They are stamped with a number near each slot which references the total mechanical advance allowed in distributor degrees (2 per shaft). The upper clip can be removed and the rotor turned 180° to engage one or the other advance limit. There are also several different upper rotors with different total advance slots in them. With a little thinkum, you can take one that is stamped "10" and open it up just enough to be a "13" with a rat tail file - or simply sort thorough a pile of 70's junkers till you find the right number you want. Get a set of springs from Mr.Gasket if you want to speed up the curve at the same time. 

The one on the bottom row of the photo is still lying on my work bench - it is stamped "13L" at one slot and "18L" at the other. These are distributor degrees - and are doubled in the engine. The 13L will net 26° at full travel and the 18L will result in 36°. The springs determine when the flyweights will push the rotor to full travel. The factory springs are usually pretty stiff and take lots of rev's to get up to full advance. This is a chart from a late 70's Ford ignition training manual - it shows a typical passenger car advance system. 

The mechanical advance data shows that the full advance won't be in there until the engine is turning fast enough to get the distributor up to 2000 or more rpm - this is well over 4000 rpm at the crank. On a stocker, the initial advance (that you set with a timing light) will be like 3°-5°. Combined - you would get a max advance of about 30°-31° with her wound right out.

To get good performance you want this total number to be 36°-38° and you want it at 2800 to 3000 rpm. You also want some additional initial timing in the engine right away to build low end torque. So you work backwards from the maximum. Put in 10°or 12° intial, and select a slot good for 26° divided by 2 = 13°. Together they will net 36°-38°. For reference, the 13° slot in my sample is .480 inches (15/32). 

If you leave the factory (stiff) springs in there they will take forever to get up to full advance. You want this to come in more quickly so lighter springs are required. Mr. Gasket has a Ford spring set #925D that has something close to what you want.

Hopefully you got more information this time around! Good Luck!   

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Just looking at your pictures - I'd say the upper rotor shaft of the Duraspark distributor will clean up and drop right down into your "new" rebuilt points style unit. Don't even change the fly weights, try it as-is. A lttle homemade prying tool would allow lifting the plastic keepers and moving the weights to the other distributor if you want to keep them together. The '77 Duraspark on my work bench has the 13L and 18L advance slots - but the weights are both marked "S1B" - don't know where the springs went, they were way too stiff anyway. 

Don't worry about machining anything on one of these style of units - just install the Duraspark upper shaft in your rebuilt distributor. If you want to increase one of the advance slots to match the other part  - I would suggest transfering the dimensions and carefully grinding the necessary clearance into the donor part.

You will have to enlarge the exit slot for the wiring pigtail - 1/4" rat tail file - and be certain you have a good ground for the black wire. It is attached to the two screw loops on the plastic wire entrance strain relief. Grounding problems are an old service issue with these Duraspark systems - that black wire is coming all the way back from the control module and Ford was "hoping" that the dizzy had a good ground to the block. Don't just hope - make sure you get a good ground down to the engine block.

While you have it apart - set the two upper rotor shafts on a sheet of paper and draw around the advance slots in the base. Do one on top of the other and you will see the differences in the slot lengths. Just an opinion - I think you will find the 15° slot is too big in the long run. These Y-engines like a good bit of early advance like 10° - 12° is pretty common for the "tuned" cars. If you use the 15° slot and some lighter springs, you'll suddenly find yourself with 40° total and if that's a bit much and she knocks - you'll have to fix it by backing down the initial and that will immediately hurt performance. If you can transfer the length of the 13° slot onto the Duraspark upper shaft (a little file work will do the trick) then you will have a quick choice.

The white wire in the schematic is a "detector" that will cause the system to automatically retard when the solenoid is actuated. So the high initial advance gets out of your way when you want the engine to crank right over.   

You can find these at the yunk-yard for cheap - with a snap-on connector, bracket, rain hat, and the condensor. The connector on this unit had a red #12 positive lead wire and two green #14 negative wires. One of the green wires is for the condensor - the other goes back to the electronic ignition box (where a transistor is used to ground it - just the same as the points in your old set-up). Best one to find is the one from a Ford Probe - it has a nice, easily adaptable bracket. 

The Reluctor pries off your donor with a couple of screwdriver blades and steady pressure.

There is a little clip down in here that has to be opened to release - hold your mouth right while trying this..

The length of the 13L slot is what is critical to getting the 13° of travel - if you get a 10 or something else, you could modify a shorter one.

This diagram uses the Ford distibutor/trigger - a GM HEI type four pin controller - and a low OHM primary coil from a V6 GM of later vintage still. 

The four pin module just screws down on an aluminum plate with some heat transfer paste under it......hide it where you wish, out of the water - and where it can cool a bit.

And the coil is out of an S-10 pick-up (V6) or something of that type. Bring the two connectors home with it. Make sure you have a way to feed the coil lots of AMPs - the diagram shows using a lighting relay to switch the power.

The HEI module will turn on the coil and run all of the AMPs the switching transistor can handle without cooking. The trace above shows just how long the coil might be saturated.

       Many of us use the 10 degree cam or fill the 13 with braze and start with 14 degrees initial. depends on your actual combination, and what you run for gas. They do like a lot of initial advance.

This one is reduced to 13° (26° at the crank) so that an intial static timing of 10°/12° will sum up to 36°/38° at the crank.

The cam was removed to fit a Ford Druaspark II reluctor onto the Autolite distributor.