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There never was a TDC mark on the rotor, it is a mark for fully advanced timing.
If you're lucky, there may be a TDC slot on the crank flywheel (like a Trumpet, with a hole behind the cylinders, I don't know that BSA provided such a convenience).

Get off the TDC trip. Why would one need to know TDC, when it is so simple to find it anyway?

In your post #818096

You show the pointer and rotor line aligning allegedly at tickover.

Except I don’t understand this comment:

“While it was strobing at idle, the rotor appeared to be frozen at roughly the point it is now in this photo,
except that only the advance mark (cross) could be seen. I could not see the TDC mark at all.”

So do you mean that the situation looked like this (with the pointer aligning with the rotor line) OR that the pointer aligned with the cross mark? Please be clear.
Please forget any idea of a TDC mark.

Just trying to get to the matter!

The A65 should have a fully advanced ignition at around 2500rpm with the points setup.
This equals about 8mm or 5/16 BTDC = 34 Deg BTDC
The standard centrifugal advance setup will retard the ignition about 12 degs cam or 24 degs Crank.
SO, at idle you should be somewhere around 34 minus the centrifugal system figure (say 20 degs @1000rpm)=14 degs BTDC

very patient this evening Nick?
and a good evening to ya mate!

Thanks, i understand the timing and the advance. Not asking about that.
I must have a problem getting my message across.
I like having the 34 degree advance mark and the TDC mark visible as a reference knowing that at idle the pointer will be a bit counter clockwise from TDC.
I suppose it would be smarter to make a mark at 10 degree BTDC (34 minus 24) where timing should be at idle but as I have been reminded several
times here it is the full advance point that is critical.
As NickL mentioned this is only a start point and many variables such gasoline used, carb setup,compression, altitude, etc., etc. will effect and determine the best points timing. Can I get an "Amen"?

It’s not odd at all. The flash of the strobe is at the point where the spark happens. As the timing advance the sparks relation to the pointer moves. You just adjust the timing until the mark aligns where you intend it to be.

Looked back through this whole thing and looks like you're sure of timing marks being correct and it strobed at your mark. If all these thing are correct, why don't you just ride it and enjoy. If you're not sure of timing marks, use piston stop and degree wheel to verify.

I’m curious about the red mark, it looks like it’s on the original factory mark.

Nope, that's a red marker of mine from when I was using a different reference on the stator.Excellent advice!

Salutations, old chap, hope all is tickety boo.

If you can read Lucas on the rotor is on the right way round.
There are NO TDC marks on the rotor as stock.
The correct alignment is the pointer freezes against the line on the raised pad at full advance , roughly 3, 000 rpms with points.
I know your flywheel is on wrong so the notch is no use.
However so long as the alternator rotor is fitted to its key way relative the crank the line on the raised pad is definitely 34 BTDC, the crank is separate from the flywheel.

If you assumed the line on the raised pad was TDC, then you have been chasing your tail with wildly off ignition timing, if the rotor is correctly fitted and your pointer is in the correct hole ( which it is, LHS is correct)
then your paint marks are completely unnecessary. Do you have a decent manual, ? this is all explained in the book.

???
Do I have a decent manual? Now that's hitting below the belt, Gav.

My problem was COMPRESSION!

My BSA rotor does not have a raised pad like this one on my current Triumph project:

Here's another photo:

Don't know why it upsets people so that i mark off TDC.
Don't give a rat's ass if it's stock.
Gavin, you have been immensely helpful but give me some credit for brains will ya?

OK , do yourself a favour, fit the correct type rotor, sorry if I insulted your intelligence. It should look like the triumph one, the crank key way puts it all right. Best of luck

I can only guess that the rotor is an earlier one than would have originally come with the motor (what’s the purpose of a timing pointer without a timing line on the rotor?)

Did you ever check to see if the rotor is on backwards (as you suggested as a possibility)?

It’s difficult to tell from the angle of the photo, the advance mark is fully 34 deg before the TDC mark?

I agree with KOAN, if the T mark is TDC, the I mark does not look like 34 degrees before. More like 15 after.
How exactly are you establishing these marks.? Also the marks are arsey facey, the I mark should come before the T mark, the rotor rotates anti clockwise looking from the primary side . As shown in the pic the I mark will be after TDC. So far your brain credit is in the red. Wise up.

Nick, I think you over reacted to Gavins comment about the manual, so many will ask a question before reading the manual, I’ve even been guilty of that myself. If you read the manual and go through it properly there is little reason for anyone to ask most questions on the forum.

I could be wrong but I Believe the early rotors are thinner, you might want to check this as it might effect power output.

The above comments are also correct, the Fully Advanced Mark should be right of the TDC mark, as the crank turns in a forward motion it will spark before TDC is reached, as the crank rolls as if it was pushed forward on the ground, the mark right of TDC will reach the top before the other mark will.

If the magnet is good on the other rotor, fit that... otherwise buy a new one, it’ll save you a lot of headaches.

Also, if your timing light has an advance dial on the back, turn it fully left and glue it in position, some (least mine does) has the habit of moving too easily and you find the timing is way off when you have set it. A glue gun is ideal for the job

Don't get me wrong, I'm grateful for the help I get here. That people would take time out of busy lives to help me out of the kindness of their hearts, I can't really fault them if them don't read my every word carefully and remember it (and they don't) , however, I did note that the mark on the left is the TDC. I created confusion by my advance mark looking like a "T".
I concede that the two marks do not look to be 34 degrees apart and this has troubled me. I have a printed degree wheel sized to the rotor and my piston height measurement conflicts. So I F'ed up somewhere.
I'll take Allan's advice and put on the other rotor and see where that timing mark lands me.

Are all rotors supposed to have an advance mark from the factory? I have a Sparx on my Triumph and it doesn't.
Also, don't T120's and A65's have different timing advance degrees?

Service manuals are free to download so everyone has access to them. Some things, like valve timing for example is barely mentioned in the manuals. They give the opening and closing points and suggest setting tappets at .015" for testing only. That's it! I didn't really catch the meaning of that until I asked here and I believe it was Gavin once again who spelled it out.

Anyway, it's all good and my starting problem was COMPRESSION (and timing)!

The marks on the rotors came about the same time they put inspection windows on the primary covers, I think this was c1967. As all rotors with timing marks are made equally The factories had to either index the position on the primary case to suit alignment of the timing mark on the rotor. Another thing worth adding is that the early cranks with 2 sludge trap plugs, do not have the same key way position on the crank as the later cranks, why bsa couldn’t have kept this the same and used a different position for the pointer is anyone’s guess, however they didn’t and if using an early crank this part needs to be made aware of.

allen covered this , so mostly saying the same thing in a different way



a rotor can go on either way , but only one face is usually marked .
early rotors had no mark ... later ones had 1 mark ... and the last ones have 2 marks , 180° apart .
but these are not timing marks
they are .... Centerline of the rotor-keyway marks .

its the crankshaft and its corresponding rotor- keyway
that has ... a degree angle ... to the cranks TDC

the rotor is a generic part that can transfer the ...crankshaft keyway slot .. to a readable line
on the face of the rotor . .. the same rotor will work with various timings of a BSA , Triumph or Norton

The primary cover holds the last part of the timing information .
with no cover ... the rotor mark has no reference .