I recommend you have an aircraft mechanic help you. There seems to be a lack of understanding overall.
Thanks for this Grant, I do have a brilliant engineer that I can ask for help, but I do like to do my research first before asking him
To respond to your earlier, very comprehensive and helpful message:
I noticed two things when re reading this thread.
I know this might be a hassle if your nose bowl is still on & depending on what cowling/nose bowl you have, but does your airplane have a starter? if so the timing mark should be lined up with the starter after you locate TDC on the #1 cylinder. This will help you immensely when finding your starting point. To find TDC on the #1 Cylinder you need to put the lower plug back in and put your thumb over the top plug hole on the #1 cylinder and turn the prop (Mags off for safety). Turn the prop until the pressure starts to build behind your thumb, you'll feel it.... Now the #1 cylinder is on the compression stroke.
Next, get a flashlight and keep moving it until the piston gets near the top of the stroke. Now go to the starter ring, and on the front you will see a mark on the ring and a small hole on the front of the starter. Line that up with the TDC mark.
Yes the aircraft has a starter, but it doesn't have a timing mark stamped or marked on it. Other starters I've seen do have such a mark. Even if it had a mark, as the propeller covers the starter in the TDC position when the propeller is installed in the conventional position, it wouldn't help in finding the TDC, which is why I'm using the crankcase join lines and the TDC mark stamped on the upper rear of the Ring Gear. I'm finding TDC by rotating to feel compression with the upper plug removed and double confirming that by checking that both rockers on #1 have the .007 - .009 gap they should have when both valves are fully closed. The nose bowl is off.
With the crankshaft set at #1 TDC, I zeroed the digital protractor to 0 and then used this to set the crankshaft at the 25 degrees timing angle and confirmed by checking the upper crankcase halves line up with the 25 degree mark on the rear of the ring gear.
Now, DON'T MOVE THE PROP.
Pull the magnetos out. Index them with the pin. (Make sure you get this right, they can fool you into thinking you have it pinned when you don't. Use the right tool. It is dead soft for a reason.) Next, once they are indexed install them in the accessory case being aware that you will still need to rotate them one way or the other to get them timed. In other words, give yourself room to rotate them. Now, secure them just enough to keep them from rotating, and you can reinstall the plug wire cap if you have not already. (always do this to prevent turning the prop with the pin left in the mag) (been there-done that)
I removed the plug leads, inserted the 2 Slick locking pins upto their first shoulder in the L hole (rotating the mag gears to allow them to go in) and then offered the mags to the engine and loosely secured them with the locking nuts, removed the locking pins and put them back in the slick magneto box as a safety measure BRILLIANT tip on reinstalling the plug wire cap as a safety measure 
The mag gears are now timed to the engine gears, but the timing has not been adjusted.
Next, go ahead and turn the prop backwards quite a bit but less than 180 degrees. and bring it back, holding your thumb over the #1 cylinder. to identify and confirm you are still on the compression stroke of the #1 Cylinder. Once you have determined you are on the compression stroke, keep advancing the prop until the impulse coupling snaps. STOP as soon as it does and back the prop up to the desired position before TDC (Usually 25 degrees) There will be a corresponding mark on the starter ring to line up with the hole in the starter.
I then moved the prop backwards, maybe 180 degrees, maybe a bit less and then inched it to 25 degrees on the digital protractor. Checked the crankcase halves join lined up with the 25 degrees mark on the back of the ring gear (it did)
Tomorrow when I do this again, I'll be making a pop mark on the starter ring and a corresponding pop mark on the starter body IF I CAN, so in future I'll be able to find the 25 degree position with the nose bowl on and from here I knew where the lights should be coming on
Now, DON'T MOVE THE PROP.
Loosen the mags enough to turn them freely, hook up the timing box, turn the mags switch on and rotate the mags until the light turns out. Then snug them down.
Now,to get it perfect, once they are tight, you will want to move the prop backwards and then forward confirming both lights extinguish at the 25 degree mark. If they are off a little then keep making minor adjustments by loosening the mags just enough to "Bump" them with a hammer handle or something similar until they are just right. (Whereby they extinguish together at the 25 degree mark) The way to do this is to get them as close as possible to perfect then "Bump the prop" toward the timing mark and adjusting the mags so the points are opening right at the timing mark. the confirmation of perfection is both light extinguish together at the appropriate mark on the starter ring. When bumping the prop you will never need to move it more than about 10-15 degrees back and forth.
Yes, this describes what I did, although I was aiming to get the lights to
COME ON at 25.0 degrees (and not the 24.9 you seem to suggest?) and I suspect the bit about snapping the impulse and then coming back to the 25 is where I went wrong. I've now ordered and received the Dead Blow hammer
and I'll be using this to tap the mags tomorrow, rather than using my hands to make the small movements.
If any of this does not make sense I would recommend you have someone come teach you how to do it in person. It is much easier to do than it is to describe.
Link to timing marks https://www.lycoming.com/sites/defau...ng%20Marks.pdf
Makes perfect sense Grant, thank you
