NOW FOR SALE: I'm happy to make a couple of batches for those who are interested. I'll do it for $25 shipped-send me a DM if interested.

I'm currently rebuilding an engine from an SAI equipped truck to put in my non-SAI truck, no big deal just have to plug the SAI ports on the heads, right? Easier said than done- turns out the holes are a 5/8-20 thread, which is technically a standard thread but is so esoteric you can't find it anywhere, even the big name fastener vendors don't have it. I was able to find a few places selling 5/8-20 nuts (at $13 each) and taps and dies, but no bolts. Time to break out the Machinery's Handbook and get to work! I designed a threaded plug on Fusion360 and manufactured it on a CNC lathe and then finished it up on a CNC Mill to make the 7/16 hex head on the top. I 3D printed some custom soft jaws to the vise wouldn't mar the threads and so I could keep the height consistent in the mill for when I swapped in the next plug to machine. I'll probably put some high temp RTV on the tapered plug portion as the plug is designed to pull that seated face down in to the head. I'm really pleased with how they turned out and had to share, I also recorded video of the manufacturing process if you want to follow the link to YouTube and check that out!



My completed plug.

The plug installed, it fits great!

 

Would you consider making and selling a few batches for your fellow D2 owners?

 

I'm open to a limited production run for folks who want them, I'll have to figure out pricing. The big thing at this point is that they haven't actually been installed on a running engine yet- so while the fit is very good and I have every reason to believe that they will work (I'm putting them on my truck, after all) I can't offer any guarantees!

 

Worse comes to worse, a little JB weld will seal em right up!

 

You've got me curious about the lack of 5/8-20 bolts now... Time to pull out the McMaster Carr Catalog!

 

I couldn’t find them on McMaster. That’s when you know you’re in deep!

 

Here is everything I know about the thread as near as I can figure out: The 5/8-20 is part of the 20 Thread series, it actually took me a while to find it in the Machinery's Handbook because it wasn’t with all of the common sizes, but sure enough there is a full table with 20 Threads per Inch for all different diameters. So the question, of course, is why 20 when there is already an 18 TPI so commonly available. It seems that the reason why 20 thread series exist is for use in adjustment screws, not fasteners, because 100 divides out evenly by 20. This, in no way explains why Rover engineers decided on this thread size, but I know next to nothing about the SIA system or how it goes together because my truck doesn’t have it.

 

No doubt! The closest I could find as well was 5/8-18.

I've been pondering on this. I recall a thread a few years ago where someone was trying to ascertain the exact size and I did take some measurements and uploaded them. In that thread there was a mention of the possibility of a metric size, but the details are very very fuzzy now.

Could it possibly be a metric thread that works out to be the same as 5/8-20?

 

M16x1.5 works out to be very close, but no cigar. I tried that first, it threads in 1 or 2 turns then locks up.

 

This is one of the all time coolest things I’ve seen on this forum. No posts about thinking about doing it and endless speculation from people that have neither the tools or talent to get the job done. This guy just did it and posted his results, well done sir.