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MG MGB Technical - Tom's knob

I cannot see the drain from inside the cavity below the the windscreen on my 1975 gt but have located the outlet. I poked it with a wire probe and it seems full of dry dust. I resisted the temptation to push the wire too hard because I don't know the route of the tube and don't want to risk tearing it. In a previous thread someone suggested a pipe cleaner. Is this safe to use from the bottom and how hard can I push to free up the dirt?
R E Merrall

It is pretty straight! It is where the rubber tube fixes to the metal outlet tube that can be the problem, if the metal has corroded. The bottom end is just a rubber tube with a split in the end, and all it's associated problems! I'd suggest a 6mm (ish) stick. AFAIR the tube is about 5" long. I cut a couple of grooves around the stick to add a bit of "bite" as I twiddled and pushed and pulled. Once it has mostly cleared a hose should shift the rest. (Just check it is draining out.....!)

However it is likely to have (maybe not full) dust in it, as the slit in the end allows water out but not the crud. If you pour a bottle of water in the top and it comes out the bottom, it works. If it dribbles, you are no worse off than if it had rained (like today!).
Michael Beswick

The tube is almost straight but you should still use as little force as possible. It’s probably better to clean from the bottom as trying to force it from the top against the small slit opening is going to very difficult. Removing it would be the best approach but that's a right pig of a task!

Best of...
MGmike
M McAndrew

A pipe cleaner i.e. those woolly things on a bit of thin wire are going to be far too flimsy to make any impression on Tom's Knob.

The hole should be clearly visible from the top on your 75 - the first two pics here http://www.mgb-stuff.org.uk/drain.htm#1 are of my 73 but my 75 is the same. Your box may have a layer of dried silt at the bottom covering the hole.

Incidentally it is still visible from above on the early air box with the flap below the grille, on the Gaydon sectioned car at least.
Paul Hunt

Hi guys. No success; At the bottom of the drain it seems to take an L bend about 6 inches up. I tried a flexible drain clearer which went up 9 inches or so and a fair amount of dirt come out but it seems compacted. I tried warm water but it is kust lying. Anyone got a plan B? I can't see the drain hole at the top because a plate is in the way; see photo.

R E Merrall

Any L-bend must be below the tunnel i.e. by the bell-housing as the spigot in the bottom of the air-box points straight at the hole in the lower skin of the tunnel. If you can't see a bend from below then I suspect it is just a very determined blockage you are encountering.

9" must be pretty-well all the way through Tom's Knob itself, although I've never actually handled one ...

Have you tried probing from above? I'm pretty sure I can see the hole in your photo, even through a layer of water.
Paul Hunt

The tube doesn’t have an L bend. It's just a slight kink of about 10deg just inside the bulkhead section.
Options are to keep at it in the hope that ever increasing levels of compacted material will be loosened. Try getting a long flat blade screwdriver in there and twist it drill fashion to loosen material as you push.
As Paul says you should be able to see the outlet in the bottom of the air box and perhaps a flexible extension/probe might do the trick.
Last option is to remove the thing! The pictures in the link Paul posted show the access point(s) but be warned, it's a bugger to get at and even harder to get removed.

Best of....
MGmike
M McAndrew

Will your flexible drain cleaner go in from the top (under the plate in the photo?
Michael Beswick

On a 75 (and for some years earlier) it is straight down the exit hole in the air-box even with the grille in place, as shown in the link I gave earlier.

I've rodded mine out with a 1/4" dowel in the past, so I can't see any bend being a problem either.
Paul Hunt

Thanx for this post.. The cowl in my 79 never drained the water it would accumulate. After Paul's post showing the "cutaway" view of the cowl cavity, I too tried to rod out the drain with a thin bamboo rod, and only got about 4' before finding it clogged solid with dirt.

Dropped the top and went to the hardware store and got caught in a Florida deluge of rain. When I returned home, with a flooded cowl cavity, I worked the bamboo rod again and penetrated another 2".

Waited 45 minutes for the rain water in the cowl to soak the dirt clog. Got my electrician"s snake and rodded the drain and success. Good puddle of water and crud spilled onto the garage floor.

Cheers

Gary
79 MGB
gary hansen

Had to give in on this one; just couldn't shift the blockage. Took car today to my local mg specialist who took over an hour to clear it, but done now.


Rod
R E Merrall

Rod,
Was that in Cardenden? If so I know it well :-)
Shame I'm not at home at the mo as I would have run over and helped.

Give me a shout if you need help with anything else.

Best of...
MGmike
M McAndrew

Thanks for the offer,be careful what you offer Iseem to have a succession of problems. Yes I took it to chick Doig.
R E Merrall

This thread was discussed between 30/08/2015 and 27/09/2015

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