Lotus Elan

TC cylinder head plugs

PostPost by: HCA » Fri Jun 19, 2020 9:31 am

The screw in ones on the side and back of the head, not the welch plugs...

Does anyone know the thread size of these, or does anyone have a head on the bench with the plugs out, would they measure the thread please?

(I have an idea to use one of these to insert a Triumph type heater control valve that has a 3/8NPT thread as I need the existing heater inlet under the thermostat for a temperature sender for my ECU.)

The plug above the distributer I want to use is well and truly rusted in place, so before I start with a drill, thought it might be wise to ask as I cannot see anything in the archive.

Thanks, Hal
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PostPost by: rgh0 » Fri Jun 19, 2020 9:51 am

Al the plug threads in the Lotus head are BSP. i don't have any of the cooling passage plugs out currently to measure but you should be able to tell what BSP size by measuring the OD of the plug.

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PostPost by: HCA » Fri Jun 19, 2020 10:44 am

Thanks Rohan - ½". Do you know if it is taper or parallel? And how deep? If I do risk drilling out the plug, then I want to be sure to keep away from the thread wall at the taper end... :)
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PostPost by: silverlink » Fri Jun 19, 2020 11:21 am

Hal,
Give Simon at QED a ring he has all sorts of parts for your head I'm sure he'll be able to advise you.
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PostPost by: rgh0 » Sat Jun 20, 2020 2:07 am

Pulled a plug out of my sectioned head. Plug is 8.5 mm deep and tapered. I have a very tight fitting square shank filed into a bolt end that I tap into these plugs to ensure I don't round of the internal square when trying to remove them.

Head water passage plug.jpg and


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PostPost by: HCA » Sat Jun 20, 2020 7:44 am

That is good news Rohan - thanks! I am not sure if the TR4 thread is BSPT or NPT, but on the basis that the sytem is not high pressure in the true meaning, the threads are often compatible with a suitable amount of PFTE tape.

I might just risk drilling out the plug. It has to loosen up at some stage. I just have to be careful not to drill into the threads at any point.

I shall use the plug on the other side, the one over the distributer as the tap mechanicals sitting between two exhaust outlets might not like the heat.

In theory, the water will not be as hot at the plug level compared to just below the thermostat, but I am thinking only theory... :)

IMG_0530.jpg and
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PostPost by: ElanDNA » Sat Jun 20, 2020 9:58 am

HCA wrote:(I have an idea to use one of these to insert a Triumph type heater control valve that has a 3/8NPT thread as I need the existing heater inlet under the thermostat for a temperature sender for my ECU.)


Hi Hal
See the arrow on the picture.
That's my place for the temperatur sender from the TECll (Total Engine Control).
Works fine dynotested since 27 years.
TEC2-EFI.jpg and

2020EFI.jpg and


Cheers Urs
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PostPost by: HCA » Sat Jun 20, 2020 10:11 am

This is really kind of you Urs - I never even thought to look down there for a suitable tapping point :D I shall go out soon and investigate further! If it does pan out, then you have saved me some frustrating time in drilling out a core plug - having said that, this will only work if there is enough clearance between the normal heater take-off point at the thermostat and the servo...or just use a simple in-line heater tap...

Thanks again :)
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PostPost by: HCA » Sun Jun 21, 2020 2:17 pm

Well, turns out that I do not have a take off point as you do Urs! :cry: There is a boss just ahead of No1 plug, but it is not drilled, it just has a capital 'N' stamped on it! Maybe something to look at when the head is next off as it would be an ideal place to position an ECU sensor as it will not get any hotter than there!

I bit the bullet and started to drill out the plug. Took it in .5mm steps and finally got pretty close to the threads, but what is left will not budge! Waiting for a larger easy-out that I hope will coax it out...
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PostPost by: HCA » Sun Jul 05, 2020 4:14 am

rgh0 wrote:Pulled a plug out of my sectioned head. Plug is 8.5 mm deep and tapered. I have a very tight fitting square shank filed into a bolt end that I tap into these plugs to ensure I don't round of the internal square when trying to remove them.

