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Sep 17, 2021, 04:24 PM
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OS Max 15 FP Prop Shaft Thread Size


I am looking for a prop nut for an OS Max 15 FP. I can't seem to find a nut that fits the shaft threads properly. Does anyone know the crankshaft thread specification. Thanks in advance.
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Sep 17, 2021, 05:08 PM
Registered User
I seem to remember they had metric threads on the small ones, something like M5 or M6, but not sure. I do unfortunately not have written data on the small ones.
Sep 17, 2021, 05:08 PM
Registered User
Possibly M5 ? metric.
edi
Sep 17, 2021, 05:22 PM
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Thread OP
Thanks but I tried those sizes. Don't fit. I heard a roomer that it is a weird SAE thread size. I will keep investigating.
Sep 17, 2021, 05:45 PM
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Quote:
Originally Posted by clmflyer
Thanks but I tried those sizes. Don't fit. I heard a roomer that it is a weird SAE thread size. I will keep investigating.
There is Metric coarse, Metric standard and Metric fine thread. Did you try those too?
Sep 17, 2021, 06:12 PM
Registered User
I don't have an FP 15, but I'd try a 1/4X28 nut, that's what's on my OS max 20 r/c from 1973. I have never seen an OS engine with a metric prop shaft.
Sep 17, 2021, 06:23 PM
Let's Go Brandon!
Eugene Chernosky's Avatar
It is a 7/32"-32tpi
Sep 17, 2021, 06:50 PM
********
coriolan's Avatar
Might be the not so common 12-32 SAE which are a tad bigger ( .2150") than M5
https://www.aspenfasteners.com/conte...cification.pdf
Seems confirmed here:
https://www.rcgroups.com/forums/show...43&postcount=7
Last edited by coriolan; Sep 17, 2021 at 07:00 PM. Reason: new detail
Sep 17, 2021, 06:51 PM
AMA 46133
SeismicCWave's Avatar
Quote:
Originally Posted by Solomaster
I don't have an FP 15, but I'd try a 1/4X28 nut, that's what's on my OS max 20 r/c from 1973. I have never seen an OS engine with a metric prop shaft.
I am playing with three O.S. .10 FSR. They all have M5 X .8mm threads on the shaft.
Sep 18, 2021, 01:36 AM
Registered User
Quote:
Originally Posted by SeismicCWave
I am playing with three O.S. .10 FSR. They all have M5 X .8mm threads on the shaft.
That is where my thought originated from, but I have no reason to doubt Eugene.
Sep 18, 2021, 03:17 AM
ffkiwi
Quote:
Originally Posted by 1967Brutus
That is where my thought originated from, but I have no reason to doubt Eugene.
Quite true-but the plain bearing OS 15's right from the start (ie Max-I, Max-II, Max-III, Max, and 15FP have always been the 7/32-32 (or 12-32) US thread....and hence a bit of an 'orphan' amidst the wider OS range-the only exception in the plain bearing 15 series being the LA 15 which is M5, as is the higher specc'ed 15CV. No idea WHY OS chose to go with a 'wildcat' thread through most of their 15 engine production history...

ChrisM
'ffkiwi'
Sep 18, 2021, 04:54 AM
Registered User
Quote:
Originally Posted by ffkiwi
Quite true-but the plain bearing OS 15's right from the start (ie Max-I, Max-II, Max-III, Max, and 15FP have always been the 7/32-32 (or 12-32) US thread....and hence a bit of an 'orphan' amidst the wider OS range-the only exception in the plain bearing 15 series being the LA 15 which is M5, as is the higher specc'ed 15CV. No idea WHY OS chose to go with a 'wildcat' thread through most of their 15 engine production history...

ChrisM
'ffkiwi'
That is something that has always confused me as well... No idea what dictates a thread size, or even the choice between imperial and metric.
Sep 18, 2021, 07:51 AM
Registered User
Early on the Japanese catered to the US market and went SAE. The smaller motors really had no standard size so I guess they just picked a number off the chart. I think #10-32 would have been a better choice than a #12-32. Even a #12-28 is more common in that size. I had to buy a #12-32 tap to make some nuts for a few older .10 and .15 motors. At least everyone agreed on the glow plug thread till lately with the turbo plugs and Nelsons. Then there are the old British threads with the 50 degree threads. MA BS?
Sep 18, 2021, 05:01 PM
ffkiwi
Quote:
Originally Posted by aspeed
Early on the Japanese catered to the US market and went SAE. The smaller motors really had no standard size so I guess they just picked a number off the chart. I think #10-32 would have been a better choice than a #12-32. Even a #12-28 is more common in that size. I had to buy a #12-32 tap to make some nuts for a few older .10 and .15 motors. At least everyone agreed on the glow plug thread till lately with the turbo plugs and Nelsons. Then there are the old British threads with the 50 degree threads. MA BS?
Actually its much worse than that-you'll find British engines using BA threads-anything from 0 to 10BA-and BA used 47.5 degree thread angles, plus the larger engines use 1/4"BSF for the shaft thread-which is a 'fine' thread series-but uses the long established Whitworth threadform which is 55 degrees....plus a few engines such as Mercos use 1/8 BSW for assembly screws.....then of course the UK went metric, so things like Irvine and later brands started to use metric threads....but just to really mess you about....some early Irvines used US threads-as Ron Irvine, the founder originally started in the business assembling K&Bs under licence-and in the process offering a few 'local' variants not available from K&B.

And how could I fail to mention the ME series?.....no not the well known diesels from the Isle of Man...but the 'Model Engineer' thread series with a long history dating back to the 19th century-and used on some early British engines-the Mills diesels spring to mind...

Truly-you step warily into engine restoration and repair-as the area is a thread minefield....

ChrisM
'ffkiwi'
Sep 18, 2021, 08:00 PM
Registered User
Quote:
Originally Posted by Eugene Chernosky
It is a 7/32"-32tpi
Correct! I just acquired two new 15FPs and was bummed to find the OS spinner nut that fits the 15LA will not fit the FP.

Tru Turn does offer spinner nuts in the correct 7/32-32 thread


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