Engine Rebuild (Part 10) - Fitting the Distributor Tower: Aligning the Slotted Head

Refitting the distributor tower is only a small job in the overall engine reassembly, but the steady flow of questions raised on forums shows that it's very easy to get this wrong......

Distributor Tower

It helps to understand how the bits and pieces relate. 

The spindle in the distributor tower rotates the distributor arm which causes a spark to occur at the spark plug of each cylinder in turn. The spindle in the distributor tower has a pinion at the base that is driven by the camshaft. 

The pinion inside this bit.........

.......Needs To Be Correctly Engaged With This Bit.

The camshaft also determines when the engine valves open and close. The camshaft is in turn driven by crankshaft - which controls the movement of the pistons. Through the distributor tower spindle, the camshaft is seeking to manage the correct point for the distributor to make a spark for each cylinder - meaning the spindle needs to rotate the distributor arm to the right position at the right time for the firing sequence in the correct cylinder.

To help ensure this happens, the drive socket on top of the distributor tower spindle has a slot in the top: the slot determines how the distributor is fitted to the spindle in the tower and so influences the point (relative to piston and valve movement and spindle rotation) at which a spark is produced....... 

Drive Socket in the Distributor Tower.
Note that the slot does NOT pass centrally through the axis

The slot in the drive socket seems to be the cause of much confusion and error as the slot does not pass the centre point of the drive head - it is very slightly offset  - giving a 'big half' and a 'small half' - thus when fitting the tower, the alignment of this head socket is crucial for correct ignition timing. 

From background reading, having fitted the distributor tower, any problems typically seem to fall into three scenarios:

1.  The distributor tower has been refitted with the drive socket of the spindle aligned - but the wrong way round (180 degrees out of alignment)

1. Looks okay - but 180 degrees out....

2.  The distributor tower has been refitted without the drive socket of the spindle being orientated in any particular way at all (anything up to 360 degrees out of alignment)

2. No Chance.....

3.  The distributor tower has been refitted with an attempt to align the drive socket, but the drive socket itself is not correctly orientated to the pinion wheel on the other end of the spindle and so will not rest in line with the camshaft. (can be anything up to about 15 degrees out alignment).

3. Nearly - but not parallel.......

Of the above, scenario 1 seems to be the most common and the most likely. It tends to reveal itself only when people come to refit the distributor and set up the timing - by which time the gearbox and clutch housing will have been bolted over the distributor tower - making it impossible to remove and correct properly without stripping the engine down.....However there is a workaround and a fix. The workaround: scenario 1 affects the firing order of the cylinders: rotating the distributor 180 degrees and refitting it restores the correct firing order. The solution also necessitates swapping the long and shot HT leads around a bit. BUT, one issue you might have with a "stacked" (side entry) distributor cap is the matching notches in the distributor and cap: if the distributor is 180 degrees around, then the plug leads will be facing the nose of the car rather than the back! Solution? Fit a 'top exit' distributor cap! The fix: remove the distributor, remove the roll pin holding the drive head on the bottom. Rotate the drive head 180 degrees on the shaft and refit the pin. Firing order will be restored and your plug leads will still point towards the back of the car.

The fix for scenario 1: remove and rotate the drive head of the distributor

The other scenarios should be obvious as soon as you fit the tower, but if you're not paying attention, problems will only come to light when you try to start the car.....

Scenario 3 only really come about if you have stripped the distributor drive tower to replace the oil seal and messed up the reassembly. If you have stripped the distributor drive tower to replace the oil seal, then you will have removed the slotted drive head from the spindle. When reassembling, it is vital that this is correctly oriented on the spindle - to preserve the relationship between the line of the slot and the teeth of the spindle that engage with the camshaft. Mark the drive head and the spindle so that these can be matched up on reassembly.

