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The cast iron gear on my 1974 302 is chipped. I got a replacement at the local engine shop it is an MSD gear. I thought I'd just pop the old gear back on but the instructions call for a lot of measuring and actually drilling a new hole in the shaft for the pin. I miked up the old and new gear and they are identical. Any advice appreciated!
The hole in the new gear is not in the correct position. What I do is use the old gear to get the location of the hole and then drill a new hole in the correct position in the new gear. The depth is pretty critical. You don't want the gear down too far on the shaft(forced against the block) or up too high(hanging by the pin above the surface of the block) you want it so that the gear can rest against the block when loaded by the rotation.
I got a replacement at the local engine shop it is an MSD gear.
The hole in an aftermarket gear is normally a strait shot through the gear and shaft centerline. On a small block OEM distributor, the hole is off center to one side of the shaft centerline.
On the depth, you will notice the shaft has in up and down play in the housing. IIRC, In a perfectly set gear depth, the shaft will have .017" of upward play when the housing is pressed down flush with the block. At the same time, with the shaft pressed down firmly against the boss on the block, the housing can be lifted .017". IMO, anything where you have play on the shaft AND the housing is good. You can copy the depth of the original gear and replicate it as a starting spot. The new gear will (should) be plenty tight on the shaft for a pre check and adjust before drilling new hole. Your instructions probably say to drill the new hole perpendicular to the old hole.
TIP: Make sure the shaft is clean inside the housing. They tend to get crudded up in there.
Thanks for the input....the oem hole is definitely off to one side for the roll pin..I wonder what the logic is there.
I downloaded the Ford gear install instructions and checked my distributor against them and endplay is good. I'm going to take the notes and distributor to a machine shop to get drilled my Ford buddy strongly recommends getting a pro to do it.
the oem hole is definitely off to one side for the roll pin..I wonder what the logic is there.
My logic as to why the offset: Not based on any research.
The gear has an odd number of teeth so if the pin were inserted on the centerline, the chances of two gears installed the same would be 50/50. With fifteen teeth, that would be 12° difference. If you look at the base of a properly timed distributor in an engine with a new timing chain, you will notice that the notch on the base lines up precisely with the end of that ridge around the distributor opening.
At the factory, all gears are thus exactly the same. The distributor can be dropped in and indexed as shown above. The engines will all be in time instead of half being 12° out. This would be critical for breaking in the flat tappet cams on initial start up. Just makes sense to me.
If there is a better explanation, I would be interested to know it.
I bet you're right and it is just how their tooling worked at Ford. Before I mentioned a little fixture that I use to copy an old gear. It's just a chunk of aluminum that has a dowel the size of the ID on the gear. I use the Bridgeport mil and just use a straight edge to align it in the vice. I pick up the hole on the old gear and then setup and drill the hole in the same spot in the new gear. A careful guy could do it accurately enough in a drill press pretty easy.
I believe you are right about the offset hole indexing the gear to the shaft. My son who is an engineer immediately thought of this when I asked his opinion about it. I guess the only question I have is that the Ford tech sheet I have for replacing the gears (attached to this post) does not appear to reference this when replacing the gears. It seems the only critical measurement is from the mounting boss to the bottom side of the gear (I assume the machined surface on the bottom of the gear that sits on the block above the oil pump). Does the tooth location on the shaft matter when replacing the gear? Just wondering before I take it to a machine shop this week.
Does the tooth location on the shaft matter when replacing the gear?
I can't see where it would matter at all. It's impossible to install more than 12° different from stock. Most guys have a timing light at the ready when firing up a new engine. The instructions for the aftermarket steel gear I bought made no mention of trying to align the gear with the rotor.
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