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MG Midget and Sprite Technical - What thread in Vertical Flow Radiator

My 1966 vertical flow radiator has a threaded bush in the top tank where I would like to fit a thermostatic switch for a radiator fan. Does anyone know what thread it is? I suspect it is tapered pipe thread. It is the same thread as can be found in the thermostat housing on the cylinder head on the 1275 engine.
Mike Howlett

Further research leads me to believe that it isn't a tapered thread, but is 5/8" UNF. Does this seem correct?
Mike Howlett

I would love to know because I plan to do exactly the same!
If you find a type of switch would you please share it?
Onno K

Sometimes the temperature sensor bulb is fitted to the cylinder head, with an adaptor to stop it being crushed, and sometimes it is fitted into the top of the rad - no adaptor needed - hence the same thread. Not sure if it helps but i have blanked the cylinder head opening using the same plug that is used to blank the threaded inlet on the inlet manifold. From memory this plug is hollow and could therefore be easily drilled and taped
S G Macfarlane

Mike I am convinced the fitting is a parallel thread and is 3/8ths BSP

This would be labeled 3/8th BSPP (British standard pipe parallel)
Robert (Bob) Midget Turbo

Bob, I would expect it to be a pipe thread, but if you look at this link on the Mini Spares site you will see a plug for the thermostat hole and it is 5/8 UNF. And the thread in the radiator is the same as the thread under the thermostat housing.
http://www.minispares.com/Product.aspx?ty=re&pid=37975&sku=0&tc=1
Is a 3/8 BSPP thread the same as 5/8 UNF?
Mike Howlett

Mike,

A quick look in Machinery's list 3/8" BSP as 19 TPI and 5/8" UNF as 18 TPI so not the same thread. The BSP table was not for BSPP I think but the pitch would be the same for BSPP or BSPT. The major diameter of the 3/8" BSP was given as .650" and UNF will be .625 ish so the 2 threads might be confused if one tried a 5/8 UNF in a 3/8BSP hole with a short thread engagement.
David Billington

Surely someone must know this?

Onno, I have bought one of these intending to fit it into the threaded boss in the rad header tank....
http://www.cbsonline.co.uk/product/Adjustable_Fan_Controller_Thermostat_STAT

....with one of these
http://www.cbsonline.co.uk/product/Brass_Union_For_Adjustable_Thermostat_Capillary_STATUN

When the parts came, I found that the thread for the brass union was too small for either the radiator or the thermostat housing. The supplier calls it a 1/4" NPT. I rang them to see if I could get an adapter and they said I would need to know the thread in the radiator/thermostat housing. Hence this query.
Mike Howlett

Mike,

Do you have any engineering suppliers near you that could supply something with a BSP thread and a UNF thread for testing the fit. 3/8" BSP is common enough and 5/8" UNF is a standard size so you might find someone that has some inch fasteners and has one. Anyone near you with a thread gauge that can measure the thread ID in the boss.

I would not have thought the thread to be NPT as that is the US pipe thread and differs from BSP. Is the union you have been supplied a compression fitting and if so what is the diameter of the thermostat bulb that fits it. BSP compression fitting are readily available in many sizes so that may work if the thread is BSP.
David Billington

I've sorted it I think - I hope! I emailed Peter Burgess and he said that they always use 5/8" UNF in the threaded hole under the thermostat housing. So I looked around for something that is definitely 5/8" UNF and found the steering tie rod end, where the TRE screws on. That is 5/8 UNF and screws happily into both the hole on the stat housing and the hole in the radiator top tank.

The fitting supplied with the temp sender is a compression fitting with a 1/4" NPT male tapered thread as shown in the link I posted earlier.

There is a lot of meat on the male end, so after speaking to my engineering pal I have bought a brass adapter from Demon Tweeks which has a 5/8 UNF male end and an M10 female end. My pal says there is plenty of material to re-thread the fitting to an M10 thread and then it will all go together. Whooppee. I hope he and I are right!
Mike Howlett

This thread was discussed between 05/01/2012 and 06/01/2012

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