Thread size for heater valve and temperature bulb

[fattogatto]

Unfortunately, due to a total lack of foresight, I have installed the body and secured it before installing the heater valve. There is room to install the heater valve in the forward hole of the water outlet neck on the head and put the temperature sender bulb in the rear hole. However, I am having trouble starting the adapter for the temperature bulb (from RD Ent) in either hole. The threads on the heater valve have been buggered by PO, requiring another heater valve. Questions: are the threads the same size for both holes? Is there any reason the heater valve should not go in the forward hole? Does anyone know the thread size for these holes? I'm sue I can find the appropriate tap to chase the threads.

I'd really rather not pull the body off the chassis again.

Thank

[Lyn7]

I know the thread is either npt or bsp, but can't remember which. They are supposed to both be the same, but whatever works. If you undo the engine mounting you should be able to jack the engine up high enough to clean out the threads. Good luck.

[ericbushby]

As the cylinder head is a British design the thread will be BSP, unlike the block which is a USA design.
I have the heater valve in the forward hole deliberately as then both fittings can be removed easily.
They both go into the same water chamber anyway.
Eric in Burnley
1967 S3SE DHC

Just been to check a tap against a spare heater valve. It is 3/8" BSP

[fattogatto]

Thanks to all. Amazon promises the tap by Sunday. Marvelous system. Happy Christmas to all.

[elansprint71]

Both threads are different. One is taper, the other straight. Check the archives before making a mistake.

Purists, look away now.....

Charlie, as a (former) Precision Tool-maker I simply could not countenance fitting that horrible heater valve to my car. There is a thread on here (somewhere) which shows how I fitted a brass elbow to the thermostat housing and placed a lever ball-valve inside the passenger foot-well, which allowed the driver to reach over and turn on/off.

[englishmaninwales]

One doesn’t have to be a precision tool maker to feel the shudder of horror looking at that awful valve stuck in the side of my Twin Cam. Often thought about an alternative set up like you describe, Pete, just never got around to it. I recall the later Mk2 LC had a simple elbow fitted, presumably with a 3/8” BSP thread, with a remote valve incorporated in the heater.
Malcolm

[promotor]

I've just been through this with a number of cylinder heads and here are my findings :

Both threads in the head (ie female) are 3/8" BSP 19TPI, and the majority of adaptors that go into the head being 3/8" BSPT (taper) 19TPI - there is no way for them to seal if they are just parallel BSP and don't have any other way of sealing such as a Dowty or aluminium type seal ring. If you look at the water pump unit the outlet for the heater uses a brass adaptor and that uses a washer - the adaptor has straight threads so has to rely on the washer to seal. Again, from what I have measured that is also 3/8" BSP 19TPI on the female thread. There are some exceptions that use a 3/8" BSPT 19TPI male adaptor but I seem to think they only came in late Escort Twincams?

I read the archives a long time ago and if I remember correctly it seems to suggest that the left hand thread is one thread type and the right hand thread is another - that would be extremely confusing and not good engineering practice to have two threads seemingly the same but in practice not being so. I can't see Lotus doing that?! What benefit is there when we had all of the sealing capabilities we needed in our own engineering practices?

I've tried all manner of thread taps and brass / aluminium adaptors (I know, aluminium isn't to be used in an aluminium head, it was just for testing!) in both BSP and NPT (3/8" NPT is 18TPI) and I'm confident there are no NPT threads in a Lotus Twicam head - as mentioned, the T/C is a British creation and therefore would use BSP. The cylinder block on the other hand although for British cars is at its roots an American designed or owned design and uses NPT.

I'd be interested to see if anyone can supply me with an original 3/8" NPT adaptor found in their engine! Happy to have my mind changed but for now I believe 3/8" BSP 19TPI is the size.

[1owner69Elan]

There appears to be a lot of confusion on the thread sizes in the head, below is what is on my car:

This is for a newly cast SAS Weber head. The two holes were provided already tapped by SAS engineering. So, I am sure they would have the answer as to how these are tapped. But, I have never asked them, as what I have seemed to work.

1. The left hand hole where the temperature bulb is normally located is using a straight thread adapter that seals with a aluminum crush washer. This is what was on my original head, from factory, as well. Note that the threads on the adapter (head side) are rather short as well and not suitable for a tapered pipe thread type of seal.
Short threads into head shown on top:

Installed with aluminum sealing washer:

2. The heater valve normally inserted into the rightmost hole has a tapered pipe thread sealed with teflon tape (or equivalent). Note, no crush washer like for a straight thread is even possible. So pretty definitive IMHO that this right hole has tapered pipe thread.

