289 Otter Tube Threads?

[LMH]

Hey all,
Does anyone know if the threads for the fan switch on a 289 Otter tube are metric, BSW or what they are?
I'm working on mine and can't quite find the correct thread. I don't want to force a tap in it. It did have a broken bolt, which I removed w/o issue and it runs through a 10-24 die but a 10-24 tap seems to want to cut threads.
Anyone have any knowledge of these threads?
Larry

[Dan Case]

Originally Posted by LMH

Hey all,
Does anyone know if the threads for the fan switch on a 289 Otter tube are metric, BSW or what they are?
I'm working on mine and can't quite find the correct thread. I don't want to force a tap in it. It did have a broken bolt, which I removed w/o issue and it runs through a 10-24 die but a 10-24 tap seems to want to cut threads.
Anyone have any knowledge of these threads?
Larry

If your are asking about original Cobra production parts:
Do you mean the three small screw holes in the steel fabrication (very early Cobras) or aluminum casting (most Cobras) used in the lower radiator circuit? Original parts should have British BA series threads I believe. Reproductions have whatever thread the maker of the part liked or wanted.

[LMH]

Hey Dan,
Yes, those three screw holes. Mine is aluminum but I don't know if it's a repo or original. It is used but of course, that doesn't mean original.
Would the British threads be close to SAE, so that a bolt "sort of" threads in? That's what it feels like when I try threading a screw in.
Larry

[Dan Case]

In that range several different thread systems can be forced to work at least once. Your problem is there are so many choices somebody could have used to modify an original or create a replacement.

[strictlypersonl]

3/16 BSW has 24 tpi, but the thread profile of the British thread is a little different than USS standards. A USS tap might not go in easily. There's also a British fine-thread (32 tpi) that may be used there, so I would check the internal pitch. Either way, I'd change to a US standard screw unless you are obsessive about originality.

[blue sky]

Is anyone knowledgeable of a person or business that is reproducing the cast aluminum tube/switch housing of the Mk2 cooling system? Thanks

AJ

[MAStuart]

What does the housing look like. I have a hobby foundry . Do you have one to copy. Will I need one for my coupe project.

Mark

[FWB]

a hobby foundry? now that would be something i'd like to see pics of

[LMH]

They were made of cast iron early on and cast aluminum later. I looked a long time before I found mine though I do think it's a reproduction. The machine work on the switch mounting surface is machined differently than the one in Dan's photo. Mine is cut by a pass across the gasket surface and Dan's looks to be cut on center.
I don't know if the coupes used them or not.

The broken bolt that was in mine fit in a U.S. 10-24 die quite well. I ended up running a 10-24 tap in the holes and it turned out that I really didn't cut much of anything thread wise. Another reason I think mine is a repo.

The tube could be fabricated out of tubing and that was my plan but I was lucky enough to find one used, so didn't take that route.
Larry

[Dan Case]

Originally Posted by LMH

They were made of cast iron early on and cast aluminum later. Larry

The remains of early ones I have seen and or held were fabricated mild steel. (Upper bleeder tubes were also thin wall mild steel fabrications. SA did design drawings for the upper bleeder tubes. ) Few seemed to have survived. Owners in warm climates often did not use anti-freeze or other corrosion inhibitors it seems as badly corroded parts in the whole of the engine/cooling system were/are common. The steel one I borrowed for pictures and a reverse engineering study was severely corroded and the car owner had to have a new one fabricated.


The AC Cars supplied cast aluminum part used on most new street cars was mounted on chassis at their factory before shipping to America. I have held cast aluminum examples recreated by two companies and have seen pictures of ones made by others and heard of even more makers . Copies are not very rare. The new made copies I have held were heavier castings (thicker walls) than originals.

[LMH]

Steel not cast iron. Thanks for the correction Dan.
Larry

[MAStuart]

Where are these located on the car? I feel dumb. I cant say I have seen one on a car . I guess I'm not paying good enough attention.

