IH-Prestolite distributor to Ford Dura-Spark II by Thom Singer

[Bill usn-1]

Thanks to Thom for the conversion years ago!!

IH-Prestolite distributor
to Ford Dura-Spark II

By Thom Singer


Please read all the way through this manual before starting on this project. There will be goods and services that you need to confirm the availability of before you start.

The advantages of this conversion is that all of the parts that will ever be likely to need replacing are available at any cheap auto parts store. Additionally, if you want to run an MSD 6 or a Crane HI-6 (I don’t like Mallory, personal preference) with a magnetic trigger, then all you need are the distributor related parts. Ignore the module and wiring, they have their own.

This is not a simple conversion, however, it is well worth the effort. A friend tried the Mallory Unilite and was unhappy with the drive gear shavings in his oil. He sent it back, they ‘fixed’ it, but it never was right. The other problem with Unilite’s is that they are, or least used to be, very sensitive to voltage surges. Extreme care was needed in handling them. A spark from disconnecting the battery killed one that I know of. Plus, if one of them goes bad, or you just want tune-up parts, Kragen/ Checker/ NAPA/ etc. can’t help you.

If you have trouble keeping the parts straight, then make drawings or take Polaroid’s as you disassemble. A note of concern, if you have trouble with this, you are probably in over your head.


You need to start with a points distributor. The Hall-Effect (stock electronic) distributors are not a good candidate for this conversion, although they may have fewer miles on them. So if you need to replace parts, they may be good for cannibalizing.


Parts you will need:

Ford Dura-Spark 6 cylinder application Pick-up Coil

Ford Dura-Spark Pick-up Coil base-plate

Ford Electronic Ignition Module, Blue grommet/strain relief

Ford vacuum advance arm ‘E’ clip (On pin on the pick-up coil)

Ford wiring harness from distributor to module (Should be 4 wires @ module, 3 @ distributor, w/green wire going to the coil “-“)

Ford wiring harness 2 wire plug {Should have a white wire & either 1 or 2 red wire(s)}

Ford 8 cylinder Reluctor (Use 2 screwdrivers, I mean prybars, to carefully remove the reluctor. The way to do this is to put the tips under the web, not the lobes, of the reluctor and push down with a back and forth wiggling motion against the top of the distributor housing. A little penetrating oil may be called for here. You do not need the tiny roll pin that will come out from between the shaft and the reluctor.) You could instead buy the MSD billet reluctor; last retail price I saw was $60.00.


All of these parts, except either the reluctor or the pick-up coil can come from one donor car or truck. I have obtained all of the parts for the conversions I’ve done from a ‘You-pull-it’ type junkyard. Average cost of all of the parts has been around $20-25. You could of course, start out with a new module and pick-up coil. If you elect to go with new parts, don’t bother trying to get the wiring pieces or the pick-up base plate from the dealer. The only way my local dealer would sell me these parts was if I bought the whole engine bay loom or distributor.


1: Remove Distributor from engine.

I find it easier to re-install if before I remove the distributor, I rotate the crank to the TDC mark, firing on #8.


2: Completely disassemble the distrbutor.

After you remove the cap and rotor, remove vacuum advance diaphragm. Then remove the upper and lower points plates. Remove the felt disc in the center of the top of the shaft with a dental pick. Removing the 3 pointed upper shaft retainer is the first real challenge. Use care, it is easy to break one of the 3 legs off. I use a dental pick to carefully pry one of the legs out/up, and one of those little screwdrivers that the Snap-on and Mac dealers give out, to pry another up/out. When you get it loose, be careful, those little dudes can sprout wings and fly. You are wearing safety glasses, aren’t you ? Slide the upper shaft up and off the lower shaft. Notice the 2 plastic bushings that are sometimes missing from the pins on the lower end of the shaft. These fit between the pins and the advance weight slots. Some distributors also have little plastic bushings between the weight and it’s pivot pin. Make a note if you need to replace these. You may be able to get them from one of the IH parts businesses, or you will have to have them also made when you visit the machinist later.

Use a center punch and mark one side of the bottom of the shaft and a point adjacent on the gear. Prestolite distributor drive gear pin holes are notoriously off-center, this will aid alignment during re-assembly. Use the proper size pin punch to remove the distrbutor gear roll pin. The shaft should slide out through the top. Sometimes there is enough varnish build-up on the shaft to make this difficult. The trick here is to take your time. Use plenty of penetrating oil and work the shaft back and forth until it comes out.

