Coolant Leak

I have a cylinder head gasket coolant leak on my 1936 SIlver Eagle... and I realise that it should not have any coolant going between the head and black through the gasket, so it must be one of the aluminium pluds leaking. I replaced them when I rebuilt the engine a few years ago, but I noticed then that they were quite a loose fit, and used a sealant on them... Clearly it is not working! Has anyone done anything to improve this, I know that the threads are a parallel BSP thread , has anyone tried tapping them out for the next size up tapered thread NPT pipe plug for instance, or any other cures for this? Posted by Hardaker 05/11/2016 16:46:23

Have you tried plumber's Teflon tape? Posted by johnlayzell 06/11/2016 21:26:36

Hi John, I think I had Teflon tape on some of the plugs, but not very tight, so I used a sealant as well... Can't remember what!  I want a permanent solution next time I do it. Also is there a reason that the plugs are aluminium? Doesn't that give an electrochemical reaction with the iron head and block? Posted by Hardaker 06/11/2016 23:11:23

Hi,  I used PTFE plumbers tape to increase the resistance and make the threads appear tight, but you could use a screwcutting lathe to manufacture some oversize plugs to suit your engine block. It would be difficult to manufacture these without the block to trial fit the new plugs - however this is tricky because the registration of the machined thread will be lost each time it is removed from the lathe for test fitting!   The plugs are aluminium to act as a sacrificial anode - they are designed to gradually erode from the inside to reduce corrosion of the iron block and the (copper / brass) radiator. I would hope that they last at least 10 years but this assumes that you are using anti corrosion inhibitors (within the anti freeze) and replacing all the coolant every 2 to 3 years. I know there are environmental concerns about the "old style" antifreeze but newer OAT type anti freezes are not suited to older engines and the components used within their cooling systems. Also check the battery earth connection to the engine - this may contribute to more rapid erosion of the anode, but is likely to be evident when starting is problematic. This is easily checked with a parallel jump lead from battery terminal to engine block - does the engine turn overfaster? are there more volts available to the ignition?  Hope this helps Dave Posted by DaveT 07/11/2016 09:12:01

Thanks Dave, I think I will try and make up some custom tapered thread plugs. NPT pipe threads are tapered, but the wrong thread. I believe that the Alvis ones are BSPF, but I can't remember the sizes. I think they are 3/8" BSPF and 1/2" BSPF. Can anyone confirm? I figured that if I make some tapered ones like NPT, they will tighten up and seal when they reach the thicker end of the thread. I have access to a good machine shop and CNC machines which should make this easy! Incidentally, why do they need to be sacrificial anodes? Posted by Hardaker 12/11/2016 03:04:18

Ian,  I believe that they were sacrificial due to the dis-similar metals and the electrolysis that would take place. We mitigate those factors today with the coolant chemicals that inhibit corrosion. These cocktails were not available really until the later 60's. I remember the days when soluble oil was added to mitigate corrosion.   Richard,  Melbourne Posted by RichardWallach 13/11/2016 05:19:34

But why not make them out of cast iron, then they would be the same metal, and no corrosion. I know you are on the right lines Richard, but why do they have to be sacrificial? What are they saving? Posted by Hardaker 13/11/2016 14:48:53

Ian,  Radiator tanks were often made of brass and with the early Alvis motors the block to head water transfer ports were made of aluminium. To add to this the chassis was running at a positive potential. Rolls Royce went negative earth to help mitigate corrosion. Hope this helps.  Richard Melbourne Posted by RichardWallach 14/11/2016 07:06:26