(Built for, and tested on, 1989 Aerostar 3.0l; it is assumed that it will work for the 2.3l and 4.0l, but I’d appreciate feedback from folks who try it on those engines.)
The problem: These engines use a modern, full lip seal, which is easily removed…. . . . but not so easily installed. You might try to beat the seal into place, but without a way to spread the insertion force to all of the seal’s outer shell evenly, it’s very easy to permanently damage the seal by deforming the outer shell.
Ford & Chilton’s recommend using a Ford rear main seal installer. The problem is that this tool is fairly expensive. The OTC equivalent is No. 7786, and runs nearly $100 (sourced via NAPA, without discounts). Faced with this, and not wishing to lay out $100 for a (I certainly hope!) one-time-use tool, I visited my hardware store and started brainstorming. As usual, the plumbing section was very useful.
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As it happened, the bolts could have easily been only 35mm long and worked fine, but I didn’t know that when I was at the store. Hence all the flat washers. The 10mm bolts are extra fine pitch, and probably aren’t stocked everywhere — I just happen to have a very good hardware store near me (they’re getting rare).
I also needed a 27/64" drill bit ($7) (see below).
The seal’s OD is close to 4.25"
When I was at the hardware store dreaming this up, I wasn’t certain how much clearance around the seal I’d have. For the 3.0l engine, it looks to be a safe bet that you could use a section of pipe, or a different fitting, rather than a pipe plug, and this might buy you more "throw" — more on this later.
Then I "machined" the flat bar a bit, to reduce the corners:
For the 3.0l, this shaping isn’t really necessary, as there’s plenty of room at the rear of the crank, but the 4.0l might really need it.
This is the resulting "fit":
Now for some layout work:
10mm = .394", roughly. The crank’s bolt pattern is asymmetric, and I’m not exactly using precision equipment for the hole layout, so I want a nice, loose fit for the draw-in bolts. I used a 27/64" bit (.422"), yielding almost .030" slop. That should be enough If your drill index doesn’t include a 27/64" bit, well, 7/16" is only .016" larger, and should be fine.
I began by using the long bolts I purchased (and the flat washers to effectively make the bolts shorter), to pull the seal into place. When the long bolts bottomed out in the crankshaft, I switched to the OEM bolts to draw the seal in further. The "throw" of this tool is just a smidge over 1/2", which was enough for the 3.0l seal installation; I assume that the 4.0l seal, which must be pressed in below flush by about .413", might require some careful "finish work" with a suitable punch.
I found that the tool cocked slightly, but that tapping the non-bolt quadrants a bit (that is, the sides of the cap that do not have bolts) corrected that. I also switched the bolts to different holes, which helped.
The finished result:
A $20 tool that did the job for me.