All three scenarios can come about if you fail to follow the Manuals and check your work.....The correct procedure for refitting the distributor tower correctly is covered by step 35 of Operation DX.100-3 of Manual 518. However the photograph there is not at all helpful and you are better off looking at the equivalent, but updated, photo in Operation D.100-3 of Manual 814 - as it is much clearer and easier to understand the process: it more clearly shows the offset of the drive socket. In BOTH manuals the photos can be misleading as they show the tower bolted home, but also illustrate the two ('before' and 'after') positions of the drive socket.

'a' is orientation before bolting the tower on. 'b' is after it is fitted

STEP ONE

Identifying The Compression Stroke. The first stage of refitting is to find the compression stroke of number one cylinder:  the stroke on which its compressing the air/ fuel mix in the cylinder. Number one cylinder is the one nearest to the gearbox. I used a short length of polythene tubing threaded tightly into the spark plug hole and and a balloon on the end as a 'tell' to show when the piston was compressing: it inflated the balloon.

Tool for Identifying the Compression Stroke. It works with any colour of balloon.

With four of the flywheel bolts loosely fitted to the end of the crankshaft I used a bar to rotate the crank until the balloon inflated. 

Rotating The Crankshaft

Turning the crank a  little more I waited until the balloon was at the point of deflating. In this way I found an approximate position for TDC. 

Compression stroke identified

STEP TWO
Finding TDC (top dead centre). Removing the tube and balloon, I inserted a length of dowel through number one spark plug hole, making a reference mark level with the top of the spark plug 'funnel'. I popped a plastic bottle top with a hole drilled in it over the funnel. This centred the dowel (held it vertical) and accentuated the movement of the dowel and my reference mark.

Using a Dowel to Confirm TDC

Slowly turning the crank back and forth again I was able to find the highest point for the dowel - TDC for that cylinder.

STEP THREE

Aligning and Fitting The Tower. I had rebuilt my distributor tower (see related post from January 2018). Although I had re-orientated the slotted drive socket to the pinion when I refitted it to the spindle, this was a little rough and ready and I wasn't sure it was properly aligned. With TDC of the compression stroke found and set, I did a few dry fits of the distributor tower to check alignment. 

The following instructions assume you are standing on the inlet manifold side of the engine - with the camshaft end sticking out to your left.

Before fitting the spindle in the drive tower, the spindle was rotated so that the slotted head was at an angle of about 45 degrees (bottom left to top right - position 'a' in the photo in manual 814), with the 'small half' of the slotted head (segment 'b' in the photo in Manual 814) closest to the water pump. 

That Diagram Again

Drive Socket Rotated Before Fitting

Keeping the tower in the position it would be when bolted on, I introduced the drive tower 'eye' over the camshaft - stopping short before the pinion on the drive tower spindle engaged the teeth on the camshaft. I loosely fitted the tower bolts to act as alignment guides for subsequent fitting.

Bolts used to Guide the Tower Into Position

The drive tower was then slowly pushed home along the 'guide bolts' - at which point the slotted head rotated clockwise by about 45 degrees as the pinion engaged with the camshaft - leaving it (1) 'horizontal' or in line with the camshaft (back of engine to front of engine) and (2) with the 'small half' of the drive head (segment 'b' in the photo in manual 814) closest to the water pump housing. With the number one cylinder at TDC of it's compression stroke, this is the correct position for the slotted drive head.

With The Tower Pushed Home, The Slotted Head Aligns To The Camshaft

Satisfied with the test fits, I did it for real: I gave a last squirt of oil to the pinion at the bottom of the drive tower and similarly oiled the surface of the camshaft and lubricated the large camshaft oil seal. I fitted a new paper gasket to the back of the drive tower (it came in the Glaser engine gasket set). I reset the spindle so that the slotted head was at 45 degrees and fitted the assembly over the camshaft - stopping short of engaging the pinion. 

I loosely fitted the bolts (smeared with threadlock) and then tightened the bolts up - as you would the wheel nuts of a car. As the pinion engaged the camshaft the slotted head rotated........ ending up in line with the camshaft and 'small half' nearest the water pump.

Job done!