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  1. #1

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    os 15 III RC prop nut ... 5.5x.8 ???

    I was given an old OS 15-III RC engine. Appears unused, but several parts missing. Well got everything together, but when it came time to bolt the prop on.... "What the H*** is this?" Near as I can measure, appears to be a real odd ball. Any such thing as a 5.5 x .8 metric; or, a 7/32 x 32 ? can't even locate a tap so I can make a nut. Any suggestions? ..... George K.

  2. #2

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    7/32x32 should do it-depending on the age of the engine, as they changed the thread during the long production of the Max-15 from 7/32-32 for the Max-III 15 to 12-32 UNF for the later drop in liner model Max-15. Either nut will work on either version, but may be a little loose or tight if the 'wrong' nut is chosen. Both taps are available, but the 12-32 one is a little harder to find.

    ChrisM
    'ffkiwi'

  3. #3

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    This company sells quite a few oddball taps & dies at reasonable prices
    http://www.midwayusa.com/find?sortby...mensionid=1168
    12-32 plug tap:
    http://www.midwayusa.com/product/911...p-12-32-thread

    bottoming tap:
    http://www.midwayusa.com/product/631...p-12-32-thread

    I have ordered from them in them in the past, no problems.

    Dave

  4. #4

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    It's been a long time since I had one of those but I think that it used the old Japanese metric thread about a 5X.6 mm.

    Dennis

  5. #5

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Thanks for the input guys. The measurements of the different screws are very close. I'll probably go with the 12x32 since the cost of the tap is only 'bout 10% of the 7/32 unit. This "free" engine is "nickle & dimeing" me some, but because of its low weight, I'm anxious to see what its' output is compared to the Enya .09. Weight of the two is just 'bout equal. ..... George K.

  6. #6
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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Ya, I had to splurge on the #12-32 tap too,    $12. but they had it in stock. One motor.  I went for the tap for the Turbo plugs too.

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    That old OS15 will eat the Enya 09 for lunch. They were suprisingly powerful for their day and are still a nice engine to have and run today. Only the scarcity of quality runners has removed it from many modellers inventory. OS won in the 57? FAI world FF event with one of those engines and contrary to opinion FF flyers use the hottest engines available, and did so even then.

  8. #8

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Actually it was '56-and it was the UK's Ron Draper who won the FAI Power World Champs with the Max-1 15 (itself an unusual occurrence as 2.5cc diesels were still dominant in FF power at that stage) and the publicity surrounding this win gave OS their first big commercial break on to the world market-in much the same way as the Webra 61 winning the (1968?) World R/C Aerobatics Championship did for Webra a decade or so later.
    The early OS boxes from 56 on actually made mention of the win on the label. The Max-1 was rapidly superseded by the Max-II, and then the archetype Max-III 15-which must have been one of their best sellers since it remained in production for about 15 years (1961- 1975)

    Pic below shows the four OS 'Max-15' models from Max-1 to the last plain Max with drop in liner.

    ChrisM
    'ffkiwi'
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  9. #9

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Chris & Dennis; Interesting info guys. Now, if only I can get the prop hooked up to the engine. No luck at the local "hardware" store for a nut or a tap. Heading to the city next week. Should find something there. ............. George K.

  10. #10
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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    I just made one with your name on it.  3/4" hex with a radius on the front like an acorn nut drilled through.  I made up a bunch of 1/4" 28 for myself.  I don't like spinners when I am in a hurry, and nuts screw up with the starter, so I make them for all my motors.  If you email your address I can send it.  My old OS motor is like the one on the far right picture.  It is pretty fast compared to most .15's, even the schneurle's.

  11. #11

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    I had to invest in some 7/32X32 taps a couple of Nationals ago when it became clear my Dixielander needed more noseweight to trim out, and the best option was to machine up some heavy brass spinner nuts-which I duly did-varying the length to achieve a range of weights (from 40-odd to 60-odd grams). Luckily the OS Max-15 and the OS Max-19 share the same shaft thread, so they are usable on either.
    Sadly George Fuller, the Dixielander designer, left us on Dec 30, and we've lost several big names of late in the FF fraternity-on both sides of the Atlantic......

