Well you seem to be doing a very nice job. Hope all goes well with the project and keep us posted.

[aseem]

While waiting for parts for the belt job, I started on the oil pan gasket and motor mounts. As usual, there is a lot of "while-you-are-in-there" jobs as well. I don't like to re-install dirty and corroded parts. So, after a lot of cleaning, parts have been sand blasted and either sent out for re-plating with zink and yellow chromate, or powder coated. I just bought a powder coating gun, and found a free oven on the local counterpart to Craigslist, so this is my first attempt. Turned out great for the motor mount housing and a few other parts. Really wish I had this opportunity before.

I have tried zink plating at home before, but didn't get the chromate to the level i wanted, so I sent the parts out for re-plating.

Here's the result. pretty happy with it:

[aseem]

Things are going fine putting everything back together. Oil pan gasket, new motor mounts, timing belt etc, all in place.

However, I cant figure out what this threaded hole is for, if anything. Just underneath the lifting bracket, in the center of the lower belt cover. It is part of the coolant pump See arrow on picture.

[aseem]

FWIW. The engine is all buttoned up, and everything seems fine. However, the Idle Valve is not working, so now I have to pull the intakes.

Not much activity here in the 928 Forum. Where is everybody? On Rennlist?

I used to be addicted to the 911 and 930 forum here, and it was a part time job just to get through all the new posts every day. It was one happy gang. One guy even lent me his expensive special gearbox tools if I payed for overseas shipping.