Adding a Calorifier to a BMC 1800.......

[Tracy D'arth]

4 minutes ago, TheSaintlyOne said:

 

Hi just found the old thread and on this you said it was NPT 

 

Just want to make sure I get it right 😎

 

Head tapping on a 1.8D is NPT thread, on a 1.5D its 1/2" BSP.  Calcutt can provide the NPT one.  Some heads are reworked ones from the factory so check first, BMC were fond of making unannounced changes!  NPT is the same thread as electrical conduit if that helps.

Edited June 29 by Tracy D'arth

 

 

 

Oh horror!!! I got them the wrong way round.

 

The 1.5D is NPT/conduit 20mm thread.  I would check first with a BSP because many of the heads have been reworked over the years and I have no idea what the Indian factory tapped these threads on the later production. If you get a 1/2" BSP male thread try it in the head. If you need the NPT then Calcutt have them but its a plain steel end where the hose goes on and needs two hose clips or an olive pressed on to stop the hose blowing off.

 

Its a senior moment, can't help it. You will find out when it happens to you.

[Rincewind]

Be very careful about this heater connection thread size on the 1.5 cylinder head.  I have 3 cylinder heads - not all on the same block of course 🙂 and have measured the threads on all three heads. I do not know what year these heads were manufactured but the engines were salvaged from scrapped fridge road trailers in the early 90's as the trailers did not meet new regulations introduced by the government.

One can be forgiven to assume that being a British engineered product of the post war era that all the sizes would be imperial but it appears that this is not always the case. 

Using imperial measurement, the "Nominal" size (Diameter) of the male thread is, 5/8" or - to be precise - 0.620" measured with a digital Vernier gauge. (0.625" being 5/8")

Using the 55 degree angle thread form of a thread gauge (Whitworth) the closest number of threads per inch (TPI) measured was 16. (Thread gauges above & below this number of TPI did not match).

There is NO such thread form – (5/8” x 16 TPI) in any of the imperial systems, including: - BSW, BSF, BSP, UNC. UNF, NPT or even the Japanese system of threads.

However, if you convert the "Nominal" diameter of 0.620" to metric you will find it is as close as possible to be 16mm nominal diameter. (15.74mm, measured tolerances apply)

The thread form itself, if you measure it with a metric thread gauge of 1.5mm pitch, (60 degree angle thread form) you will find it will have an almost identical TPI as the imperial equivalent. (16.93) i.e. 25.4mm divided by 1.5 = 16.93  This thread form - 16mm x 1.5mm pitch - DOES exist

Conclusion:-   It is therefore, a metric (Fine) thread: - 16mm x 1.5mm pitch

Further, if you purchase a 16mm x 1.5mm pitch (second) tap you may well find - as I did - that you can recut (clean up) your threads perfectly.  Do not cut all the way through with the tap, use the taper on the tap to provide somewhere for the male thread to “bite”.

A quick search on eBay will bring up numerous pipe connections for sale with this particular (metric) thread form for various nominal hose sizes.

BMC may have chosen this metric thread form rather than the coarse BSP or NPT thread form, for two (or more) reasons:-

·         it would offer a smaller footprint on the head where space is at a premium

·         It would increase the thread contact area in the relatively thin wall of the head.

Check carefully, you may well find that, someone in the past has rethreaded these ports to accommodate an alternative thread size, so be wary and do not assume the "obvious".  

This observation applies to the two ports on top of the cylinder head: - the thread underneath the thermostat housing to accommodate the temperature sensor is of a different thread form. 

Hope this helps

Rincewind