Saving Old Seagulls

Can someone enlighten me about what sort of thread size this is? All I wanted was a thread chaser that would clean the whole female thread!!

It seems to be 22 mm across flats, but with an 11/16 of an inch diameter shank (0.6875) - but my thread gauge will not fit - it seems to be between 18G and 16G, but my zeus book does not match up - the closest would seem to be an 11/16-17 UNF, not in the book

The nice thing about Whitworth is that the non expert has a good chance of understanding it - but with this one I'm lost!

Donald A

Donald A. 18mm thread size.

Spark Plug Thread Repair Chasers ​18mm, search on ebay for this.

1.5mm pitch, just had to recoil one of mine

Thank you all for the replies - I'll pursue it accordingly
So what thread in my zeus book does this take me into?

I'm not sure if it's listed in Zeus as the book only covers common sizes. Spark plug is M18x1.5 which falls in to metric fine. There is also 18x1, 18x2 and metric coarse is 18x2.5. On top of this there are special pitches and left hand too.
If your wanting to know if it's in the book for tapping drill size it's just 18mm-1.5 = 16.5mm near enough.

Thank you for that - two more bits of the jigsaw. I did not begin to think that a spark plug "made in the usa" for an engine designed round whitworth would have anything metric on it - and did not realise that zeus does not cover everything....
I am now on the hunt for an 18 mm chaser, and a workshop book that lists all the threads, not just some of them!

Why a thread chaser, just get a tap. If the thread is stuffed don't loose heart as you can repair it by helicoil or better still timeserts.
I doubt you will find one book that will list all threads, there's just too many and then there's specials by manufacturers that want you to buy their parts. You just have to measure them.
A trick for threads you can't get or see you thread gauge in is to screw a bit of soft scrap wood into the thread then use your pitch gauge on that.

The reason a chaser is preferable to a tap, firstly cost but secondly & perhaps more importantly is that many Seagulls were fitted in the factory with Helicoil inserts, you will run a greater risk when using a tap that it the tap will bite on the helicoil & damage it or tear it out, giving you a more complicated & costly repair when all you wanted to do was clean the thread up!

Thank you - I had already found out that the chaser is about 1/3rd the cost of a tap, so it's on order!
Why did the manufacturer need helicoils - doesn't quite sound right?!

Donald A wrote:Thank you - I had already found out that the chaser is about 1/3rd the cost of a tap, so it's on order!
Why did the manufacturer need helicoils - doesn't quite sound right?!

The idea is that the coil is usually stainless steel and there fore gives you a stronger thread by both material and larger surface area of the larger thread of the coil. This is common practice for quite a bit of machinery and in plastics. I feel I would rather a thread in aluminium and when that is buggered then recoil it for if the coil is stuffed it makes for a harder repair. I prefer timeserts which are a solid insert.

Good point on the chaser Charles I didn't really pick up on that as we don't use them because of the large amount of taps lying around, good and buggered

Charles was right - there is a helix insert into the original thread, which I only recognised after looking at a u tube video of how to insert them - it has a sharp point at the lead in, unlike ordinary threads with a smooth lead in - and the whole thread has 2 diameters - the one deeper in is wider. The chaser worked well, although it's a pity I don't have a parts cleaner to finish of the job
Thanks to all for a good result
Donald A