406 brake disc bolts

[David C]

I just got some 406 hubs from Graeme Payne to complete the parts for a disc conversion. I am missing the bolts to attach them to the hub. They seem to be a really odd size. By my measurements M11 x 0.75.
I found a bolt in my parts bin that seems a good fit. The shank is approximately 11mm diameter, the thread 10.8mm and roughly 20 TPI. Nothing imperial seems to come close.
Can anyone help with some bolts or point me in the right direction to find some? I also need 4 bolts to attach the brake callipers but I hope they are a standard size.

[peterg]

As the disc brakes were aircraft derived they may be an American thread.

[Geoff Dowdle]

David ,
I have a selection of New Old Stock 6 cylinder parts from the two Australian Bristol agents and the Arnolt Corporation , plus a variety of used parts .

Included in these are New Old Stock

406-1-23047 Front Caliper fixing bolt. 4 per car. High tensile with head drilled for lock wire.
1/2 inch x 20tpi x 1 1/16 long


406-1-23008 Front disc attachment bolt. 10 per car. High tensile
7/16 x 20tpi x 1 1/2 long

Contact me if they are of interest
Geoff

[innellan House]

The bolt in your parts bin is American 7/16 UNF - a common size.

0.75mm pitch (approx 33.8tpi) is a fine thread more suited to small holes in soft alloy. A manufactured bolt in 33.8 tpi simply does not exist.

BA-3 which is approx 34.8 tpi and 4.10mm in diameter is the nearest imperial equivalent to 0.75mm pitch.

vc

[David C]

I had a senior moment and worked out metric pitch by dividing 20 TPI by 25.6 rather than vice versa. Silly me!

The bolt I found that fitted came out of my metric bolts pot but could easily have strayed. I will check the head with both AF and metric spanners. Not conclusive, I know, but interesting to know.

7/16" works out as 11.2" The bolt that seemed a good fit was 10.8" The thread in the hubs are a bit grubby but it didn't feel as though it was under size.

I will have a gentle go with a 7/16UNF tap to see if I can clean the thread without removing new material.

Thank you, Geoff. Very kind of you and I will send you an email.

[JoeF]

You'll find UNF fittings on the Bristol disc brakes. I would expect those bolts to be 7/16 20 tpi UNF. The smaller size, such as on the flexible hoses are 3/8 UNF 24 tpi.
They were Dunlop brakes, so the UNF thread would have come from Dunlop's use of it.
Those bolts that Geoff mentioned match the part numbers in the 406 spares book. They are probably similar to what was used on Jaguars such as the XK150 around the same time .

[David C]

I'm happy to report that Geoff is helping with these. Several mistakes my end. A UNF bolt had strayed into a pot of miscellaneous metric and the 7/16" taps in my UNF tray turned out to be 24TPI UNS. Better glasses needed...