Dumb question about a rotor

I'm in the middle of this rebuild on a 71 Commando, when I pulled the original rotor out it had a bunch of different lines on it, and they each had an A or a B on them, and on the rear of the rotor it had Lucas with the number and the two raised markers, I bought a new rotor and it has nothing on the one side where the A and the b and the lines are on my original one , but it does have the same Lucas marking with the two raised lines on what I assumed was the rear, but every video that I've seen the Lucas with the two lines goes facing out and I'm guessing thats the timing mark, just wondered why it was facing the rear on my original rotor ???

Scottyv said:

I'm in the middle of this rebuild on a 71 Commando, when I pulled the original rotor out it had a bunch of different lines on it, and they each had an A or a B on them, and on the rear of the rotor it had Lucas with the number and the two raised markers, I bought a new rotor and it has nothing on the one side where the A and the b and the lines are on my original one , but it does have the same Lucas marking with the two raised lines on what I assumed was the rear, but every video that I've seen the Lucas with the two lines goes facing out and I'm guessing thats the timing mark, just wondered why it was facing the rear on my original rotor ???

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That's what the timing marks for the Trident/Rocket III look like, maybe someone used a rotor off one of those bikes? Alternatively, is the same rotor is used on all the bikes, with just a different face presented for different models?

Yes, that end (with the 'A' and 'B' markings) is for timing BSA and Triumph triples, often found on the other end of later Lucas rotors. Those marks are not correct for a Commando so use the end with the two raised pads/lines but note that only one is the correct timing mark.

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That's too funny, whatever shop my uncle bought the bike to originally 20 years ago when he had it bored out and worked on must have put that thing in there, and I wonder how they timed it because the A and the B side were facing out, and apparently as you say the marks are wrong for a Norton, bizarre ???

Scottyv said:

I wonder how they timed it because the A and the B side were facing out, and apparently as you say the marks are wrong for a Norton, bizarre ???

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With the rotor fitted back to front, the 'B' mark adjacent to the key slot is only a few degrees out, so it's possibly what they used.
Edit: Thinking about it, I'm not sure that is correct for a Commando, but it is for (some) Triumph twins.

For truly accurate timing, the marks might not be correct/needed. You use a degree wheel to find the timing position and mark it yourself, or you time using the degree wheel - a PITA but accurate.

I recently rebuilt a 1969 BSA B25 and the timing mark was off by 5 degrees - made the ignition too advanced. The rotor and primary covers were both the correct parts.

I have a 67 Triumph TR6C whose rotor keyway in the crankshaft is wallowed out. To resolve this, I used an energy transfer rotor and front primary sprocket - has a pin in the sprocket that fits into the rotor in one of three places. Using a degree wheel, I made my own mark at the fully advanced location.

I've already timed the motor with a degree wheel, I was just wondering about the markings, thanks