AlanG Posted March 6, 2017 Report Share Posted March 6, 2017 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. Quote Link to post Share on other sites
RogerH Posted March 7, 2017 Report Share Posted March 7, 2017 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. Quote Link to post Share on other sites
AlanG Posted March 8, 2017 Author Report Share Posted March 8, 2017 Thanks Roger. I will have to do some measurements before drilling. Alan. Quote Link to post Share on other sites
iain Posted March 8, 2017 Report Share Posted March 8, 2017 I can not remember the tap size but do know I have one( its not clearly marked before you think dumbo ).......Stuart will surely help us here. I will loan it out for the cost of postage and a refundable deposit. Iain Quote Link to post Share on other sites
stuart Posted March 8, 2017 Report Share Posted March 8, 2017 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. Quote Link to post Share on other sites
RogerH Posted March 8, 2017 Report Share Posted March 8, 2017 WE had quite an in depth thread about this. The thread is almost 16mm but not quite. I'll try and dig my notes out. Roger Quote Link to post Share on other sites
Grahamgl Posted March 8, 2017 Report Share Posted March 8, 2017 Roger, Can't believe that Triumph were using metric threads fifty years ago. Graham Quote Link to post Share on other sites
stuart Posted March 8, 2017 Report Share Posted March 8, 2017 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. Quote Link to post Share on other sites
Grahamgl Posted March 8, 2017 Report Share Posted March 8, 2017 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 Quote Link to post Share on other sites
mike3md Posted March 8, 2017 Report Share Posted March 8, 2017 (edited) Well, look what I found (start at ROGER'S post #98) http://www.tr-register.co.uk/forums/index.php?/topic/54483-the-4a-engine-rebuild/page-5 how we forget so soon..... Edited March 8, 2017 by mike3md Quote Link to post Share on other sites
stuart Posted March 8, 2017 Report Share Posted March 8, 2017 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. Quote Link to post Share on other sites
iain Posted March 8, 2017 Report Share Posted March 8, 2017 Stuart for Reference what size is the drill.....as I say I have the tap! Cheers Iain Quote Link to post Share on other sites
AlanG Posted March 8, 2017 Author Report Share Posted March 8, 2017 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. Quote Link to post Share on other sites
Rob Salisbury Posted March 8, 2017 Report Share Posted March 8, 2017 I can confirm that a M16x2 works fine, I cleaned up the thread of a reverse light switch in a top cover some time ago, the switch is still working and I just now checked the markings on the tap I used. Cheers Rob Quote Link to post Share on other sites
stuart Posted March 9, 2017 Report Share Posted March 9, 2017 Stuart for Reference what size is the drill.....as I say I have the tap! Cheers Iain 17/32" Ian. Stuart. Quote Link to post Share on other sites
RogerH Posted March 9, 2017 Report Share Posted March 9, 2017 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 Quote Link to post Share on other sites
stuart Posted March 9, 2017 Report Share Posted March 9, 2017 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 Quote Link to post Share on other sites
iain Posted March 9, 2017 Report Share Posted March 9, 2017 17/32" Ian. Stuart. Thanks Stuart. Iain Quote Link to post Share on other sites
RogerH Posted March 9, 2017 Report Share Posted March 9, 2017 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 !! Roger Quote Link to post Share on other sites
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