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Philip Burley03/04/2019 12:12:31
198 forum posts
1 photos

I am having problems with a fine thread , the dia is 0.156 inch ie 5/32 The pitch fits on my metric gauge marked as 50 , which I assume is 0.5 mm pitch, but seems finer than that , too fine to count till I borrow grandsons microscope . The screw is an adjuster in a model aeroplane engine ,

any thoughts ?

Ian P03/04/2019 12:21:35
2590 forum posts
114 photos

Rather than trying to count the threads to find the pitch, Just measure haw far the adjuster travels over say, 10 turns. Simplest is with digital calipers because you can set zero over the whole assembly (exposed end of screw to opposite side of engine or carb.

No requirement for the manufacturer to use a standard thread size (although they probably did) but establishing the pitch accurately will give you the answer.

Ian P

Lambton03/04/2019 12:28:55
694 forum posts
2 photos


0.156"= 4.0 mm

4.0 mm diameter X 0.5mm pitch is a standard metric Fine thread. Yes it will look very fine.

I find it is best to avoid any confusion by sticking to either imperial or metric dimensions.

Best regards


Hopper03/04/2019 12:57:15
6393 forum posts
334 photos

Or could it be 5/32 x 50tpi?

What make, vintage and country of origin model airplane engine is it?

Might give a clue as to whether it could be metric or imperial.

JasonB03/04/2019 13:02:20
22747 forum posts
2653 photos
1 articles

Yep M4 x 0.4 is standard needle thread, I had to buy a suitable tap when I used an OS needle on the carb I made for my Firefly

Philip Burley03/04/2019 15:48:53
198 forum posts
1 photos

hello Jason yes its for a old Bluebird 45 engine , the needle valve . I have one for a pattern . I was told by an "expert" that it would be an AF , I could have a try on the lathe but sure to need a die to make a clean finish

Thanks every one for information

Philip Burley09/04/2019 18:16:24
198 forum posts
1 photos

made one , and it works !!! amazing !!

Jon Lawes09/04/2019 20:06:59
926 forum posts

Well done Philip

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