Good evening. This is thread is both a forum intro for me, and hopefully a useful reference for others. I'm working on a '72 Nova project- I plan to completely replace old 307/th350 drivetrain with something better including an LS engine... but for now I'm working on the rear axle. And I've come upon an idea that I THINK could help a lot of people out.

I'm building a Ford Explorer 8.8 for this car. They're an easy choice these days- 3.73 gears, big 31 spline axles, limited slip diff, disc brakes, the width I need, and CHEAP. I'm getting away from the leaf spring suspension anyway, so I saw no reason to keep the old 10 bolt when I can get all these features in a stronger axle for under $200 (well actually free since I sold the old 10 bolt for about the same price I paid for the 8.8).

Before my second career as a mechanical engineer, I was a mechanic for 16 years. I don't claim to know everything- but I do know something about differentials... and I HATE those little cross shaft retaining bolts with the fiery passion of a thousand suns. Those damn things always break- if you've worked on more than a couple differentials, you know what I'm talking about. Now I plan for this car to have say 450-500 hp to start off... maybe more in time if I decide I want that. So I thought: surely there's an upgrade for these sorry little bolts. And I sincerely HOPE that one of you will make me look like an idiot- and show me an upgrade that will keep these stupid little bolts from breaking. But I couldn't find one- no matter how many google searches I did, all I could find were threads posted by people trying to remove a broken bolt... or people saying to replace that bolt twice a week so it doesn't break. So I got to thinking... and I got to looking at this diff carrier:


Notice where the cross pin bolt goes in.




Now take a look at the carrier 180 degrees from that bolt. Notice the similarity? The casting is the same on both sides- it just isn't machined on this side. I don't remember is GM carriers are like this, but it wouldn't surprise me if that was the case:






And so this got me thinking: if one bolt is strong enough (barely)... well then wouldn't two bolts be PLENTY strong?

 

I don't have a mill. Or even a lathe. I plan to one day- I'll consider myself a success in life if I have a CNC bridgeport mill, and a lathe so big it can only be moved with a forklift. But for now I have bigger priorities like student loans and mortgage payments... so all I have is a drill press.

But you can do a lot with a drill press- as I intend to show here. I live in the mecca of mediocrity that is Wichita Kansas- and one of the redeeming qualities of this place is that there's quite a lot of heavy industry. Aircraft to be specific. So there's a really cool store here in town that sells surplus aircraft tools and such. You can buy this stuff on ebay too and it isn't that expensive... but I wouldn't have got this idea had I not been poking around said surplus store:





This piloted end mill (.65 diameter) and a drill bit to fit the center cost me about $5. They'll be a little harder to find on ebay, but still cheap compared with the price of a broken cross pin bolt. The spot-face for the original bolt was more like .72" diameter, but I don't see that it really matters much so long as there's room for the bolt head. The pilot drill was somewhere in the neighborhood of 3/16... point being that it's smaller than the 1/4" hole that you need.

Did some measuring with my $20 verneer calipers, and center punched the spot where I wanted a hole:





I built a precision fixture out of 2X4's, deck screws, and a couple strips of aluminum flat-bar, and clamped it to the table of my Harbor Freight drill press:







And cut- slowly, carefully, with a lot of oil. I measured and got it to roughly the same depth as the spotface for the other bolt- within .020" or so:

 

From this point it's pretty obvious what to do: Drill a 1/4" hole to the same depth as the other side (mark the bit, be slow and careful). Cut in a little counterbore so there's no burr sticking up from the threads (same as the other side of course), and tap the 'top' (in this picture) section of the 1/4" hole to a 5/16x18 thread. Looks like this when those steps are done:






I missed a picture or two on the next step, but it's easy enough to explain: Now you need to drill the cross pin. These things are hard- you need a good bit. Use a quality high-speed bit... a cobalt takes too big of a bite and actually doesn't drill as well (high-chrome and/or stainless steels tend to work harden AS you cut if you cut to fast- and cobalt bits are generally angled to cut FAST). If you get in trouble and your bit won't cut the cross pin, here's a trick I learned when I was a heavy equipment mechanic: If you need to cut something hard- so hard that no ordinary bit will touch it- what you need is a masonry bit. I know... you think I'm crazy. And you're not entirely wrong. But a masonry bit- even a cheap one- will cut through the most ridiculously hard steel I've ever had the pleasure of conquering. It's not the prettiest cut you ever saw- but not as bad as you may think.

To make sure the hole in the cross shaft was drilled exactly where I wanted it, I installed the cross shaft back into the carrier (still attached to my high-tech fixture). As you'll notice, the hole in the cross shaft is a bit bigger than the retaining bolt- it's a little loose. So I wrapped a couple layers of scotch tape around the bolt just to center up the shaft. And drilled right through that new hole I just put in the carrier. The pilot hole drilled easily with my new high-speed bit (same bit I used to pilot the end mill). And I thought at the time- I'll just finish the cut with one of my cobalt bits. My cobalt bit cut for about 1/8", and then just screamed in pain. So I got out the cheap 1/4" masonry bit (as mentioned earlier) and finished the cut. Worked fine. There WILL be burrs when you're done- you need to cut them off.

And while I was still set up at the drill press, I realized that it has always annoyed me that they put BARELY enough clearance around these bolts to remove/install them with a 6-pt wrench. I think... well I kinda have a mill here. I'll fix that! So I added some wrench clearance. Final result here:






And with the pin installed:





When finished, the pin should fit with some clearance. You should be able to move it around a little even with both bolts installed.

This project isn't for the faint of heart- it's easy to cut metal off, but not so easy to put it back. But working slowly and carefully, it only took an afternoon... and it's an upgrade that you just can't BUY. Like lots of projects on internet forums, my nova is a long-term one. So it will be a while before I get to put some power to this and try it out (I'm saving up for an LS2 or the like). But I think it's a worthwhile upgrade- and can prevent a lot of heartache down the road, particularly when you start dumping the clutch and fun things like that.

Good night and good luck.

 

Nice work fella