3: Clean all parts.

IH parts:

The hard part is getting the inside of the bottom of the distrbutor housing clean. Try a 20 ga. or 12 ga. shot gun bore cleaning brush. Sometimes soaking in a carburator dip cleaner is the best. Just make sure that no plastic bushings go in. I use a stainless bristle brush to clean the vanish and thick crud off the housing and in between the gear teeth. The bore brush works good for the gear’s bore also.

Ford parts:

There is small wire clip that holds the pick-up coil to the base plate, remove it carefully, and put someplace where you can find it later. Clean the grooves in the center collar’s outer surface. Clean the pick-up coil carefully. I use a 50/50 mixture of water and Castrol Super Clean for things like this. Make sure that if you use Super Clean, that you never let it dry on the part. Rinse it thoroughly.

Use hot, soapy water and scrub brush to clean the wiring. This simple trick is something I learned in vintage race car restoration. You’ll be surprised at how supple a formerly stiff loom can get. Notice the Lithium grease, it’s there for water protection. When done you may want to replace it.

The easiest way to clean the reluctor is glass bead it. Notice I said Glass, most engine re-builders use something much coarser for speed. If it’s not bad, run it through the 50/50 mixture and rinse.


4: Inspect the lower shaft and bushings.

Look at the lower shaft just under the advance weight plate, is it worn, scored or is the diameter reduced ? If so, time for another distributor. Put the lower shaft in the housing all the way down. How does it fit ? Will it move side to side ? If you can feel it move, but can’t really see it move, it’s OK. If you can see it move more than a tiny amount, time to find another distributor and start again. How does the lower end of the shaft feel ? It needs to be as good as the top. If either moves, but the shaft is good, find another housing. The need for close fitting bushings goes beyond timing accuracy. If the bushings are sloppy enough, the reluctor will contact the pick-up coil’s core, which is bad and makes this whole conversion a complete waste of time.


5: Installing the Pick-up Coil and base plate.

Slot the housing at the location of the original wire hole. Keep the slot centered on the hole, and make it deep enough for the grommet on the pick-up coil wires to slide in far enough so that the top of the grommet is flush to the top of the housing. With the pick- up coil off of the base plate, install the base plate in the housing. Notice that the plate fits in almost exactly like the stock lower plate. The only problem is that the mounting holes are 180 degrees apart on the housing and more like 160/200 degrees apart on the plate. Temporarily slide the pick-up down onto the base plate. Align the base plate so that one of it’s hole lines up with one of the housing holes. Align the grommet in slot you just created. Check for interference between the 2 screw tabs on the bottom of the grommet and the bumps and holes in the base plate. Play with the base plate alignment until there is no interference and one of the plate holes lines up with a housing hole. Start one of the Prestolite lower plate screws in that hole. Mark the holes in both grommet tabs on the base plate. Remove the pick-up coil. Screw the plate screw down snug.

Start the other base plate screw up from the bottom of the housing. Run the screw in until it contacts the bottom of the plate hard enough to mark it, then remove. Drill a hole the same size as the existing holes in the plate at the mark on the bottom of the plate. Notice that there is clearance underneath the base plate only for one of the two tab holes. Drill and tap for a 6-32 screw at the mark of that grommet tab. A note on the importance of this screw, this is the grounding screw for the module. It does not ground through it’s case. It does not matter if the module is mounted to something electrically conductive or not. Use a screw that barely, if at all, projects below the base plate. Too long of a screw can very easily interfere with the advance weights.


6: Modifying the upper shaft.

Here you will need the aid of a competent machinist. Most, but not all, engine re-builders lack the correct equipment for this operation. The breaker point cam is heat treated. To anneal it, heat it evenly with a torch to a dull or brick red. Let it air cool in still air. When cool you can machine it to size. The material that this part is made out off does not machine well on a lathe. I use a rotary collet holder on a surface grinder, this gets me a good surface finish. Hold the part by the machined rotor surface. This is another reason I use the set up I do, you have to get in behind the flying advance pins to machine the breaker point cam area down to size. See drawing for size details. The reluctor’s inside diameter is .625” The diameter of the upper shaft in the reluctor area has to be 0.625”, +0.000, -0.002” While you’re at the machinist’s, if the advance pin bushings are missing or worn past reusing, have them make some new ones out of Delrin, Celcon, Turkite or similar acetal. They don’t have to have the flat spots, they can be flat sided ‘donuts’. This might make them really thin, if so, have them made .100” thick and file the flats yourself. Their outside diameter or across the flats has to be .005-.010“ smaller than the smallest point of the slots. The inside diameter should just fit on the pin. You can use a dollop of J-B Weld on the bottom of the pins and bushings to retain the bushings if so desired.


