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Imperial bolt sizes
quadra - 15/10/09 at 09:03 AM

Sorry for my imperial ignorance, but how do you determine the size of an imperial bolt.

I am trying to replace a bolt on the steering rack of my mini, which requires a 1/2 inch spanner to remove. It is about 25mm in length.
When I look online for replacements they appear to be listed as No. 4 x 1/2 inch etc.

Can anyone give me some guidance please.

BenB - 15/10/09 at 09:52 AM

The 1/2" spanner is the size of the head of the bolt whereas you order them on the width of the shaft.

I've only ever heard of No sizes being used for wood screws....

If it's 25mm in length it'll be a 1 inch bolt.

Try measuring the length of the bolt (not including the head bit).

MikeRJ - 15/10/09 at 10:06 AM

Just like metric bolts the size is determined by the diameter of the shank. Imperial bolts used on cars generally use two different thread pitches, UNF (fine) and UNC (Coarse).

An imperial bolt with a 1/2" head will most likely be a 5/16" bolt.

[Edited on 15/10/09 by MikeRJ]

britishtrident - 15/10/09 at 10:08 AM

If its a Mini it won't be an Imperial thread it will be Unified thread

5/16" x1" UNF set screw
ie 5/16" dia UNF thread 1" long and threaded full length.

If it is the pinch bolt for the steering column if you use a bolt that isn't fully threaded it runs into danger of running out of thread before it clamps properly --- common MOT failure.

[Edited on 15/10/09 by britishtrident]

britishtrident - 15/10/09 at 10:10 AM

Having said that unless it is going in a threaded hole I would use an M8 set screw and nut.


[Edited on 15/10/09 by britishtrident]

britishtrident - 15/10/09 at 10:17 AM

ebay items
380161081936

130088675011

[Edited on 15/10/09 by britishtrident]

quadra - 15/10/09 at 11:08 AM

Ok thanks for the info and links. I need two 5/16 x 1 countersunk bolts for the steering rack gearbox housing. This is to allow a little more room for the slightly oversized engine I am trying to squeeze in.

Nick Skidmore - 15/10/09 at 01:30 PM

trident racing supplies at silverstone will have them

britishtrident - 15/10/09 at 02:06 PM

quote:
Originally posted by quadra
Ok thanks for the info and links. I need two 5/16 x 1 countersunk bolts for the steering rack gearbox housing. This is to allow a little more room for the slightly oversized engine I am trying to squeeze in.



If they are screwed into aluminium alloy they are likely to be 5/16" UNC thread not UNC

rusty nuts - 15/10/09 at 06:15 PM

Invest in a set of thread gauges and a Zuess? book. Will last a lifetime and you will never need to ask what a thread is again .

quadra - 16/10/09 at 07:18 PM

Yeah a Zeus book would be good, I have some thread gauges but they are only metric. All sorted now thanks.