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Old Feb 28, 2014, 02:38 PM
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OS crankshaft thread

I have an OS 91 four-stroke.

I have seen info that says the crankshaft thread is 5/16 - 24 .... but, what threadform is it?

I am assuming that it's an Model Engineer thread.

Can any of you chaps confirm this for me?
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Old Feb 28, 2014, 02:49 PM
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There is no 5/16"-24 in ME series. It's a standard UNF.
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Old Feb 28, 2014, 02:51 PM
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BB,

5/16"-24 is a UNF thread, just like smaller 1/4"-28 of .36-.56 cid engines.
UNF is the acronym for Unified National Fine thread.

There is no such thing as a 'Model Engineering thread'.
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Old Feb 28, 2014, 03:24 PM
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Yes, actually there is, but it is not a defined standard as such.
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Old Feb 28, 2014, 03:35 PM
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I stand corrected, Greg.

However, the thread BB asked about is UNF.
The MET seems not to be used in any model engine I know.
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Old Feb 28, 2014, 03:44 PM
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a better site for thread standards than the one above: http://www.gewinde-normen.de/en/index.html
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Old Feb 28, 2014, 04:07 PM
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Quote:
Originally Posted by bogbeagle View Post
I have an OS 91 four-stroke.

I have seen info that says the crankshaft thread is 5/16 - 24 .... but, what threadform is it?

I am assuming that it's an Model Engineer thread.

Can any of you chaps confirm this for me?
Typical 60Deg thread form Class 2 fit.
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Old Feb 28, 2014, 04:08 PM
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OK, thanks everyone.

UNF it is.


I did think that glow plugs were 1/4 - 32 ME. Am now a bit confused.
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Old Feb 28, 2014, 05:00 PM
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1/4-32UNEF (unified extra fine)
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Old Mar 15, 2014, 11:57 AM
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The OS .75 2-stroke uses the 5/16 x 24 adaptor for spinners, and the screw that fits into this Dave Brown adaptor is 10-32 and about 3/8 inch long.

I don't know what the name of the thread form is, but it's what 10-32 hex machine screws bought from the local Ace hardware store use.
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