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MG MGB Technical - Rusted Fresh Air/Heater Plenum

I have discovered a rather disturbing problem which I would appreciate some views on. To cut a very long story short, I discovered some fairly serious rusting inside the fresh air/heater plenum, despite some preventative waxoyling of the plenum chamber some years ago. Tom's Knob appears to have failed in its duties and got clogged up despite there being mesh under the intake scoop to prevent the intake of leaves etc. Having looked with a torch it would appear that the floor of the plenum has some pretty bad holes and will require repair. I tried clearing UT's knob in the time honoured fashion and it promptly dropped into the chamber below! I have managed to get the loose rust out of the plenum, treated the area with Fertan rust killer as a temporary measure and have sealed the air intake with some heavy plastic sheet to prevent water (and air) getting in. The heater is currently functioning through the expedient of drawing air into the heater box via the internal fresh air vents. Having discovered that works, I wonder why they put a plenum there in the first place, it just seems to be a first class rust trap - if you need fresh air, open the quarter lights!!
Option 1 - I am assuming the only practical way of dealing with this is to remove the heater unit and cut out the section of bulkhead behind the heater to get access. Has anyone got any practical experience of doing this and any advice?
Option 2 - Close up the air intake forever and use the dash mounted fresh air vents to feed the heater and forget about it?
Any thoughts would be appreciated. Thanks!
PJ Eades

The heater was provided long before the fresh air vents on the dash, so the intake at the base of the screen was the only way of getting any heater function at all. If you have fresh-air vents, then they can be used as you describe, and that is how to get a 'recirculating' function which may boost heater output a little in winter.

Something need doing, as there is a box-section under the intake that will trap water itself and rot through into the cabin unless it rots through the tunnel first, see the attached.

Note this is a Mk1 (the Gaydon cut-away) and does not have the curved shields in the centre of the box that diverted incoming water away from the cold air entry point, added later.

That shows you *might* be able to access the floor of the air box from a combination of the holes for the cold air flap, and the screen ducts, and the intake aperture. Other than that it looks like one of the two options you mention.

Paul Hunt

Whilst, as Paul said, it is accessible (to some extent) via the cold air flap etc, all that is very well tucked under the dashboard.
If you want a neat job, you'd really be looking at removing the dashboard so you could get in easier at the angles you'd need to aim a welding torch at the base of the area.
The other option is, as you said, cut a hole behind where the heater sits. This is a bit brutal but if you cut carefully, the heater can cover any mess you make on reinstallation.
Last choice, again from you, is to seal up the cowl intake. To do this effectively you'd be looking at welding in a plate and fully seam sealing and repainting the area. A bit of a mess again.

Depending how handy you are at removing the dashboard, i'd still consider cutting a flap behind the heater box and going in that way. It can be bent back and welded over, then covered by the heater box. You'll know it's there but it won't affect the presentability of the car.
You could also look at reshaping the whole base of the plenum area for a better drain-off angle. It's rather flat in there as standard and a basin aiming at the 'knob' would be much better at avoiding standing water.

I tackled reclaiming the plenum floor and the drain tube to which the knob attaches. Work on the former was done through the cold air flap hole (flap and speaker box removed) and the latter through the drivers heater flap hole (again, the flap removed) and, IIRC, the heater space - heater was out with the engine at that time. I'm a little hazy on the details here, sorry.

Dash was in place but seats were removed so task was done lying on side reaching in and up to work.

This was a real cow of a task. It took a bucket load of hours of work to do but I reasoned it was worth doing as well as possible because I would not want a second go.

The floor of the plenum had quite a degree of surface rust, though the sides were clean and there were no holes. I used a 'dental' type of extendable mirror with a small torch to check it all. I sanded the floor down and brush painted (brush handle shortened to fit in) undercoat liberally followed by a coating of weather proofing compound - type for underbody use.

I refer to the job as my "seventeen bandaid task". That's about the number of decent slashes I copped on hand and arm, almost to elbow, on the very rough edges of the speaker frame and flap hole. If you go in via that route I'd strongly recommend taping all likely contactable edges as a protection.

My plenum floor was shaped to throw water towards the knob drain - 69 local production but body would have been factory made between late 68 to around March 69 latest.

R Taylor

Thanks for your input everyone. I will probably have to bite the bullet and cut out the bulkhead panel in the engine bay behind the heater unit so I can get reasonable access and assess the damage properly. At least this way if I also bypass the heater unit I can still run the car if need be. Does anyone have any thoughts on the best way to cut out the bulkhead panel as I imagine using a hand hacksaw would be difficult at best? Cheers!
PJ Eades

If you have access to air tools, a reciprocating air saw will do the job. If not a "nibbler" should work or, a cutting disc. Messier!!
Allan Reeling

Was thinking about a cutting disc...!
PJ Eades

"If you have access to air tools"

Perhaps air shears after a starting hole is drilled, or maybe even tin snips, to keep sprayed metal to a minimum.
Paul Hunt

The other point about using snips is that there is no metal lost as there would be sawing or using a cutting disc. This means that it would be easier to refit and weld up when the job is done. If you have access to air tools then an air chisel would also be ideal
Iain MacKintosh

Hadn't thought about a nibbler, particularly in regards to avoiding spreading metal shards everywhere! Had a look on Ebay and you can get electric drill driven nibblers that will cut steel up to 1.8mm thick. I haven't checked, but wouldn't have thought that MG panels would have been thicker than that, even the bulkhead. 16SWG maybe at most? Comments anyone?
PJ Eades

Drill nibblers easily cut panel steel in an MGB. They will leave about a 1/8" gap though if reusing the panel you have cut out as in this case.
Paul Hunt

Yes, nibblers leave the 1/8" gap, but they also make lots of metal chippings which get everywhere. I would sooner use a thin cutting disc. When I rebuilt my car I used a combination of disc and air chisel. You must wear ear defenders because the noise is quite extreme at times.

I actually cut a square of metal out of the firewall inside the car, into the heater box on my '69 GT, and let into the space a similar piece cut from a wrecked later car. That way I was able to fit fresh air vents to my car, valuable on hot summer days.

Mike Howlett

I used a very thin cutting disc on my rear wing & sill.

Dave O'Neill 2

For small cuts, a Dremel is very handy.
Mike Howlett

My "nibbler" reference was meant for something like a Monodex, which removes a 3mm wide continuous strip. Air shears do the same, without the effort!
Allan Reeling

If I replace the bottom floor of the plenum chamber, I wonder whether it would be possible/desirable to MIG weld in stainless sheet instead. At least it would reduce the risk of it rusting out again!
PJ Eades

Can you MIG weld stainless steel to mild steel? Just asking 'cos I don't know.
Mike Howlett

Allan Reeling

Meant to add, "but the weld and the immediate area, will still corrode.
Allan Reeling

Properly protected i.e. Waxoyling in the box-section as well as in the air box (OK, it will pong in hot weather, maybe underbody seal), and shaping the base to avoid any water traps, should be good for another 40 years.
Paul Hunt

Just a thought! Perhaps I could use 22swg stainless steel sheet with s/steel blind dome rivets and seal up the gaps with silicone.
PJ Eades

PU sealer would be better if you go down that road. It's a strong and flexible bonding agent.
Allan Reeling

Don't use silicone, you'll regret it sooner or later.
Use PU sealant and even forget the rivets, just bond the sheet in with the PU.

This thread was discussed between 19/02/2015 and 13/03/2015

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