Head water passage plug.jpg


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Rohan - may I be a pain and ask if the female thread in the head is also tapered?

I am drawing a blank in getting mine out. I have drilled the plug as much as I dare and still cannot get what is left to budge.

I now wonder if it is time to re-tap the thread. I remember this subject coming up before a long time ago, that for engineering it is best to insert a taper plug into a parallel thread to prevent galling of the threads, and the argument went on and on!

thanks
Hal.
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PostPost by: rgh0 » Sun Jul 05, 2020 8:01 am

Hi Hal

The threads in the head are pretty short so hard to tell definitely but they look parallel as best I can measure. Only between 4 and 6 threads in the head, an uneven amount due to variations in the casting thickness from top to bottom.

If all else fails I would keep drilling going up in the finest increments possible until you just get to the crest of the head threads on one side and then you should be able to collapse whats left to get it out. Alternatively use a Dremel with a parallel carbide tip or stone to remove one side of the hole gently until you see the crests of the head threads

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PostPost by: promotor » Sun Jul 05, 2020 8:25 am

HCA wrote:The screw in ones on the side and back of the head, not the welch plugs...

Does anyone know the thread size of these, or does anyone have a head on the bench with the plugs out, would they measure the thread please?

(I have an idea to use one of these to insert a Triumph type heater control valve that has a 3/8NPT thread as I need the existing heater inlet under the thermostat for a temperature sender for my ECU.)

The plug above the distributer I want to use is well and truly rusted in place, so before I start with a drill, thought it might be wise to ask as I cannot see anything in the archive.

Thanks, Hal


Are you sure the Triumph is 3/8" NPT and not BSP (British Standard Pipe) ? It'd be unusual for a solely British car manufacturer (at the time) to use NPT which is an American thread. I bought a Triumph heater valve recently (brass one, not like yours) and it is definitely 3/8" BSP ie 19tpi thread compared to the NPT 18tpi.
The Lotus cylinder head (British) used BSP for sealing threads where as the Ford cylinder block - although for the British market is an American company at its roots - uses NPT for the oil galleries and water outlets.
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PostPost by: HCA » Sun Jul 05, 2020 9:07 am

Thanks Rohan - I am pretty close already to the thread crown - in fact I was able to prise out a little bit of a thread with a metal scribe, but could not get any more out. I might try a dremel and see if I can get any closer. I think I have some BSP taps somewhere but I know they are not tapered, so if the head thread is parallel, then good. My only fear on re-tapping is that the tapping process might not follow the original thread and dislodge the remains leaving me with no thread at all!

Thanks Promotor - no, I am not sure at all if it is NP! I am not sure about any threads outside UNF/UNC/Metric and even these give me headaches! You are right though - highly unlikely Triumph would have used a US thread size. So BSP definitely :)

There was a picture posted on another thread showing a US two seat Elan with the same Triumph heater valve I have in mind screwed into the outlet under the t'stat housing, so what I propose doing is not new (albeit I want to tap it into somewhere else), that begs the question why Lotus did not continue with this thinking instead of piping hot water into the cabin and then rely on a very primitive mechanical flap to prevent the heat dissipating. I can only guess that Lotus were offered a job lot of antiquated Consul heater boxes and someone thought what a good idea to pass them onto their unsuspecting customers... :lol:
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PostPost by: rgh0 » Sun Jul 05, 2020 1:17 pm

Just came across this old photo ( it came up in a reference on another post) I have of a scanned head section that actually shows the plug. Thread in head does look parallel !

Full scale head section 1 resized.jpg
Full scale head section 1 resized.jpg (88.56 KiB) Viewed 101 times


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PostPost by: Craven » Sun Jul 05, 2020 2:02 pm

A guide is to look at how the plug is fitted on the outside, a parallel thread will need to be sealed usually a sealing washer against a machined face. A taper thread seals be virtue of the reducing clearance between the plug and hole.
FWIW.
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