    As I indicated earlier, it is generally understood that OS changed from a 7/32-32 thread to a 12-32 UNEF thread when they introduced the last Max-15 model, the 'Max' 15. As also pointed out earlier, the two thread forms are so similar (only a couple of thousanths difference) that either nut will fit on either shaft thread-but due to tolerancs etc (and possibly thread angles) you may get a good, loose or tight fit of the nut on the shaft.

    Most of the last model of Max-15 I've encountered over the years have been fairly well thrashed in R/C models by the time they passed into my hands, being nice, easy handling and with no bad habits-but run on pressure, on a large bore venturi (or even no venturi insert) -with a decent amount of nitro they really put out.

    I'd agree that they's leave an Enya 09 in the dust (and an Enya 15 for that matter)-but the Enyas will far outlast them for longevity. Somewhat surprisingly, the last model Fuji 15 will beat the OS Max 15 quite handsomely on smaller props-but despite being cheap and plain bearing it does use a sort of schneurle porting system-unlike the Max which remained crossflow with a baffled piston right till the end. Mind you the Fuji 15-IVS didn't appear till 1976-by which time OS had only just (1975) introduced the last version of the Max-15. In any case the Fuji never really caught on, despite its much lower price-which explains its relative scarcity these days-35+ years on.......

    ChrisM
    'ffkiwi'

  12. #12
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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Isn't the 7/32" the same as the #12?I have the spec somewhereon the #12, but it isn'thandy for the 7/32 thread. It is sort of like the 1/8" coarse being a #5, like on a stovebolt or a Baby Bee. Mtntopgeo, do you need the spinner nut I made?

  13. #13

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Aspeed-more or less! This topic cropped up on the Barton forum last year, and was thoroughly thrashed out. Apparently there is only a couple of thousanths of an inch difference between the 7/32-32 thread and the 12-32 UNEF thread major diameters-and the pitch is the same-so in theory nuts from one should screw on the other..........however I'm not sure that the thread angle is the same (it will almost certainly be 55 or 60 degrees)-if it is different then that explains why-depending on which way round you have it-a cross-matched nut will be either loose or tight-in other words a nut with a 60 degree thread angle will fit on a shaft with a 55 degree thread angle easily-perhaps slightly loose, whereas if you try it the other way round there will be a lot more friction from a 60-degree male shaft thread trying to fit into a female 55 degree nut thread.
    I'm not an engineer-so I'm not 100% certain of what the exact angles involved are in this particular example, but 55 degree and 60 degree thread angles are common [though the BA series are 47.5 degrees]-the unified thread series uses a 60 degree thread angle, so the 12-32 thread will be 60 degrees-I'm not sure however what thread series the 7/32-32 belongs to-BSW, BSF, BSP and the ME series all use threads of the Whitworth form, which is 55 degrees.

    ChrisM
    'ffkiwi'

  14. #14

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    RE: os 15 III RC prop nut ... 5.5x.8 ???


    ORIGINAL: aspeed

    Isn't the 7/32'' the same as the #12?Β*I have the spec somewhereΒ*on the #12, but it isn'tΒ*handy for the 7/32 thread. Β*It is sort of like the 1/8'' coarse being a #5, like on a stovebolt or a Baby Bee.Β* Mtntopgeo, do you need the spinner nut I made?


    PM sent. ............ George K.

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Well, I received the 12x32 prop nut from "aspeed". Thanks Alan. Ten min. after I got the package open, I was ready to the start the engine (mounted on test stand). Prop was a generation 2, MAS 7x4. Fuel was 15% nitro & 20% castor. Soon as the engine fired up, even before leaning out & using the tach, I knew that this thing was a rather serious runner. Tach read 19,200, ... steady!!! As "ffkiwi" & "dennis" stated, this engine will eat an Enya .09, (and even a lot of 15s). Throttle response was good. Makes me kinda wonder what this thing would be capable of on c.c. pressure & venturi. A real plus is that the engine only weighs 4.5 OZs. (less muffler) think that I'm gonna like this thing. ....... George K.

