upper ball joint castle nut gone past split pin holes?

Hi all,

I’m just putting the front suspension together, on my MK2.5 sport, after de-rusting it all. Tightened up the castle nuts and found, on the top ball joint, the split pin holes are now way to far from the castle nut. They’re new nuts from MX5parts (they claim it fits both the upper wishbone ball joint and the track rod end) and I can’t find the old ones to compare size but these new ones are 10mm long and would have to be at least 15mm long to reach the holes.

mx5 upper ball joint.

mx5 near side suspension

Has anyone any idea what’s going on here?


You said that the parts have been de-rusted. Have you ground away rusty metal under where the nut sits against the upright?

It also appears that there is no washer under the nuts you’ve fitted. Either or both of the above would make the problem worse.

I think the best way round the problem (if you can’t source the correct parts,which would be better) is to use a machined steel spacer in lieu of a washer, under the nut.

Hmm, it looks like you have the wrong ball-joint.

The boot should be higher up and further from the top of the stub axle, the lip of the boot might even be damaged now. Compare it with the two lower ball joints which look correct.

Does it have the correct size taper to fit the stub axle? Is it too small with the wrong diameters and has it gone in too far?

The parts were sand blasted so I don’t think it took much off, and I don’t think there was a spacer on there originally (it was so long ago I took it off, I can’t remember rightly but it would’ve had to be a thick one) but I guess that would help now.

My main worry is that something is clearly wrong but I don’t know what and it’s on a part of the car which I don’t want failing.

It seems that the early MK2 knuckles and upper joints are a different size to later MK2 models. Perhaps someone has swapped it over with a wrong part sometime in the past?

Yes looks like you have a mismatch of ball joint taper and stub axle top hole size. Some photos below if they are of any help. NA tappers the same on track rod end and upper ball joint so yes the same castle nut fits.

I found a thread here: http://forum.miata.net/vb/showthread.php?t=574237 which says “it is the upper mounting boss on the NBs that is thicker and is the difference requiring a longer ball joint stud. From what I have seen, it amounts to about an 1/8 inch”

the taper certainly seems the same so I’m assuming that I’ve got the right upper control arm but the wrong knuckle - Ugh.

I couldn’t find any actual measurements anywhere but on my stud, the thread is 20mm long, the entire length from tip of thread to top of ball joint is 76mm and the thickness of the upper mounting boss, that the stud goes through, is 18mm.

I guess it’s probably best to replace the knuckle…


 Good work finding that. I would have thought that a spacer between the castle nut and the stub axle face [stack of washers] would get the job done, after all its not going to foul on anything. come to think about it that maybe why some have been fitted with nyloc nuts? see below. Be aware that the nylon lock collar will spin the tapper unless locked by clamping or by  the weight of the car forcing the tapper to bight into its seating. Also if you do convert to Nylocs you mush make sure no old bits of split pin or any burrs around the split pin holes are present. If thats the case the Nyloc collar can be stripped and rendered useless for the application intended.



I found some thick washers to space it with and will go with that until I find cheap replacements. The only thing I’m not sure about is whether it will affect the alignment or handling at all - I’m not racing it so not fussed about tiny amounts.

And also, like RichardFX mentioned, the boot looks a bit squashed so might not last as long - but it’ll do for now.

cheers all!