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MG Midget and Sprite Technical - Midget MK 3 Vertical Flow Radiator Blow Plug

The blow plug on the side of my vertical flow radiator started to leek and then part of the plug blew out.
Is it possible to replace the plug assuming I can remove the part that is still in the side of the radiator, also assuming this part is still available?
Regards.
J Bond

Do you have a photo, as I’m not sure which part you are referring to?
Dave O'Neill 2

Photo attached which shows the head of the bolt placed on top of the radiator and the side of the radiator where the head of the bolt has broken away from.
Part of the threaded bolt is still inside of the radiator.
Regards.


J Bond

Ah, I see what you mean now.

A solid blanking plug should be available. It’s the same thread as the bypass hose stub that fits to the bottom of the cylinder head on some A+ engines, which weren’t fitted with a bypass.

https://www.minispares.com/88g619-by-pass-tube-blanking-plug-for-cylinder-head

You may be able to remove the remains with a stud extractor.
Dave O'Neill 2

I'd be careful with the thread information given on minispares site as I've seen it wrong before and they were adamant it was correct. 5/8" Whitworth fine is 14TPI not 16TPI, it may be Whitworth form but the pitch they quote is wrong for BSF. Just checked and it was the bypass hose adaptor https://www.minispares.com/12a2075-by-pass-hose-adaptor-long-later-type-on-cyl-head that also gives the wrong information.

It's not clear from the image but normally threaded bosses into the radiator will be brass and soft soldered into place so easy to remove with a torch, the fitting will be easier to work on if out and easily soldered back on.
David Billington

With regard to the threaded corroded blanking plug which was fitted in the top side of the radiator on my early 68 midget as shown on the photo previously attached.
Does anyone know the reason for the blanking plug and what purpose the plug serves?
I have been searching the usual mg spares suppliers for a matching replacement plug but none appear to stock this item.
Regards.
J Bond

Originally, the sender for the temperature gauge was fitted there. It was then relocated to the cylinder head and a blanking plug fitted to the radiator, so on later models it serves no purpose at all…other than keeping the coolant in.
Dave O'Neill 2

This would suggest the thread is 5/8"-18 UNF https://mgaguru.com/mgtech/dash/dt102d.htm . That should make life easier finding a replacement.
David Billington

The same blanking plug was fitted to the cylinder head when the sender was fitted to the radiator.

This is a 1098 Midget engine.

I also have an early 1275 head with one fitted.

They were also used on a host of other A & B series engines and the part number is ADP210.

The second image shows the Minor engine in the Moss online catalogue.





Dave O'Neill 2

I have measured a genuine Rover cylinder head plug - 88G619 - at 18tpi.


Dave O'Neill 2

https://www.ebay.co.uk/itm/354394673694
David Billington

Broken bit in original picture doesn't look like a bolt head, is it in fact the remains of a temperature sender?
AdrianR

No, it’s just a hollow plug.


Dave O'Neill 2

Thanks for all the help on this matter & especially the link to the threaded plug and sealing washer which have now been ordered.
Regards.
J Bond

5/8 x 18 is normal unfortunately, so should be easy to replace. However, although logically it would be the same as the temp fitting in the head, I wouldn't write off 5/8 x 16, as BMC used this in the odd application - most notably the front of the crankshaft. I think the bypass fitting may be another.
So whichever you opt for try it carefully first.
If it turns out to be 16 tpi then you could always make one up out of a crank pulley retaining bolt/screw.
Paul Walbran

hmmm--always thought those temp gauge fitting threads were 3/8 BSP Straight which is 19tpi but if it's 18 then yes unf
Interesting one that crank bolt Paul, I had to block off the bypass hose fitting hole on a head once for pressure testing and a crank bolt screwed straight in there as well

willy
William Revit

I've got a 34B Jacobs chuck sitting in front of me and that has a 5/8"-16 mounting thread. These days that would be called UNS but it's likely that thread usage predates the Unified Thread standard and Jacobs chose it and stuck with it to maintain consistency.
David Billington

Ehehehehe just reread my post and see one of the better autocorrect gaffes. It picked up UNF and made it unfortunately. And I didn't notice. 🤣😂
Willie, yes, definitely 18 pitch. I always thought bsp too until I had to clean a thread up and measured it.
Paul Walbran

The 5/8"-18tpi unf (ADP210) blanking plug with fibre washer arrived today and its a perfect fit to seal off the threaded aperture at the top of the radiator.
Thanks for all the help.
Regards.


J Bond

This thread was discussed between 25/04/2026 and 28/04/2026

MG Midget and Sprite Technical index

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