7: Modifying the reluctor.

Be careful with the reluctor, it is really soft. If you spent the $60.00 for the MSD part, this probably doesn’t apply to you. A centered hole in the web of the reluctor should be drilled and taped 6-32. Do not put the hole in the two web areas that have the v-groove opposite of them on the inside of the center hole. Use a cup-point stainless steel set screw in this hole. the screw should be stainless steel (i.e. non- or low magnetic signature ) so that it does not give a false signal to the module, unless you want it to run like an 8.5 - 9 cylinder. The screw should not stick out more than one whole thread from the hole.



8: Pre-assembly.

Lubricate the lower shaft and the housing bushings with grease or assembly lube. I prefer the pink engine assembly lube from Isky. Insert the lower shaft into the housing and install the gear (Align by your center punch marks) and roll pin. Temporarily install the upper shaft. Do not install the retainer clip. Install the base plate and pick-up coil. Install the vacuum advance can, and hook it’s arm over the pick-up coil’s pin. Do not install the E-clip, yet. Put a distributor cap on the housing. Mark where the center of a cap tower is on the housing. Remove the cap. Notice that the reluctor lobes are not centered on the web. Install the reluctor to the upper shaft with the long side of the reluctor pointed down. Put a rotor on the upper shaft just far enough for the rotor to index. While aligning the rotor with the mark you made, align the reluctor so that it is centered horizontally and vertically with the center core of the pick-up coil. You may not be able to get the reluctor all the way down and aligned vertically with core without the reluctor hitting something. Leave some clearance, but get it as low as possible. Tighten the set screw hard enough to mark the upper shaft. Remove the reluctor, pick-up coil and base plate, and the upper shaft. With a small file (I use the edge of a mill bastard), file a flat at the location of the mark. Be very careful to not rotate the flat around the upper shaft from the original mark location.


9: Assembly.

Lube the upper end of the lower shaft and the bore of the upper shaft. Lube the advance weight pins or bushings on both the upper and lower shafts. Assemble the advance weights and springs to the lower shaft. Align the advance weight slots and bushed pins and install the upper shaft to the lower shaft and install the retaining clip. Install the pick-up coil and base plate assembly to the housing. Install the wire pick-up coil retainer. A small wire tye may be necessary to keep the wires between the grommet and the pick-up coil from contacting the reluctor. Twist the grommet so that a loop is formed in these wires and install the tye where they cross. Adjust the loop so that full vacuum advance movement is possible with out reluctor contact and cinch the tye down tight. Install the 6-32 grommet hold-down screw. Re-align the reluctor and tighten the set screw with blue Loc-tite on the threads. If you want to overkill it, use red Loc-tite on the threads and between the reluctor and the upper shaft. This will mean that to replace the pick-up coil you will probably destroy the reluctor getting it off. But you’ll have no fear of the phase changing, not that it ever has with blue.


10: Bench testing.

It is fairly simple to bench test the system. You need several jumper wires, a battery, a spark-plug, a coil, and a short spark-plug lead. Connect the distributor housing and spark-plug threads to battery “-”, the coil “+” and the white, or red (try both) wire to battery “+”, and the coil to the spark-plug. Plug the module into the 4-wire and 2-wire plugs and the distributor into the 3-wire plug. When you spin the distributor gear, the spark-plug should spark. It probably sparked when you made the last connection.


11: Wiring.

The wiring is pretty simple, only 3 wires to connect. Mount the module somewhere convenient. Locate away from as much heat as possible. Plug both plugs into the module and the one into the distributor. If you have to extend the wires in the module to distributor loom, do so with secure Western-Union soldered splices and cover with adhesive lined heat-shrink (You can get this from good marine supply places like West Marine, they’re on the ‘net). There are valid reasons for being extra careful here. The signal that the pick-up coil generates is less than, or equal to one (1) volt. Any resistance here is an immediate and major problem.

