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MG Midget and Sprite Technical - Gearknob thread

Hey guys, i've looked through the archives and can't get a firm answer on this one.

What is the gear-knob thread on a 72-74 ribcase?, I've tried putting both UNC and UNF re-threading dies on to check the size but no joy. Are these BSW like the windscreen bolts?

Nick

Nick, IIRC they are 5/16" UNF on the ribcase.
Bill Young

Thats what the archives said, however I tried turning a 5/16 Nut, a 5/16 UNC and UNF thread restorer on. None fit. It looks quite a course thread to me, so hence why i'm thinking BSW.
Nick

I wonder if it's been modified by some DPO? It's often done when one breaks off, they just rethread the shaft with something close, perhaps it's even a metric thread?
Bill Young

Nick - I have used a 5/16 UNC half nut to successfully lock an after market gear knob on my '72 midget Mk 3 RWA
John M Platt

Nick. I have seen problems with nuts and bolts of modern manufacture--they are often just a hair off size.

The best way to find out what you actually have is to check the diameter of the part and use a thread pitch gauge to check the number of threads per inch.

Measure the diameter across the threaded portion of the item. We know that 5/16" is a nominal .312" in diameter. We also know that the bolt, stud, or shaft will be turned to very slightly smaller than this so that the nut, or other locking mechanism can be turned on by hand. Thus, look for a .305"-.310" diameter across the threaded portion.

Number of threads is best measured with a thread pitch gauge, a device similar to a feeler's gauge, but having the leaves with a series of V notches cut into them. You try the various leaves into the threaded piece until you find one where the points of the V on the gauge go fully down into the bottom of the V on the threaded piece and the tops of the threads fit fully into the bottom of the V on the gauge leaf. If you do not have a pitch gauge, nor easy access to one, measure the length of the threaded section of the gearshift/bolt/stud. If it is one inch, or longer of threaded section, simply lay a ruler on it and count the number of threads in one inch. If it is shorter, count the number of threads in a one half or one quarter inch section and multiply to see how many threads there are per inch.

Hope this will help. Your local library should have a copy of "Machinery's Handbook" in their reference section. With the diameter of the threaded section and number of threads per inch you should be able to determine what the thread is if it has been re-threaded to something other than the 5/16" UNC that people have found before.

Les
Les Bengtson

Nick,

BSW and UNC have the same TPI in the the size ranges you are likely to be looking. While BSW has 55 degree angle and UNC 60 degree angle this doesn't normally prevent them being interchanged although it is not recommended but for a gear knob not likely a problem. One option to check might be that someone has rethreaded it with BSF which are all different from UNF IIRC. So if finer than UNC and coarser than UNF, BSF is a possibility.
David Billington

thanks for the responses guys.

I've been using a snap-on rethreading kit, with various UNC and UNF sizes, none go on. I also have a spare gear lever assembley and they don't fit on there either. The rethreadign dies are pretty high quality and normally turn right on to even heavily damaged threads.

I'll try the tread pitch gauge i think, thanks for the thorough description if one les!!

Nick

Nick. My website, www.custompistols.com/ has a description of such things under the article "Bolt Sizes". The website also has a contact e-mail address which is much better than the one that I use here to "catch spam". Contact me if I can be of assistance. Now, you have me wondering also.

Les
Les Bengtson

This thread was discussed between 11/02/2008 and 12/02/2008

MG Midget and Sprite Technical index

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