Bypass Stub in Cylinder Head - how to get it out

[Declan_Burns]

Bob,
It' a strange thread 5/8" x 16 UNS not UNF. The S refers to special and if you look at the cost of a UNS tap you will soon realize that you could buy a cylinder head for the same money! I doubt if you have such a tap lying around your workshop.

Regards
Declan

[bmcecosse]

I'm not convinced about that Declan. Where did you get that from? Pretty sure most sell 5/8" UNF stubs and plugs for the job - and for vacuum servo take off connectors.
Looking it up seems 5/8" UNF = 18tpi. UNC = 11 tpi, UNS = 14 tpi, and BSF = 14 tpi.
So if indeed it is 'UNS' - the BSF thread is likely to be nearest although I appreciate there will be some differences in thread form, but unlikely to be a problem putting the parts together.

[les]

I'm surprised it's not bsp as katy mentioned earlier--- 18tpi.

[bmcecosse]

I really don't think it is BSP - although as a pipe fitting, you would imagine !

[Declan_Burns]

I'm not convinced about that Declan. Where did you get that from? Pretty sure most sell 5/8" UNF stubs and plugs for the job - and for vacuum servo take off connectors.
Looking it up seems 5/8" UNF = 18tpi. UNC = 11 tpi, UNS = 14 tpi, and BSF = 14 tpi.
So if indeed it is 'UNS' - the BSF thread is likely to be nearest although I appreciate there will be some differences in thread form, but unlikely to be a problem putting the parts together.

The whole issue had me totally confused at the time. I bought one with the 18tpi and it didn't fit. Sombody was selling stainless steel ones at the time which I thought would be better. The stainless steel one fitted perfectly so I measured them up and found that the stainless steel one had 16tpi. I just hope my case was an exception. Perhaps the OP can throw some light on the subject when he goes to fit the ESM one.
Regards
Declan

[gtt1951]

I've chickened out - I've left my original stub in. I have talked to ESM and told them that their stub is twice as long as need be and their answer was "we don't sell many of them".
Anyway, I've kept the new stub as I have another project (recommisioning an 803 engine) thaty it may get used on.
George.

[gtt1951]

Folks,
I've just checekd my spare 803 head and its stub is near perfect, so I won't be taking that one out. The one on the head that is still fitted on the project 803, has not yet been looked at.
I can confirm, however, that the ESM stub is 16tpi, but I cannot tell you the size (apart from that its bigger than 1/2 UNF).
The protrusion of an original stub, from the head casting is 13.5mm. The ESM stub is 27.5mm - quite a difference!
G.

[katy]

FWIW, an S designated thread (UNS) is not any specific thread count, but any count that is different from UNF or UNC. That's why it's "S", special.

[bmcecosse]

Well tables of UNS threads are listed on the net. None are 16 tpi at 5/8" diam.

[Declan_Burns]

I think I may have solved it. It could be 5/8" UNR ASME B1.1-2003. That's the only 5/8" thread I can find with 16tpi.
UNR = Unified Thread Series with external thread controlled root radius.
Regards
Declan

[MikeNash]

Hello chums!
Declan gives the dimensions as 5/8 inch dia with a 16 threads per inch. Now that made me think 'cos if I remember the story right Morris bought a load of Hotchiss factory machinery just after WW1 and so the threads in the engine of my 1935 Oxford are metric although the nuts and bolt heads have BSF/Whitworth exteriors. And if you consult your 1940 Machinery Handbook (i.e. pre ISO) you'll see that for a dia of 5/8 inch (which is exactly 16mm) the Metric Fine Thread of the day is a pitch of 2mm, i.e. 16.93 per inch, which is close to Declan's observation! So if you're with me still, do we have here the only metric thread on the A Series engine?

I rest my case, MikeN.

[Declan_Burns]

Mike,
It's the same on the XPAG engines in the MG T-Series. Metric threads and whitworth heads. They call them the mad metrics. Apparently the tooling came from a French ammunition or gun company retreating in WW1.
But probably the real reason why the by-pass was eliminated was that they lost their UNR tap so it was easier to do away with the by-pass rather than find a replacement tap:roll:
Regards
Declan

[bmcecosse]

Good theory - except the still needed it for the vacuum servo tapings on cars so fitted.......