Mystery tool [ tube spanner] in MK IV/ SS tool tray

Sorry Peter, I was quoting word for word from an old USA magazine that just said a C305 was a 7/16 x 9/16 Box Spanner, with no indication whether they meant across-flats, Whitworth of BSF or SAE or anything at all, regardless its wrong. But I do note the C178 Box spanner also supplied to SS Jaguar is in fact 7/16BSF x 9/16BSF so I am guessing the USA magazine has confused the C178 with their advised C305.

For the tool in my Mark V box which I take to be the C.305, here are a few more ideas.

The square end has outer 0.500" AF approximately, there are a few thousandths dimple from the process which made the square cross section. I’ve not identified anywhere to use the outside for turning a female square hole.

Inside span on square end is about 0.314" AF which covers a 5/16" square head pretty well. I have used the inside span on my clutch. It does not fit the adjuster wedges on my later-type Salisbury rear end brakes (and I can’t say about whether it fits earlier brakes).

The C.305 hex end is about 0.580" AF. A 1954 Blue Point Supreme S-1820 9/16" open end wrench has 0.580" clearance between the flats. I’ve not used the hex end of the C.305, I usually reach for 9/16 modern sockets when that is needed. There may be a 9/16" AF nut with up to an inch of exposed thread somewhere, but it does not come to mind.

Roger,
I have checked the tappet locknut size. !/2" AF too tight and a Snap-On
5/16 BS double-hex ring/ open end rather loose. Is it possible that a single
hex box@ 5.80 was specified for this purpose?
Peter B.

Roger McW - (and whoops, I notice you have a Mark V, not a Mark IV as I guessed for my C31/C179 response).
You raise an interesting thought - my immediate reaction was that AF (SAE/ANF) size spanners and hexagon nuts are of no relevance to an SS-Jaguar/Mark IV usage of a Box Spanner - whether C305 or any others, given SS-Jaguar/Mark IV only used BSW/BSF/BA fasteners and nothing SAE/ANF.

BUT I did already notice Peter B’s comment that the rear-axle ‘U’ bolts had 3/8"UNF threads but special nuts with 3/8W/7/16F hexagons! So I started to wonder about the origins of ENV rear axles.
I am aware that the post-war Salisbury rear-axles have USA design origins (and thus potentially SAE threads/fasteners), but not ENV. So was going to look into that. Maybe the C305 Box Spanner has special sizes based on ENV rear axle origins, that may have thought necessary to introduce the special C305 for 1938MY cars on, but specification was changed, thus nothing apparent for C305 in post 1938MY cars???
Still a long shot that an ‘error’ would be perpetuated post war and also Mark V still provided with a C305 with no apparent use.

Here’s a little about ENV. They were associated with Eaton axles of the USA and the designs seem similar so they may have been a licensee or something.
https://www.gracesguide.co.uk/E._N._V._Engineering_Co

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The tappet lock nut s were 5/16"BScycle thread. Essentiall a 5/16"BSF half nut but with 26 TPI instead of 22TPI So normal 5/16"spanners ft.
But they can’t be a tube spanner as one needs to have a screw driver on the end of the tappet screw at the same time as turning the lock nut

I am well aware of all you comment on Ed!!
I reiterate, a 5/16 BS double hex ring/open end is not a good fit on these
lock nuts, as is evidenced by the many I have observed with the corners
of the hex “bruised off”. A single hex ring is preferable of course and to
this end I have “taken out” a 1/2 AF single hex ring for this purpose.
Perhaps you have !
A screwdriver can , OF COURSE , be inserted in the tube to engage the
tappet and the square end of the tube utilized to tighten the locknut.
Needless to say I have a dedicated screwdriver with a tube
“silver soldered” on to locate, without slipping, on the slot of the tappet.
No doubt you have also!
Peter B.

MY guess about the lock nuts bruised corners…
The tappets were case hardened, all over. The cycle thread is very fine and so the threads me through hardened, and so brittle. We have made over a few batches about 800 tappets and screws but only hardened the ball not the thread. But the originals had a tendency to chip off parts on the screw inside the rocker and jam , Hence the need to try excessive force. and damage the hex. After all there is unlikely to be corrosion to make them difficult to adjust.

We just use a ring spanner and so far so good… But more recently i have used MGB rockets [ and tappets and nuts]
. As just a personal preference I wouldn’t think of a tubs spanner as making the job any easier
But we all have our own ways of doing things. I just doubt the factory would have provided a tube spanner in the tool kit for this…

I agree Ed , a tube spanner not my choice either.
Just “grasping at straws” for a explanation for the said box spanner,
sometimes best to be quiet perhaps!!
I have noted your previous mention re MGB rockers etc.
In the past I have obtained enough original rockers etc to complete a set,
but in future will perhaps go the MGB route. Are there any other
contenders Standard/Triumph perhaps?
Rear axle U clamps. As I have stated 4 were obtained off a `37 saloon,
All have 3/8 UNF threads with, not a common size for the dia, nuts that take a 3/8W 7/16 BS spanner. Rob posted a photo with a 5/16W 3/8 BS
spanner applied to the standard size nut for 3/8 BSF.
Perhaps SS used whatever was on the shelf !!.
Peter B.