Winco Steve Posted July 1, 2018 Share Posted July 1, 2018 Good afternoon Gents (and Ladies), A friend of mine has kindly agreed to make me some nice Domed Prop nuts. However, we have come up with a problem. A Thunder Tiger F91S is a 5/16-24 UNF thread. By our tables this is equivalent to an M8 x 1.25 (metric). Having engineered a very nice nut (with a M8x1.25 thread) when I attempted to thread it on to the prop shaft it 'bound' within about 3/4 of a turn. However, it wound on nicely to a 'standard' M8 bolt with no problem. I say 'standard' as it has no markings as to what thread it has. I must say that, on inspection, the bolt thread looked courser than the prop shaft thread. Now, is it possible that the Thunder Tiger instruction sheet is wrong? Please note, I am not an engineer so this has explored and exhausted my engineering knowledge in one foul swoop! He is also making domed nuts for my DLE 20 which is shown, on the spec sheet, as being M8 x 1.00 prop shaft thread. The DLE and Thunder Tiger threads are definitely different. The prop nut from the DLE and visa versa are not interchangeable. I have tried! Any help or advice (not too technical) would be gratefully accepted. Regards to all, Winco Steve Quote Link to comment Share on other sites More sharing options...
Don Fry Posted July 1, 2018 Share Posted July 1, 2018 Equivalence on your tables refer to the load you can put on the thread. They are totally different threads, and are not interchangeable. You need a 5/16-24 UNF tap (5/16 inch shaft, 24 threads per inch, UNified Fine) to cut the tread for the Thunder Tigre. Take care with the metrics as well. A M8 1.25 is metric course, the 1.25 measures the distance apart of the threads . M8 1.00 is metric fine, 1.00 mm between threads. Again not interchangeable. Different threads never interchange. There was once a BA pilot got sucked out when the windscreen fell off. A mix up when an Imperial thread and a metric would appear to run together, but had no engineering strength. How come your mate has a lathe and don't know this. Edited By Don Fry on 01/07/2018 13:04:35 Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted July 1, 2018 Share Posted July 1, 2018 5/16-24 may be similar to M8 x 1.25, but it is not the same. The 24 is threads per inch = 25.4 / 24 = 1.05833mm. This is clearly not 1.25mm. You need a specific 5/16-24 nut to fit properly. Quote Link to comment Share on other sites More sharing options...
thomas oliver 1 Posted July 1, 2018 Share Posted July 1, 2018 A metric 1,25 mm coarse pitch thread is near enough 20 tpi and will not fit at all. A 1.00 mm Metrc Fine thread will be about 25 tpi and is nearer to 5/16 in but 8mm is a good few thousandths bigger than 5/16 anyway. Most Far Eastern engines were made with UNF prop shaft threads to cater for the large American market. I would think the Thunder Tiger shaft is what it says - 5/16 in UNF and the DL|E what it says as 8 mm x 1.00 mm pitch. Quote Link to comment Share on other sites More sharing options...
Winco Steve Posted July 1, 2018 Author Share Posted July 1, 2018 Hi all, Thanks for the information. I think I will just purchase the 'correct ' taps and all should be well. Being somewhat advanced in years I think we just got a bit confused with me performing and providing most of the confusion. I think I set my mate off with some incorrect info so, actually down to me, not him. I presumed (never should you do that, I know) that when it said equivalent , that was just what it was. Obviously not. Similar does not = the same! Thanks again, I think this can now be put to bed! Regards, Winco Steve Edited By Winco Steve on 01/07/2018 13:45:09 Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted July 1, 2018 Share Posted July 1, 2018 Easily done Winco, the charts suggest they do compare But any engineer will support you as thread sizes can be a nightmare Quote Link to comment Share on other sites More sharing options...
Winco Steve Posted July 1, 2018 Author Share Posted July 1, 2018 Hi Denis, Thanks for the post. To the relative layman the tables are misleading. I do repeat...to the layman! I just took it literally and at face value that they were exact equivalent . Now I know different. Best of, Winco Steve Quote Link to comment Share on other sites More sharing options...
Don Fry Posted July 1, 2018 Share Posted July 1, 2018 Just to add to the nightmare, there a taps, and there are taps. To put a tread into a blind hole, which is what most spinner nuts have, you need a bottom tap. Tis a square ended jobbie that cuts to the bottom of the hole. But it not so easy to start a thread with a bottom tap. For that, a proper engineer will use a taper tap, a spikey looking thing. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted July 3, 2018 Share Posted July 3, 2018 Posted by Don Fry on 01/07/2018 17:09:27: Just to add to the nightmare, there a taps, and there are taps. To put a tread into a blind hole, which is what most spinner nuts have, you need a bottom tap. Tis a square ended jobbie that cuts to the bottom of the hole. But it not so easy to start a thread with a bottom tap. For that, a proper engineer will use a taper tap, a spikey looking thing. I think you mean a First cut, second cut and third or plug cut Don Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted July 3, 2018 Share Posted July 3, 2018 Or, as we were taught in the RAF, Taper, second and plug Quote Link to comment Share on other sites More sharing options...
kc Posted July 3, 2018 Share Posted July 3, 2018 One of the big suppliers of taps& dies - Tracy Tools -calls them taper, second and plug. Tracy Tools are one of the cheapest supplier of high quality taps dies see them at Model Engineer exhibitions or buy by post. Quote Link to comment Share on other sites More sharing options...
Don Fry Posted July 3, 2018 Share Posted July 3, 2018 I stand corrected. I'm from the West Midlands, so don't understand these complexities. Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted July 3, 2018 Share Posted July 3, 2018 This thread reminded me of a little story. in 1965 I was at RAF Laarbruch in Germany. One of the club members needed a 3 mm tap and two of us went into town Kevelear . We stopped at the first sutable looking sho but they didn't stock them but they toldus where we could get taps, right across town. They were very nice because they also told us what taps were called Der Gevinnen Schieder fur the Innengervinder Dri millimeter (Sorry if the spelling is wrong) Jim said "I will never remember that!" so I said that I would and I walked all the way across town repeating it to myself. When we got to the new shop I asked for the taps which were duly provided and paid for. The trouble is that, although I know absolutely no German even 53 years later I can NEVER forget how to ask for 3 mm taps in German. Oh and dies would be for Der Ausen Gevnder Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted July 3, 2018 Share Posted July 3, 2018 For a minute there Peter, I thought you was going to say that they'd sent you to the local plumbing centre to buy your taps. Quote Link to comment Share on other sites More sharing options...
Former Member Posted July 4, 2018 Share Posted July 4, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Don Fry Posted July 4, 2018 Share Posted July 4, 2018 I had visions of four candles. Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted July 4, 2018 Share Posted July 4, 2018 Posted by Don Fry on 04/07/2018 07:16:57: I had visions of four candles. And I have a little story about that too. At the RAF Championships a great scale modeller called Jed Mackie had this superb Sopwith Camel with two Vickers guns with their ribbed barrels. Jed told us that he realised that ribbed hose would be perfect for these guns so he went to the biggest iron mongers in Edinburgh and asked for hose. The assistant brought some up but it wasn't quite right so the assistant went down in to the store and brought up another roll, again not perfect. At last he staggered back up with yet another roll and Jed measures it and pronounced it "Perfect" Relieved, the assistand asked Jed how much he wanted. SIX Inches says Jed. He told us that he never dared go back into that shop again. Edited By Peter Miller on 04/07/2018 08:33:06 Quote Link to comment Share on other sites More sharing options...
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