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O/D Isolator Switches


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Just fitting a J type O/D to a non- O/D gearbox. Has anyone drilled and tapped the top cover for the isolator switches? What size are the threads? Seem to think I read somewhere that they are metric but maybe I dreamt it. And how critical is the position? Is drilling the bosses centrally near enough?

Alan.

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Hi Alan,

it may be worth using the search facility to read more about these pesky threads.

I believe they are very close to 16mm - but ever so slightly not quite. Get a switch and measure it.

 

The alignment needs to be very good. The area that the end of the switch works on is quite small and you don;t want to miss it.

They are not central to the cast boss - close but not spot on.

 

Roger.

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I can not remember the tap size but do know I have one( its not clearly marked before you think dumbo :P ).......Stuart will surely help us here.

 

I will loan it out for the cost of postage and a refundable deposit.

 

Iain

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I have both the drill and the tap, unfortunately neither are marked with size/TPI ! I think Pete W knows what the sizes are

Stuart.

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Roger,

Can't believe that Triumph were using metric threads fifty years ago.

Graham

They werent but I cant remember what the size is.

Stuart.

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Had a look at a switch which I bought some years ago for an MGB. the thread looks like it might be 1/2" - 13 UNC.

Graham

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So its 5/8" X 13TPI. Like I said I have the drill and tap here.That would probably explain why they arent marked!

Stuart.

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Thanks everybody for all the info. and offers. I'm going to go the M16x2 route. Looks as close as makes no never mind and I have a 14mm drill already. A cheapo tap is not a lot of money bearing in mind it'll probably only get used one time. Well twice actually, I'll do a test piece first. Better safe than sorry.

Alan.

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Stuart for Reference what size is the drill.....as I say I have the tap!

 

Cheers

 

Iain

17/32" Ian.

Stuart.

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Hi Folks,

this is driving me crazy but I may have an idea.

 

Why would anybody make a thread size on the switch which is not standard. (5/8 x 13tpi) when they could have used 5/8 x 11tpi which is a standard thread.

 

Could it be that they make the switch as 5/8 x 13tpi because it is easy to do once set up on a lathe.

However the hole (female thread) is 16mm x 2mm (this is a standard thread.)

 

This is not good engineering because the threads are everso slightly different 13 tpi V 12.5(ish) tpi.

However as these switches are sitting on a fibre washer they are not locked in a firm position (ie no torque loading or locking washers) could the tpi misalignment make the locking possible.

 

It's only an idea to overcome what appears to be some sort of cock up.

A simple binding thread to make it lock.

 

Roger

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No I suspect it was a left over from a much older previous thread form from one of the gearbox manufacturers of the 20`s or whenever the need for a switch on a box occurred.

Stuart

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No I suspect it was a left over from a much older previous thread form from one of the gearbox manufacturers of the 20`s or whenever the need for a switch on a box occurred.

Stuart

 

spoil sport - it was such a great idea !! B)

 

Roger

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