Cambelt stretch during installation

[Roeland_Vester 5]

Hi all, 

 

I'v done a head job on a DKZ head (thanks again welshpug for the help with the valve springs), with a D6B camshaft (two marks near the distributor end of the cam) and I've followed the Haynes procedure to fit the cambelt (Contitech). The head has been shaved just to clean it up (minimal material removed), and the head gasket was a 'normal' item from an Elring seal kit. 

I've fitted timing dowels in the cam and crank pulley. I've put on the belt, and tensioned the roller by hand. I did two revolutions of the crank, and checked if the timing dowels still lined up (they did).

 

However, when I then tension the belt until I can just twist the the long end of the belt 90 degrees with two fingers, the timing marks do not align anymore. The crank timing hole is now about 3mm out. Is this correct? 

 

Does anyone have the cam timing specs of the D6B so I can check? (I'm already looking forward to finding true TDC....)

[Thijs_Rallye 87 4 Cars]

Do you mean they don't line up right after you've checked the tension by twisting the belt 90° ?

 

How does your bottom locking hole look? (behind the crank pulley?) Often they have been misused for loosening the crank pulley bolt, which breaks them and possibly leads to mis-aligning the locking pin.

 

 

 

[Roeland_Vester 5]

yep, the process of tensioning the belt up to '90 degree fingertwist', the holes align, after that, they don't anymore, due to belt stretch.

Timing recess looks okay.

[Thijs_Rallye 87 4 Cars]

It still isn't clear the me what you mean though. If you rotate the crank two full revolutions (with the belt already tensioned) do the holes line up then?

[Roeland_Vester 5]

I've attached the relevant haynes section to clarify. everything goes well until step 36. after 36, the dowels dont line up anymore due to belt stretch.

 

[DamirGTI 342]

Wrong belt ?

Early engines (with spring loaded tensioner) use 113 tooth belt whilst late ,with roller tensioner (like yours i guess ?), use 114 tooth belt .

 

If that's not the case , if you routed the belt starting from the -> crank -> WP -> cam wheel .. you'll always end up slightly off ..

Rather than that , route the belt like so staring from the -> crank -> cam wheel -> WP .. as you'll need to have pre tensioning belt slack around the WP .. then up the tensioner , spin a few times and re check .

 

D

Edited March 23, 2018 by DamirGTI

[Thijs_Rallye 87 4 Cars]

1 hour ago, Roeland_Vester said:

I've attached the relevant haynes section to clarify. everything goes well until step 36. after 36, the dowels dont line up anymore due to belt stretch.

 

2 hours ago, Thijs_Rallye said:

If you rotate the crank two full revolutions (with the belt already tensioned) do the holes line up then?

 

You still haven't answered my question ;). If you try to fit the dowels after twisting the belt it is very likely you will slightly rotate either the crank or the camshaft ;).

 

Therefore the question, can you fit the dowels if you rotate the crank 2 full revolution?

 

How much force do you need to exert on the belt for flexing it 90°? It should be a relative easy twist.

 

[Tom Fenton 1,542]

These belts do not stretch. They have very strong cords that run round the entire belt. Cut through your old one you will see. As you are tensioning the belt it is pulling the pulleys round out of exact time. If the head has been skimmed this can move the timing out. Not much you can do other than fit an adjustable cam pulley.

[SootySport 23]

3mm out of alignment is nothing to worry about.  The belt does not stretch  until thousands of miles of use.  All timing belt changes I have done on different engines end up with a few mm discrepancy and the ignition timing can cope with up to being one tooth out.   Only way you can get it exactly right is to fit adjustable vernier cam sprockets.

For example https://www.demon-tweeks.co.uk/performance/pulleys/piper-cams-vernier-camshaft-timing-pulleys  You can see the 3 hex head adjusters in slots which allow a few degrees of adjustment after the belt is fitted under correct tension.   They are of more use to optimise timing with tuned engines.  

[Roeland_Vester 5]

4 hours ago, Thijs_Rallye said:

 

You still haven't answered my question ;). If you try to fit the dowels after twisting the belt it is very likely you will slightly rotate either the crank or the camshaft ;).

 

Therefore the question, can you fit the dowels if you rotate the crank 2 full revolution?

 

How much force do you need to exert on the belt for flexing it 90°? It should be a relative easy twist.

 

I can refit the dowels when the belt is not fully tensioned, but only set up 'without slack', as per point 32 in the haynes section. After fully tensioning, I can not refit the dowels. 

