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What size pipe connector?

How to identify pipe coupling size

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Avon29/10/2020 23:35:43
54 forum posts
20 photos

i have what looks like a Stuart 5 or 7? Vertical with reversing gear. It doesn't have an 'S' on the valve cover. I'm trying to identify the air/steam connector thread size. The internal diameter is 3/16". What thread do you think it might be and where is the best place to buy fittings?

Many thanks.

Paul Lousick30/10/2020 05:40:21
1576 forum posts
594 photos

The inside diameter of 3/16" is too small for 1/8" BSP which has an 11/32" (8.8mm) drill size for tapping.

Possibly a 1/4" BSB (British Standard Brass) thread which is common on brass fittings. (3/16" = 4.76mm)

BSB uses a Whitworth thread profile with a 55 degree tooth angle. Do not confuse it with an American Cycle thread that has similar dimensions but uses a 60 tooth angle.


bs brass thread.jpg

JasonB30/10/2020 07:21:06
19098 forum posts
2100 photos
1 articles

I doubt it is a No 5 unless it's been made with seriously undersize connections as that should use flanged connections with 3/8" clear inlet and 1/2" outlet.

For the current No7A the threads would be 1/4x32ME which would have 5.6mm tapping size. Not sure if the old original No7 had different maybe 1/4" Whit. In any cast the builder may not have used the specified thread and Stuarts have always used a coarser one than most other makes of steam fittings use so only their fittings will fit. Stuarts list inlet pipe for the 7A as 3/16" and 1/4" on the exhaust

Edited By JasonB on 30/10/2020 07:23:15

Avon30/10/2020 22:02:19
54 forum posts
20 photos

Thanks guys, glad to see there's no absolute answer. It's definitely 3/16 so not sure.

Any suggestions for good suppliers?

Paul Lousick31/10/2020 02:11:23
1576 forum posts
594 photos

3/16" may be the minor diameter but you will need to establish the thread pitch and profile. A bit hard to measure in such a small hole. Try screwing in a piece of round wood and measure the pitch of the marks that the thread leaves. This will establish the tpi.

Then do a Google search for screw thread specifications and find a match for the minor thread diameter. This will give you the major diameter of the thread. Then you can find a supplier for fittings.


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