The green wire from the module goes to the coil negative. Notice that the wires in the two wire plug switch color coding, i.e. the red module wire is connected to the white plug wire and the white module wire is connected to the red plug wire. Don’t ask why, it’s Ford. I will be referring to the plug wires when I write a color. The white wire gets connected to the coil “+” terminal. The red wire gets connected to the “I” terminal on your starter. The original, power wire to the coil should be left alone. The module does not appear to be a power hog like the GM HEI module.

My 302 powered ’67 Ranchero was wired so the module gets it’s power off of the coil “+” and would pull right up to 6500 rpm. I hope that an IH engine isn’t ever going to be put up that high. If you have done a remote starter relay conversion, then connect the red wire to the high amperage ‘start’ side terminal of the relay. Note that sometimes the engine doesn’t stop when the ignition is turned off. Should this happen to you, it is the fault of this last wire connection.


Twist the key; it’s time for a test run!

[StanMcMurrey]

I modified my prestolite dist today, it's untested so I'm not sure if I did any good but everything looks good. The directions above need some pictures, I couldn't follow them I didn't have any problems cutting the upper shaft down to fit the ford reluctor like the directions above mention in number 6.

Pic 1, I chucked the housing in the lathe and lowered the lip that the base plate sits on.

Pic 2, The plate sitting in the new location with the mounting holes drilled and taped for the ford base plate.

Pic 3, The reluctor will be flipped upside down in my setup. I went too deep when I cut the lip in the base and it lines up better with the pickup if it's upside down.

Pic 4, The clip that's a booger to get out.

Pic 5, Cutting the reluctor off the upper shaft, I put the Ford reluctor in the same spot.


Stan.

[Bill usn-1]

Nice work.
I wonder if the ford DS baseplate would drop in the prestolite like it does on the holley distr?

[StanMcMurrey]

It does "drop in" but the reluctor would sit where the rotor needs to be. That's why I machined the dist so the prestolite base plate sits lower (something like .5" lower), then mounted the ford baseplate to it. Take a look at the 3rd picture again, the prestolite baseplate mounts with two tabs(I'm holding one), the screws go in from the outside of the dist not down from the top like the Ford does.

I took the prestolite reluctor off and turned the shaft just a little so the Ford reluctor to fit. The reluctor position didn't change on the upper shaft but the base plate is moved down.

I didn't see an easy way to do it without a lathe and next time I'll look for a goldbox or points dist to start with.

Stan.

[scorp1us]

I'm fixing to get this done tomorrow. I just wanted to co firm that we are disabling the mechanical advance, and ask... If the pick up coil spins on the base plate. How do we find the right position and lock it down? We're not using vac adv, because the ECU is using MAP. So this makes me think we need to drill a bolt through both plates to hold them together... Orbison this what the grommets does? I'm not understanding how that grommets would work.

[Bill usn-1]

Are you doing fuel only or fuel and spark.
If spark control then the conversion is the same as described in the duraspark conversion. No mech or vac adv.

Are you converting the prestolite with duraspark or mopar pickup?
The conversions are totally different. And they all have thier own write ups.

[Eric VanBuren]

To know where to lock the pickup in place you need to first install the reluctor and rotor. Line the rotor up with a plug wire terminal. Then while keeping the dist shaft from turning line up the center of the sensor with one of the "teeth" on the reluctor it doesn't matter which one which ever is more convenient. You are then properly phased so that the spark occurs when the rotor it pointing at a terminal and not between it.

[scorp1us]

Prestolite to duraspark into a GM ign module. For fuel & spark.

Ah, thanks Eric. I guess that makes sense as ever 45 degrees of dist rotation there is a spark. The rotor assigns the cylinder.

I'm 99% sure my engine problems are coming from a bad distributor. So replacing it with new internals and dropping the vac & mech adv will likely fix my problems.

[Eric VanBuren]

Because the computer is advancing the timing from the fixed reference point I should have noted that you want to line up the trailing edge of the rotor with the terminal on the cap. That way as the timing is advanced electronically the rotor is still very close the the terminal on the cap. So for example if you were lining it up with #1 you would want the rotor to be closer the the #8 terminal than to the #2. That way you don't run the risk of the spark going to the #2 terminal as the timing is advanced.

[Bill usn-1]

Or just read the distr conversion FAQ's!
The reluctor and rotor phasing are discussed.