Identifyting threads on your mini.
Turbo Mini Forums

Lets start at the beginning!

Thread forms are measured in OD (outside diameter) and Pitch.
On metric threads, the pitch is measured in mm and the measurement refers to the distance from the tip of one thread to the tip of the next along the axial length of the thread. This distance will be between 0.7 and 1.75 for most stuff you'll use on cars!
The 'pitch' of imperial threads is designated in 'TPI' (Teeth Per Inch). Basically, this means how many thread 'peaks' there are within a full one-inch length of thread. This will be between 13 and 28 for almost all the stuff on your mini.

So how do you tell which is which?
What you need to find my friend is a set of imperial thread gauges, a set of metric thread gauges and a digital caliper/micrometer. A thread gauge looks like this:
http://www.machinemart.co.uk/product.asp?p=060720157

You measure the diameter of the thread in mm (imperial micrometers are not something I can teach to read over the net*). Note down the size. Compare the diameter to those on the charts below. Bear in mind, threads will often be slightly under the stated size ? eg M8 will often be 7.8 to 7.9mm when measured across the diameter of the thread.

Let?s do an example: Say you find a bolt, and it measures 7.9mm OD.
As mentioned earlier, most M8 bolts will measure just under 8mm, so in theory this could be 5/16 (UNC or UNF) or 8mm; so you have to do a little investigative work.

Firstly, clean the thread with a wire brush. Now take the metric gauges and try to find the best fitting gauge that neatly sits into the thread form. The correct one will sit EXACTLY into the thread form, with no daylight visible when you hold the bolt and gauge up to the light. Note what the gauge 'number' is.
Now repeat the same with the Imperial gauges, again noting the gauge number.

In doing this, you'll be left with one metric or one imperial gauge that fitted better than the other. The correct thread will be obvious.
Now all you do is compare your findings to the tables below?
For example;
We already know the OD is 7.9mm, but the pitch gauge that matched was 18tpi, hence this thread was 5/18UNC

There are a few basic guides you can use for fasteners on the Mini:
Anything bolting into alloy (eg diff covers, clutch cover) will always be UNC. Because of the relativly ductlie nature of alloy, a coarse thread form is needed for maximum thread contact area.

Almost everything that is a nut/bolt arrangement (eg top arm shaft, ball joint, etc) will be UNF.
With the exception of the MPi minis, the only metric stuff factory fitted was associated with the combined brake servo introduced in '88.
Everything else will be imperial (watch for clever-clogs come and correct me now!)!!!



There you go! Easy Peasy ? but only once you?ve been taught!


* anyone who?s been taught and used an imperial micrometer will know exactly what I mean!

Edited by TurboDave16V on 21st Oct, 2008.

Just to add-

1/4" bolts have a 7/16" hex head
5/16" = 1/2" hex head
3/8" = 9/16" hex head

Not in the case of the 5/16" UNC & UNF bolts used on the externals of the A+ Transmission...
These have a 7/16" head - normally they'd have a 1/2" head!

Edited by TurboDave16V on 21st Mar, 2005.

Yes forgot about those.

Also some A+ timing covers have 5/16"bolts with 1/4" (?) heads I seem to remember.....

Oh don't I know some useless shit.....!!!

Indeed they do - and some 1/4" bolts with 3/8"AF heads!!!

LOL!


Glad this is a 'laid back' forum!

Edited by TurboDave16V on 23rd Mar, 2005.

I must say I have never bothered with this Miniclassic forum.

By the sounds of it I am not missing much though.

Your not any more... once it was good, then again it was the only mini forumn!

alex

AlexF

be careful what you say guys, bonnet polisher might be looking

Well if he is.......

lol, up yours bonnett polisher!!!!

hehe, think i annoyed him with one of my first posts on miniclassic. might have something to do with the fact i called him a cunt though?

he probably deserved it though

http://www.tracytools.com/

Good link for cheap taps and dies.

The best thing about this forum (this was a nuts and bolts thread) is the ability to swear yor fucking bollocks off and it dosen't get edited to rubarbs.

Goo dinfo though Dave, and Tom.

Wil

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

Metro turbo weekend driver,Mini turbo in the making again!

dangerous the thread was called "identifying threads on your mini" and not on the metro engine youve fitted to your mini

lol

no longer a series, but still 1.3 turbo.