MTFCA Forum

Have a set of Common Sense Split Radius Rods. Need to do a bit of thread repair on the long radius rods. Problem is that while the original threads appear to be 7/8" 9tpi replacement nuts don't fit. The new nut & original nuts fit on a new 7/8" - 9 bolt but only the original on the radius rod. The new nut is BSW. I also have a 7/8" - 9 NC die that doesn't fit the radius rod. Posted measurements. That's all I can get the new nut on the radius rod. Any ideas about a difference that is causing the fit issue. Shouldn't a 7/8" bolt have a .875" od on the threads?

I am not an expert on thread sizes but have an old British car that uses BSW fasteners. One thing I have learned is that BSW threads, even though they appear to match up to some Standard threads, the pitch on the threads are usually different causing a BSW nut to lock up on a Standard bolt after a few turns.

Shouldn't a 7/8" bolt have a .875" od on the threads?

Not necessarily...ANSI standard for 7/8-9 is Max diameter .875 and Min diameter .861

BSW threads are 55 degrees and UNC are 60 degrees

They sometimes are considered interchangeable but they are not

There is very likely a small imperfection in the new nut which a tap will clear out

put a machinist's thread pitch gauge on the radius rod and you'll nearly certainly find the pitch is 60 degrees (UNC)

I'm going to bet that the die you're trying to use is a fixed-size hexagonal thread chasing die. Those things are sometimes worse than junk and cannot adapt to the multiples of variation of what a 7/8-9 thread could actually be, and your radius rod is at maximum condition. As such, a thread chasing die will likely not fit (as you've found) and if MADE to fit would recut the threads to a much sloppier fit. A proper die is here: https://www.zoro.com/drill-america-78-9 ... 26EALw_wcB

as I mentioned above, run a tap through the nut and be done with it

now, for serviceability, blowing up the picture, it looks like the original part has seriously eroded threads as well as stretched threads. If that is the case, then some serious soul-searching needs to occur before the radius rod is placed into service (if it's placed into service at all). It would have to present a WHOLE lot better in real life than these pictures for me to use it as it is.

Thanks for all the replies. Scott, you wouldn't be happy if you saw the threads. Been debating on having new ones made and possible with a stronger steel, you got me 99% on the way. I have both a threading die and a chasing die. Looks like I;ll be headed to the local machine shop to see what they can do

If you radius rod has coarse threads like Whitworth it has likely been re-threaded as a patch-up repair for damaged original UNF threads.

The 55* and 60* thread angles Scott mention have little practical effect on interchangeabiloty between the two, EXCEPT in the 1/2" where there is a 1 thread per inch difference. Mixing Witworth nuts and bolts will cause confusion on a T because Whitworth bolts with the same diameter as their UNC equivalents will have larger heads and nuts.

Allan from down under.