Subject: 850 Mk3 – Ignition Timing degree Scale recociliation.

1. True TDC Verification: I used the positive piston stop method (rotating the engine forward anti-clockwise, then reversing clockwise to the stop) to split the difference and establish absolute True 0° TDC at an arbitrary point on the rotor.
2. Rotor Dimensions & Math: The total circumference of my Lucas alternator rotor measures 232 mm around the outside edge. Based on this, 1 degree equals 0.644 mm.
3. Advance Mark Layout: To set up the 31° BTDC strobe timing required for the Boyer Mark IV, I measured exactly 20 mm anti-clockwise (forward in the direction of natural engine rotation) from my calculated True TDC mark and scribed a fine white line.
4. The Discrepancy: My degree scale plate inside the inspection window runs from 16° to 44° (reading low to high, left to right). With the crankshaft physically held perfectly at my verified 20 mm (31° BTDC) position, the factory horizontal engraved line on the rotor points to 18° on the scale.

McBrewster said:

Hello Houston we have a problem,
I'm setting up electronic ignition on a Mk3 and have run into an alignment discrepancy with my primary cover timing scale.
I meticulously measured and have verified the following details:

1. True TDC Verification: I used the positive piston stop method (rotating the engine forward anti-clockwise, then reversing clockwise to the stop) to split the difference and establish absolute True 0° TDC at an arbitrary point on the rotor.
2. Rotor Dimensions & Math: The total circumference of my Lucas alternator rotor measures 232 mm around the outside edge. Based on this, 1 degree equals 0.644 mm.
3. Advance Mark Layout: To set up the 31° BTDC strobe timing required for the Boyer Mark IV, I measured exactly 20 mm anti-clockwise (forward in the direction of natural engine rotation) from my calculated True TDC mark and scribed a fine white line.
4. The Discrepancy: My degree scale plate inside the inspection window runs from 16° to 44° (reading low to high, left to right). With the crankshaft physically held perfectly at my verified 20 mm (31° BTDC) position, the factory horizontal engraved line on the rotor points to 18° on the scale.

When I attempt to adjust the loose degree scale plate on its sliding axis to align the printed 28°/31° marks with the rotor line, I physically run out of room. The plate is hit against the absolute limit of its adjustment slots and cannot physically slide any closer than that 18° reading.
When I rotate the engine anti-clockwise towards True TDC from this position, the factory rotor mark moves further to the left and completely out of the viewing window.
My tape calculations, total circumference, and directional logic are verified, but the physical components are refusing to gel with my own. It is mismatching by about 10 to 13 degrees.
Does anyone have a clear explanation for why it is mismatching by so many degrees? Is this type of error commonplace enough for me not to worry about it, or is there an underlying issue here I need to resolve?
Any insights from the group would be greatly appreciated.
Thanks in advance by many degrees.
McBrewster
NZ

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Click to expand...

McBrewster said:

Hello Houston we have a problem,
I'm setting up electronic ignition on a Mk3 and have run into an alignment discrepancy with my primary cover timing scale.
I meticulously measured and have verified the following details:

1. True TDC Verification: I used the positive piston stop method (rotating the engine forward anti-clockwise, then reversing clockwise to the stop) to split the difference and establish absolute True 0° TDC at an arbitrary point on the rotor.
2. Rotor Dimensions & Math: The total circumference of my Lucas alternator rotor measures 232 mm around the outside edge. Based on this, 1 degree equals 0.644 mm.
3. Advance Mark Layout: To set up the 31° BTDC strobe timing required for the Boyer Mark IV, I measured exactly 20 mm anti-clockwise (forward in the direction of natural engine rotation) from my calculated True TDC mark and scribed a fine white line.
4. The Discrepancy: My degree scale plate inside the inspection window runs from 16° to 44° (reading low to high, left to right). With the crankshaft physically held perfectly at my verified 20 mm (31° BTDC) position, the factory horizontal engraved line on the rotor points to 18° on the scale.

When I attempt to adjust the loose degree scale plate on its sliding axis to align the printed 28°/31° marks with the rotor line, I physically run out of room. The plate is hit against the absolute limit of its adjustment slots and cannot physically slide any closer than that 18° reading.
When I rotate the engine anti-clockwise towards True TDC from this position, the factory rotor mark moves further to the left and completely out of the viewing window.
My tape calculations, total circumference, and directional logic are verified, but the physical components are refusing to gel with my own. It is mismatching by about 10 to 13 degrees.
Does anyone have a clear explanation for why it is mismatching by so many degrees? Is this type of error commonplace enough for me not to worry about it, or is there an underlying issue here I need to resolve?
Any insights from the group would be greatly appreciated.
Thanks in advance by many degrees.
McBrewster
NZ

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Click to expand...