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MG MGB Technical - Leak from the air box - not Tom's knob

Hi all,

I have a UK RHD car, 1972. Reshelled with a heritage shell in the early 90s.

When water gets into the fresh air vent, I get a drip from a seam in the passenger footwell - see the photo where I'm pointing with the screwdriver - you can also see the drip of water forming to the right of it.

I have cleared out the drain by rodding it from above and also gone under the car to clear out Tom's knob.
Looking with the debris cover off, the box looks pretty clear.

The water drains out really quickly, but I still get this drip forming. Is there a poor seam seal somewhere in that fresh air vent box?

Any ideas about what can I do about this? Would putting some cavity wax/dynax/waxoil and hope it runs into the right places work?


Nat

Heritage shell noted but that panel looks like it has been plug-welded - my 72-built 73 model first attached. The bulkhead is double-skinned above that panel. For info RHS of the Gaydon cutaway also attached.

Have you dried it out then put a little water in the scuttle vent and seen the drip reform? How soon? If so then one of the wax treatments is easy enough to try - stinking out the cabin of course. Too cold for Waxoyl at the moment though unless you can heat the interior of the box up with a heat-gun.

It could also be water on the heater shelf in the engine bay from poor sealing of the edge of the bonnet channels.






paulh4

Interesting re the plug welds of my heritage shell compared to the OEM shell.

I've just tried to pour a smaller amount of water directly into the vent box, and so far no drips, so perhaps it's not coming from there or needs to be loads of water/continual rain.

I definitely get a some water landing on the shelf where there would otherwise be the brake servo, but the area around the heater seems to be dry.

hmmm.

From the cutaway, it is difficult to identify where else it could be coming from!
Nat

The original Tom's Knob was ball-shaped at the lower end with just a slit for water to exit (arrowed in the attached). That could allow water to build up in heavy rain and it looks like there are two overlapping panels with a couple of gaps in the left-hand picture of your PDF. Seems to be quite a way up from the bottom though.

paulh4

I had water on the shelf where the brake servo would fit if I had one. In my case it came from the corner joint of the bonnet gutters, where the side gutter on the wing meets the rear gutter on the apron. I smoothed some seam sealer over the area and stopped the water ingress.
Mike Howlett

Mike, yeah, I have got water ingress at that same location - I've put a whole load of flexible sealant in that corner where the bonnet side channel and rear channel meet (something like that 'dum dum' type material & similar to blu-tak), but still am getting some water running along the outer side wall of the passenger footwell, and also - see first photo - note the plug welds. I wonder if its also poor sealing under the T-piece between the wing and cowl... I have put a whole lot of flowable sealant in that area already.

More on the leak from the vent box - parked slightly pointing uphill in heavy rain today, and ended up getting really fat drips of water coming from a different seam! This time, right up where the panel where the wiper spindles/jets joins another panel. You can see the drip of the water landing on the corrugated heater vent tube. See second photo...

Popped the bonnet open, and the heater shelf looks pretty dry (& incidentally the area where the brake servo would be)





Nat

1st photo: That looks like it's coming from behind the inner wing. It it getting wet inside the hole in the inner wing where the bonnet release cable exits?

2nd photo: If not from the wiper/washer holes themselves then that could be the seam where the bulkhead joins the panel at the base of the screen.
paulh4

Re photo 1- it's dry around the hole for the bonnet release cable (on both sides)

Re photo 2, definitely dry around the wipers and jets. I can see the water gathering in the seam, where's it coming from the outside? Is it the vent box & means i have to somehow seal all in there?
Nat

See the attached from the Gaydon 'divorce' car from my web site showing the clips for the grille. It could well be coming through the seam between the panel the grille clips are under and the panel with the leaf trap. By removing the grille you should be able to get a flexy nozzle on something like Dinitrol run along that, get the nose of the car as high as you can to encourage it to run back into the seam.

That's a late Mk1 or early Mk2 and the sides of the air-box seem to stop well short of the underside of the panel with the wipers and washer. On my 72-built 73 model the sides go all the way up, but leaving a small hole in the corner where that seam is. So any water running along the seam could get all the way across to the side of the footwell including that panel in the photo of your original post. There is a cavity where the hinges bolt to the bulkhead under the M/C access plugs and above the heater shelf from the sides of the air-box to the inner wings.



paulh4

Thanks Paul,

That's such a helpful photo! I have some Dynax S50 so will try and warm it up & blast it into that seam.

Hopefully I can build up enough layers to fill the hole in the corners.

I'm not what you'd do with an earlier body with those short walls...

I didn't have these leak on my previous B which was a mid 73 car with original body - I wonder if the plug welds of the heritage shell is the cause of these leaks?
Nat

This thread was discussed between 12/10/2025 and 26/10/2025

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