Mag post screws: correct thread?

[Steve Jelf]

The last time I had the hogshead off my 1915 transmission I found one of the mag post screw holes stripped out. The simple solution was to fill the hole with new aluminum, redrill, and tap. The screws were 10-32, so that's the tap I used. Recently, despite my great care to be gentle, both the other holes stripped out. The one I had redrilled was OK. I intend to fill and redrill the two stripped out holes. But I'm wondering if all three screws stripped out so easily because the ones I used were slightly too small. Is 10-32 correct, or is this another case of Ford using weird fastener sizes? The Colorado list of chassis fastener sizes shows only a question mark here. Anybody know the correct size for sure?

[speedytinc]

I pulled an original. It is 10-32. The actual od is .186". FWIW I measured a new screw. It measures .185"
A 10-32 thread in aluminum in not advisable by todays standards. 10-24 would be used for its better bite.
In aerospace panels, steel thread inserts are specified.
Your 100 year old aluminum threaded holes have been long subject to corrosive forces minimizing an already minimal effective thread.

[John kuehn]

Is using JB Weld a no - no to use on an aluminum transmission cover? I agree that originally Ford should have used a coarser thread instead on a fine thread. Would a self tapping screw work? But of course that wouldn’t be right for a T. Using a 10/24 would be the best way to go.

[speedytinc]

Is using JB Weld a no - no to use on an aluminum transmission cover? I agree that originally Ford should have used a coarser thread instead on a fine thread. Would a self tapping screw work? But of course that wouldn’t be right for a T. Using a 10/24 would be the best way to go.

Self tapping screws dont cut as clean a thread as a good tap. The holes still need to be built up for a new thread. 10-24 would be OK, but if welding the holes up, might as well stay with the original 10-32. Tapping for a 12-24 screw would require modifying the mag post & probably the screw head diameter. Jb weld would possibly work. My second preference to welding the holes would be to use heli-coils.

[Susanne]

I was thinking along the same lines as John - weld the oles closed, redrill them, tap for a helicoil, and use helicoil inserts to give that old aluminum a bit more bite. Not sure if or why that might pose a problem, but considering that aluminum has 100+ years of corrosion, vibration, and fatigue, that poor hogshead needs all the help it can get!

[John kuehn]

I don’t think Steve would have really “gotten down” on the mag post screws going into 100 yr old aluminum but somewhere in the past doing that a few times to many couldn’t have been good. And cross threading a fine thread can’t help at all.

Don’t ask me about the Discount tire store here in town which gets a lot of business cross threaded 2 or 3 of the front wheels studs when they installed a new set of tires of our car.
I found out about it a few months later when the removed the tires and then came and told me they the 3 studs came out of the impact socket on their impact gun. They wouldn’t unscrew easily and they found out why. I know I got off topic a bit but ===============

[Plankhill]

Rotate it 30 degrees and redrill and tap.

[Steve Jelf]

The original 10-32 hole was good enough for several decades, and the one I already fixed with new aluminum is fine, so I expect I will do the same on these other two. I can't rotate the post. It has an oil line.

[speedytinc]

I suspect that rotating the post any will make screwhead access much worse than it already is.