Ok, so some of you may have noticed from the recent "eBay" thread I picked up some cheap wilwood 4 pots. They are old style dynalite, 120-1056, with 35mm pistons and set for 0.81" discs. Fortunately they have the external cross over tube so I can respace easily.

Anyway, I have spent today stripping them and cleaning them, in the process I had to drill out two of the bolts that connect the two halves together as the cap heads were corroded and they seemed to be superglued in rather than threadlocked. Anyway, eventually get the bolts out without knackering the threads, bonus.

So, quick question, what thread are these bolts, from a quick measure it looks like they are 5/16" UNC, I measured them a 7.9mm diameter and a thread pitch of 1.44mm. Can anyone confirm this? Also what grade bolts should I use, high strength I assume?

Will now sand down and paint and then reassemble with new seals, pistons, nipples and bolts, the dodgy welded repair seems to be ok with no visible voids so will see how this goes.

Thanks.

Rally design are the font of all knowledge on rebuild bits.

I can't help but ask would it not be worth investing in a new pair of calipers considering all the bits you need to replace anyway? With the state of the ones in your pics I don't think I'd ever risk using them.

Can't tell you the exact thread, but it will definitely be imperial not metric, all US stuff is invariably imperial, & most Willwood threads I've come across are either 3/8 x 24 or 7/16 x 20 or 24

I'd be a little surprised if they are UNC, as usually for anything like btrake stuff requiring high strength & clamping they go for a fine thread, but it's certainly not impossible

I'd say carefully try a 5/16 UNC bolt down them

Worth investing in a decent set of thread gauges and a Zuess booklet. Imperial threads are normally measured in " teeth per inch " your measurement of the bolt diameter is close to 5/16" so measure the TPI and consult a Zuess chart.

Thanks for the replies so far, with a pitch of 1.4mm gives a tpi of 18 if that help.

Also whilst I am on I have a set of pads that came with them, these are OEM wilwood ones size 7112 but no idea of compound, other markings are -

OM015
150-8850
114857-MX

Any clues?

[Edited on 19/2/17 by Ugg10]

5/16 UNC is 18 TPI UNF is 24. So looks like you were right with your first instinct. As far as the grade they don't need to be too strong as they are only screwing into an Aluminium alloy perhaps stainless would help but there will always be some corrosion taking place.