Pressure release valve in radiator ?

My Speed 25 has been running for the last few years with a modern radiator with a conventional modern pressure release valve.  I have just refitted the original radiator having had it re-cored.  It occurs to me that I have no idea whether there is a pressure release and if so,how it works.  There appears to be some sort of overflow pipe just below the screw on radiator cap. This drains at the bottom of the radiator on the offside.  It has what might be a release valve near the top.  Can anybody tell me whether I have diagnosed this correctly, and if so whether there is any way I can check that it is working correctly and whether there is any maintenance I should do on it. I don't particularly want to find out that it is jammed by blowing out a core plug ! All help gratefully received. Posted by NeilWillatt 21/04/2019 11:55:58

Under the radiator cap on the Speed 25 and most if not all PVT Alvises there is indeed an overflow/pressure relief arrangement. Visible inside the neck of the radiator there should be a flat slotted screw head with a hole about an eighth of an inch in diameter in the centre. Under this hole there is a ball held in place on its seating by a small spring. This allows coolant or steam to escape via the copper pipe exiting the header tank and leading down to the bottom of the radiator if pressure builds up due to overfilling, overheating etc. Test by inserting a piece of rod or a small screwdriver into the hole and you should feel the ball lift off its seating with light pressure. The pressure allowed in the prewar system is well short of that generated in modern (i.e.1950s onwards) systems. If the radiator is overfilled a small amount of water will be ejected at the first stop after a run. Helpful onlookers will inform you that you have a leak. Annoy them by displaying complete indifference and replying "No I haven't.". Posted by DeaneHarding 23/04/2019 21:03:23

Many thanks Deane.  That is really very helpful. I've just done as you suggest and I can indeed feel the ball moving down and being pushed back up by the spring so it would seem that all is well. Posted by NeilWillatt 27/04/2019 19:43:43

If that ball and spring is still intact, would it prevent water from being sucked back into the radiator from an overflow tank? Posted by johnlayzell 30/04/2019 15:16:40

John,  The short answer is yes. However it really depends on the "quality" of the seal that the ball would make especially given the  condition of the part(s). Owners of TA21's have a valve in the header tank and that valve is removed if an expansion tank is subsequently fitted. Can the valve be removed?   Richard Melbourne Posted by RichardWallach 30/04/2019 23:02:14

Appears the ball and spring cannot (easily) be removed. I have an expansion tank fitted, but now wonder if it's actually doing anything!  [attachment=0:3hon3zj3]<!-- ia0 -->SP25 new overflow tank 03.JPG<!-- ia0 -->[/attachment:3hon3zj3] Posted by johnlayzell 01/05/2019 14:39:27

John,  Now is the time to experiment. Without driving the car it is worth disconnecting the tube from your tank and placing it instead into a clear container such as an old coke or milk bottle and part fill with clean water and mark the water level with a felt pen. Fill the radiator right up in order for there to be an expansion overflow. Now run the engine to operating temperature. If your radiator fluid is coloured you will be able to watch the expansion at work and see the level rise and colour transfer to the make shift container. When at its peak mark again with the pen. Turn off the engine and leave it for an hour or two. If the system is working the level should now drop back towards the original starting point.   If you decide to perform this experiment let us all know the result.   Richard Melbourne Posted by RichardWallach 01/05/2019 22:39:01

Richard, et al,  Experiment conducted as per your instructions.  Starting level indicated by black felt pen line.  After about 15 minutes level was as in the photo.  Left overnight to fully cool, and no need for another photo as level remained identical.  Yes, the radiator cap was on tight! The brass slotted cap to the ball and spring valve is about 1/4&quot; thick (6 mm). Wayne Brooks tells me that on a SP25 he owned he was able to unscrew this cap. With a flat metal bar firmly wedged in the slot, I was unable to turn the screw exerting about as much force as I dared for fear of breaking something, including the solder or brazing holding the pipe to the filler neck of the radiator. Has anybody other than Wayne had success removing this cap, and if so, how?  My next step would be to apply heat before trying again.  Since posting, Wayne called me to say he was just able to unscrew his brass cap easily with a screwdriver!  I ran the engine to operating temp and tried again. Still no go!  [attachment=1:2txkyp3w]<!-- ia1 -->Alvis SP25 expansion tank experiment.JPG<!-- ia1 -->[/attachment:2txkyp3w]  [attachment=0:2txkyp3w]<!-- ia0 -->Alvis SP25 radiator overflow valve.JPG<!-- ia0 -->[/attachment:2txkyp3w] Posted by johnlayzell 12/05/2019 18:31:37

John,  Before you do anything ensure that the cap makes a good seal as this is necessary for the return of the fluid from the overflow reservoir. Now on another level of thinking if after filling up the radiator to the top and with the subsequent expansion and loss of fluid where does the level sit when cold? If it is above the core then what are you really gaining as the returned fluid is only in the header tank when cold. If on the other hand due to hot spots etc the new cold level is below the core then an overflow system can act as a makeshift solution but that said it is only a band aid solution. My TA will throw out excess water if I overfill but when cold the level sits about half an inch above the core tubes. To me that is fine.   Richard  Melbourne Posted by RichardWallach 13/05/2019 05:59:01

When cold, on both the 12/50 and SP25, the water level is just about to the nail of the middle finger when dipped in. Two more experiments over the weekend, letting the engine run in the garage for 20 minutes, and the radiator cap on good and tight had the water level rise in the clear container as in the photo above, but when completely cool had dropped to its original level.  Never was able to undo that slotted cap, even with considerable force applied! Back to the brakes tomorrow! Posted by johnlayzell 20/05/2019 00:04:18