Mark

[LMH]

Right above the steering rack on the right side of the car.
I'd post a pic but I'm on my phone and my reference stuff is at home.
I'll try tonight.
Larry

[LMH]

You can see it mounted on 2552 just above the rack boot. I wish I had a good pic showing the mount but I don't. Hoping I can get one at Coronado Speed Week in September.
Larry

[LMH]

Originally Posted by Dan Case

The remains of early ones I have seen and or held were fabricated mild steel. (Upper bleeder tubes were also thin wall mild steel fabrications. SA did design drawings for the upper bleeder tubes. ) Few seemed to have survived. Owners in warm climates often did not use anti-freeze or other corrosion inhibitors it seems as badly corroded parts in the whole of the engine/cooling system were/are common. The steel one I borrowed for pictures and a reverse engineering study was severely corroded and the car owner had to have a new one fabricated.


The AC Cars supplied cast aluminum part used on most new street cars was mounted on chassis at their factory before shipping to America. I have held cast aluminum examples recreated by two companies and have seen pictures of ones made by others and heard of even more makers . Copies are not very rare. The new made copies I have held were heavier castings (thicker walls) than originals.

Interesting Dan! Mine has hose ribs cast into it and the one pictured looks to not.
Larry

[MAStuart]

Thanks Larry for the pic. Now I have another question. I assume its part of the cooling system? What does it do?

Mark

[LMH]

Yes, it's part of the cooling system and holds the Otter switch for the cooling fan.

Wondering why it's called an Otter switch, I was told because it tells the fan to turn on "as the water gets otter and otter"! ( you have to say it with a British accent though)
Larry

[Dan Case]

Regarding Post 14:
AC Cars included the aluminum parts in the chassis builds. I don't know anyone that knows how they were attached. When made the part had a large round boss, lug, protection, or whatever AC Cars called it on its top surface. As installed in running Cobras the bosses had been crudely cut off. The tool used could have been a hack saw by hand or a power band saw.


Regarding Post 15:
As you see in my image there are no ribs or end beads of any type on original castings. Casting wise I know of only two variants; most have an AC Cars part number casting into them but some do not. Final installation wise every one is unique. The lug or sprue or boss whatever you choose to call it cast into the top surface was crudely cut off prior to car completion. There was a second hand controlled saw cut to shorten the down pipe section. Some of the copies I have noticed were cast copies of a used part warts and all. On them there was no lug to remove or no length modification perform. The best detail wise one I have seen was reverse engineered and recreated just like AC Cars parts. This more accurate copy required two saw cuts prior to installation in a car just as original parts did. After one of these gets used it would be impossible to know if it was original or not mounted in a Cobra. The clues that they are copies are in the screw threads and casting wall thicknesses which are significantly thicker than originals.


Regarding Post 17:
Calling them 'otter switches' is technically incorrect. They are thermal switches made by a company named Otter®. The logo shown below is on a generic switch with a black plastic potting, pressure injection molded, sealing feature of the electric internals. I have come across switches with red, black, and natural (almost translucent pale yellow now after all the decades) sealing. All known switches original to Cobras use red potting but be advised not all of this family of switches with red plastic centers are the correct temperature calibration for a Cobra's cooling system. I have come across two original variants for Cobras, one for early cars and one for cars after that. Both are marked with a 70°C temperature activation temperature (158°F I believe and that water leaving the radiator already cooled as much as it is going to be). Otter made switches of this style (design family) for several different type uses, including electrically operated automotive carburetor choke systems. They were made in many different temperature activation calibrations. I have been hunting originals exactly as fitted to Cobras since 1985 and never found one for a Cobra (correct every detail including temperature calibration) that was not in use in an original Cobra or with an original Cobra as a loose used part. A problem with the original Cobra application is that all the current for the electric radiator fan motor goes through it. The switches are a mechanical design. A bimetal spring changes shape suddenly as the calibration point is reached. The sudden operation slams the silver contacts together then in-rush current goes through them. I dismantled a switch that still snapped as the trip temperature was reached but it did not complete the electrical circuit anymore. The silver contacts were badly arc burned and hammered out. It not uncommon today to find original Cobras that are driven a little to a lot with either a toggle switch someplace for the driver to operate or a modern solid state switch mounted in some type adapter fixed on the water neck in place of the O.E.M. switch.

[LMH]

The one I'm going to use is an adapter plate that is threaded for modern thermal switch. While it's not original looking, it is more accurate and replacements are readily available at any auto parts store.
Larry