Installed with teflon tape

But the above, doesn't really answer the question whether both holes have different threads or the same thread. It could be that both holes have tapered pipe threads as proposed by Promotor. Perhaps It's just that the short straight threads of the temperature bulb adapter only screw in so far into the tapered hole and then are sealed with the crush washer. Thus, "subverting" the intention of the original tapered pipe threads but still working in practice. The fact that several people have reversed the temperature bulb and heater valve locations, without consequence, would also tend to support this supposition as well.

[Lyn7]

I think "oneowner69" makes some very good points.
"Perhaps It's just that the short straight threads of the temperature bulb adapter only screw in so far into the tapered hole and then are sealed with the crush washer. Thus, "subverting" the intention of the original tapered pipe threads but still working in practice. The fact that several people have reversed the temperature bulb and heater valve locations, without consequence, would also tend to support this supposition as well."
I have used 2 different heads over the years and had the fittings reversed a couple of times as well. Not had any issues. There is also the time when most owners would have just used what worked and not asked any questions as long as they got to work. Also after all this time the soft alloy would probably accept a few options before letting go.

[Lyn7]

Slightly off track. This heater valve looks pretty poor now ,but at the time it was considered state of the art. A lot of cars going into the sixties had no heater at all or it was an extra! This Smiths valve was fitted to the Mini Cooper albeit with a two bolt flange instead. On the MG Midget there was a tap you had to turn on and off , under the bonnet. It was quite the mod to use the Mini Cooper valve to be able to turn the heater on and off in the car.

[Craven]

My conclusion on this, it’s come up many times on here, is the tapped hole in head is BSP parallel thread but the fittings, heater valve and temperature adaptor are BSPT. ie,Taper. It works by the way.
Pic original temp adaptor.

[davidj]

By coincidence, I have just bought a new temp/pressure gauge for my "bottomless pit". The attached documentation suggests the thread, on the adaptor at least, is NPT.

[1owner69Elan]

Some other info to add:

The SAS head was delivered with tapered plugs, no sealing washers, for both holes:

SAS plug, visually discernible and measured as tapered:

Left to right (newly suppled adapter (Smiths), original adapter, SAS plug.

My original adapter looks to have had the first few threads corrode off. Thus, little chance of sealing without a crush washer. Even the new adapter with its short threads, IIRC, seemed to screw all the way in to the shank without any resistance that might be expected for a pipe thread seal. Thus, again the need for a crush washer.

My conclusions:

1. My SAS head was supplied with tapered threads, as evidenced by the tapered plugs. For the holes to be straight threads while using a tapered plug, while possible, seems unlikely. As far as an original head, don't really know at this point what the hole threads are. I sold my original Stromberg head and don't have it anymore.

2. The heater valve definitely has tapered threads

3. The adapter threads (old and new) are rather short and somewhat difficult to determine if really tapered or not. Moreover not enough threads to seemingly make a pipe thread type of seal. In any event I am using a sealing washer on the adapter, as was also used on my original head.

4. No idea whether BSP or NPT threads. As noted, the instructions currently supplied by Caerbont (Smiths) for a new gauge refer to NPT.

[Craven]

Interesting note, but its states adaptor fits the engine block. I think we all agree the threads on the engine block if Ford will be NPT but the head on a Twincam is UK origin. Note also refers to GAS thread which is unusual, true gas connections are Left Hand thread so there is no mix-up with BSP.
Mix of parallel BSPP & taper BSPT is a standard engineering practice.
Idea of mismatch in threads being used to seal is illustrated in the use of NTPF threads on the oil galleries, once fitted should never be undone as the distortion caused is permanent. Little wonder they are very difficult if not impossible to remove without damage to the tapped hole.
FWIW

[rgh0]

By coincidence, I have just bought a new temp/pressure gauge for my "bottomless pit". The attached documentation suggests the thread, on the adaptor at least, is NPT.

I think the documentation is wrong. The threads in the head as far as I can measure are all BSP. They are probably straight tapped also but its also hard to tell these days on original heads due to corrosion and the short length. The original style adapter thread is short and a NPT thread would probably screw in as NPT and BSP are only one thread per inch different

cheers
Rohan