Ignition - Establishing a TDC reference point

Ignition - Establishing a TDC reference point.

25/9/2000

Terminology

TDC	Top Dead Centre
BTDC	Before Top Dead Centre

Establishing an accurate TDC reference point is of paramount importance when developing and monitoring ignition timing (and cam timing come to that). When an experienced engine builder assembles an engine - no matter whether for road or race - he will establish an accurate TDC indicator that can be used for ignition purposes. Usually contrived as a rigid pointer aligning to a corresponding mark on the crank pulley in a position easy to see with the engine installed. Unfortunately, many of us have to deal with engines that haven't had such attention to detail - so either have to make do with the standard ignition timing graduated scale if it's there or sort something else out.

If it's purely a reference point you're after without needing to be desperately accurate, the easiest way is to either use the standard markers, check timing with a strobe once established with whichever pointer you decide on at a specific rpm, and use that same point each time the ignition timing is checked. Or make a pointer, fix it so it's adjacent to the crank pulley, get number one piston rough at TDC position and make a corresponding mark on the crank pulley/damper. This, of course, will mean nothing when applied to another engine, as there was no consistent accurate point established. There are a number of methods to establish this all-important reference where the engine is already completely assembled and no implicit information on the engine is known. Some are more complex than others. Following is the 'easy' method I use that doesn't involve any fancy equipment - just general toolbox stuff.

Jack the car up under the bottom suspension arm on the left side of the car until the tyre is off the ground so it can be rotated easily. Select third gear; remove spark plugs, rocker cover, grill and alternator/dynamo. Rotate wheel/tyre anti-clockwise until you get numbers one and four pistons at TDC. Doesn't matter if it's number one or number four on it's firing stroke - they both reach TDC at the same time! Check the piston is as near TDC as you can get it by sticking something through the spark plug hole to act as a 'rise and fall' indicator. When indicator 'peaks' that's near enough TDC for now.

If there are no pointers/markers at all, you need to fashion one. I generally use a strip of steel plate 3/4"/19mm wide, 1/16"/1.5mm thick, and long enough to reach the edge of the crank pulley/damper from the front plate, plus enough to act as a 'foot' through which a 5/16"/8mm hole can be drilled to mount it to the front plate. Cut the pulley end to a point. On A+ engines or those using the timing chain tensioner, the length will have to be increased again as the pointer blade will have a kink in it to get it around and across the wider timing cover. I usually use the lowest timing cover to engine block bolt as the securing point. When making this pointer for the tensioner cover type set-up, make a template out of cardboard first - saves a lot of anguish and scrapped metal! Alternatively you can use 1/16"/1.5mm welding rod bent to the relevant shape. It's not as stable as the plate one, but is better than nothing at all.

Before finally fitting your pointer in place, clean the edge of the crank pulley thoroughly as you're going to need to mark it. As there's no room for hacksaws, and we're supposed to be doing this without fancy equipment, that means using a white or bright coloured paint of some sort. I use metal marking paint coz I have it. Now fit your pointer.

Next up is establishing the TDC point. To do this we're going to use a 'positive stop' method. This is where an object is used to stop the piston in a required position either side of TDC. I use my 3/8"-drive 'T' handle, but anything reasonably resilient and longer than around five inches that will fit through the spark plug hole will do. Using tape or marker pen, make a reference line 1 1/2" to 1 5/8" from one end. This is the end you're going to stick through the plughole, the reference line indicates 'by how much'.

Rotate wheel/tyre clockwise to drop number one piston away from TDC a ways. Insert your 'positive stop' tool until the reference line is in line with the top of the plug hole, then VERY CAREFULLY rotate wheel/tyre anti-clockwise again until the piston just traps the 'stop' tool. Obviously extreme care and a certain amount of 'feel' are needed here. Any heavy-handedness will cause damage to both piston and plug hole/thread. Using a fine point of some sort (cocktail stick, scribe, very small screwdriver, etc.) dip it in the paint and mark the crank pulley exactly adjacent the pointer. The smaller you make this mark the better. Now rotate wheel/tyre clockwise a little way to release 'stop', and remove it. Then continue clock-wise rotation to turn engine backwards, bringing pistons 1 and 4 back to TDC. Before they get to TDC, insert your 'stop' again to the relevant reference point, and again trap it with the same pressure you used before. Now marl the pulley adjacent to the pointer again. Rotate wheel/tyre anti-clockwise a little to release 'stop' and remove it.

Rotate wheel/tyre until you have the two marks on the crank pulley clearly visible and accessible. Using a rule or tape, measure as accurately as possible the mid-point between these two points. Take your time. Once satisfied, mark this with paint. And there you have your TDC reference point. To get really carried away, make second and third markers below the TDC point at 5/32" (metric doesn't work here!) intervals. This represents 4 and 8 degrees BTDC.