 

Damir, I did manually count the teeth on the belt to be 114. I also rechecked with the wheels one tooth out, and those were worse. I cannot move the pulleys less than 1 belt pitch wrt to eachother.

 

 

If I were to buy an adjustable pulley to get it just right, am I correct that 'inlet valve opens 11.5deg btdc' is all the data I need ?

205gtidrivers.com/articles/articles-guides/205-gti-basics/technical-specifications-r12/

[Thijs_Rallye 87 4 Cars]

What type of tensioner are you using? The spring loaded one or the 16v like one? 

 

For setting up the camshaft you'll need to know lift in TDC. And you'll need two dial gauges and proper stands as well for doing it properly. 

Edited March 23, 2018 by Thijs_Rallye

[Roeland_Vester 5]

I have the eccentric tensioner setup.

 

My idea was to find 90 degree btdc by finding the point where pistons 1 and 2 have the same height (as measured from the plug gap). This way I'm using the fact that the crankshaft is flat plane to measure where the crankshaft motion results in the most difference in piston position, instead of where the piston position is least sensitive to crankshaft position (tdc). I then will rotate the crankshaft 78.5 degrees, and position the camshaft such that the inlet lobe associated with the piston going up is just touching the tappet.

 

In my mind, this is a rather foolproof way to setup the camshaft timing.

[welshpug 1,661]

3 hours ago, Roeland_Vester said:

 

 

My idea was to find 90 degree btdc by finding the point where pistons 1 and 2 have the same height (as measured from the plug gap).

 

 

that will be when its Pinned.

[Thijs_Rallye 87 4 Cars]

Ok, with the excentric one at least you have the correct belt :).

 

Regarding setting up your cam timing, your plan sounds feasible in theory, but you might as well just wing it with an adjustable pulley. There is quite a large toierance stack up between the crank keyway, crank pulley, drilled hole in the pulley, the cast oil seal carrier with locking hole.

 

There is no decent way of setting it up without specifications and measuring equipment unless you do a lot of trial and error. (which actually is doable since even with measuring equipment there was quite a lot of winging it going on because those pulleys often also don't really cooperate )

[Roeland_Vester 5]

Okay. After a lot of back and forth, I failed in my attempts to get an adjustable cam pulley delivered on time (I'm taking the car to scotland next monday to do the north coast 500).

 

First, I refitted the original cam pulley. Then I checked the timing. With the timing hole in the camshaft roughly lining up, I set the crank at 90 deg BTDC/ATDC by using a vernier caliper through the spark plug hole, taking car that I held the caliper the same way each time. This went really well, I could reliably locate this point within 0.01mm of piston movent, which translates to about 0.01 degree of crankshaft motion. The actual accuracy will be a bit less as it assumes a perfect cilinder head and perfect pistons. The crankshaft timing hole almost lined up. Piston 1 is now coming up in the exhaust stroke. The cam spec (11.5 deg btdc, 234.5deg duration) is not quite sufficient, as the lift at this duration is not given. I fitted a printed out protractor to the distributor side of the cam. With this, I measured a '-0.05mm' lift duration, by finding the points on the cam where I could just slide a 0,05mm feeler blade between the lobe and the tappet (both of piston 1 inlet valve, so valve clearance differences don't mess it up). I could reliably find this point within one degree of cam rotation. Half the difference between the mentioned duration (234.5) and measured '-0.05mm lift' duration (can't remember what it was, something like 280) is then subtracted from the -11,5 degree btdc (which is at degree 0 of 234.5), and setting the crank at this point (using the camshaft protractor and a 2:1 crank-cam rotation ratio), should allow me to just slide a 0.05mm feeler blade under it. Adding half the difference between the mentioned duration (234,5) and measured '-0.05mm lift' duration to the 46mm bbdc figure for the valve closing gave me the second point where I should be able to just slide my feeler blade between the cam lobe and piston tappet. 

 

Luckily, it all ended up right on spec. Much ado for nothing, but I did have a nice time wrenching with my dad. It did require me to get a paper to write all the angles down, because it gets hazy if you need to convert back and forth to camshaft and crankshaft degrees and tracking everything.

 

I got the APK (Dutch MOT) this week, coming saturday is a 500km shakedown run, and if everything goes well, I'm on the boat monday

 

 

Edited April 19, 2018 by Roeland_Vester