I finally got my valve cover back together. I drilled out the center of the outboard rivets, expecting the rivet remains to come out with the drill bit, but no luck. Instead, I ended up with a 1/8 inch hole through the approximate center of each of the four steel outboard rivets. So I tapped them to M4 x 0.7. The rivet remains still didn't come out, even after drilling and tapping through the steel rivet length, but not through the aluminum at the bottom of the rivet hole (except for one that I didn't catch quickly enough).. They are probably molecularly welded in place (steel to aluminum press fit after 50 years). I used M4 x 0.7 x 12 long Phillips Pan head screws in the outboard holes.
The inboard aluminum rivets, after the inside heads had been cut off with a sharpened screwdriver, were easy to drive out from the inside with a skinny nail set punch, leaving (7) 12 mm long by 4 mm diameter through holes. I shortened the rivets to about 5 mm and set them aside. Then I tapped the holes, from the inside of the cover, using a #10 x 24 English tap (because that's the size of screws that would work in a tapped 4 mm hole. I shortened the screws to 7 mm - 5 mm for the inside end of the holes, 1 mm for the baffle thickness and 1 mm for a #10 lock washer. That totals 5 mm of rivet and 5 mm of screw to go in each 12 mm thru-hole.
I used a little medium grip (blue) thread locker (that's all I had) and put it all together with the screws and lock washers. I even put a little thread locker on the shortened rivets before driving them back into the holes on the outside.
It looks like it's going to work since I didn't tap the holes quite deep enough from the inside for the screws to seat without some pretty heavy torque due to thread interference.
If I were to do it all again and had plenty of time, I would drill out the outboard steel rivets completely and drill through the outside of the cover (the drill will come out in the center of the bottom of the four indentations on the driver's side of the valve cover. Then I would do the same kind of thru-riveting as was done on the rivets on the top center of the valve cover). Long 4 mm aluminum rivets (or something close) are available from Spruce Aircraft Company. It would require an arbor press with some rivet setting tools to do the job right. With all aluminum rivets, one could periodically remove the rivet heads, drive out the rivets, clean the ventilation chamber and rivet it all back together again in probably an hour.
Oh Well, live and learn. Here's what mine finally looked like when I finished.
Tom Kizer