  16. #16

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Don't be afraid to up the prop size. I used an 8x6 on mine for both Rc and CL most of the time. Still have a Max-II and some Max-III's. Here's a pic of the Max-III's. The CL one is an early one. The one on the right has an early throttle system, the top one sports a newer throttle. The muffler is for the older CL version.

    George
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  17. #17

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    I recently purchased an OS .15R/C off fleabay. I don't think it's any of the II or III's but has the strap on OS 702 muffler. The engine is in beautiful shape and I plan to install it on my other recent purchase- An Ambroid kit of the "Charger", a rudder only plane. Yes, I'm regressing and using Phil Green's single channel encoder!

  18. #18

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    RE: os 15 III RC prop nut ... 5.5x.8 ???


    ORIGINAL: mtntopgeo

    Well, I received the 12x32 prop nut from ''aspeed''. Thanks Alan. Ten min. after I got the package open, I was ready to the start the engine (mounted on test stand). Prop was a generation 2, MAS 7x4. Fuel was 15% nitro & 20% castor. Soon as the engine fired up, even before leaning out & using the tach, I knew that this thing was a rather serious runner. Tach read 19,200, ... steady!!! As ''ffkiwi'' & ''dennis'' stated, this engine will eat an Enya .09, (and even a lot of 15s). Throttle response was good. Makes me kinda wonder what this thing would be capable of on c.c. pressure & venturi. A real plus is that the engine only weighs 4.5 OZs. (less muffler) think that I'm gonna like this thing. ....... George K.

    George-you're under propping it if those are your revs-especially seeing as its the R/C version. The engine tests (PGF Chinn in 'Model Aircraft' and 'MAN') show it to peak at about 18,000-so there is no point running it faster than that-and allowing for in-air unload, you'd want to prop it for about 16,000 on the ground no more (allowing for the 10% unload factor!). As I indicated previously-these engines put out-but being lightly built, and plain bearing.....are not noted for their longevity. Being over-revved is one guaranteed way to ensure a short life-and spares are few and far between these days.

    For further info check out: www:sceptreflight.net/Model%20Engine%20Tests/Index.html which is a nice compendium of scanned tests-both diesel and glow from a variety of published sources. The OS Max-I,II and III 15 test reports are all in there-including the stand ard Max-III and R/C Max-III. The standard Max-III test is particularly detailed.

    ChrisM
    'ffkiwi'

  19. #19

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    Chris & George, You guys are obviously correct in advising to use more prop on this engine. I chose the 7x4 for the test run just to compare to a Enya .09 performance. (I have several Enya .09 engines, and since I almost always end up with a 7x4 prop on them, I had a good standard to compare with.) Normally, for .15 work, I use an XLS for the sport jobs, and a Conquest for the quick stuff. But for a couple of recent acqusitions, a Fairchild 22 (pictured) & a TRI* canard, from a friend, I wanted to save a little weight. The TRI* got a .15 HB (reversed fo pusher use) and the Fairchild is gonna get this OS .15RC. My friend ran the Fairchild with a 10, but the plane is rather large, dirty, & heavy. So I wanted to use a 15 that wouldn't add any more weight. I'll probably start with a 8x4, & go from there. .... George K.
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  20. #20

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    RE: os 15 III RC prop nut ... 5.5x.8 ???

    I think you'll find an 8x4 a very good choice George, and they'll turn something smaller with a bit more pitch if needs be. Peter Chinn noted that a 9x4 was about the largest practical size-and that the engine didn't being overpropped either. An 8x5 might also be useful in certain roles. In FF (and I have several of the Max-IIIs in use)-I use an 8x3 or one of the 'oddball' APC sizes like 7.625x 3.75-but I have the luxury of short runs, plenty of nitro and pressure feed.

    ChrisM
    'ffkiwi'


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