Have a look at the photos I posted under a early item “MK IV fuel tank construction”. There are 2 baffles (running front to back) - one forming the reserve tank section, and the other with small cut outs to allow petrol through the main tank section but limiting the amount of “slushing” when cornering on the road.
As mentioned - the exact positions are revealed by the spot welds on the tank (probably hidden under paint, etc.
That’s it, there are two internal walls so three chambers if you will.
The left chamber is the reserve.
The center and right chambers are the main.
That’s really interesting Rob.
Thanks for posting.
Peter.
Would that explain why the fuel in my reserve tank was old and dirty while the fuel in the main tank was fresh and clean?
Ron
Interesting point. In theory when you fill up, the reserve gets filled first, and should mix with the old in there. Then after you get a few gallons in there, it should start spilling over into the main center side, and flow to the right side as well.
Maybe if the fuel in your reserve got really really old, like approaching gummy goo, it would stay at the bottom, and the fresh clean fuel would not mix, but flow over the top into the main side. Just a guess.
It brings another thought, that we should make it a habit to use the reserve once in awhile, and use it up before putting the car into hibernation every winter.
Earlier in this thread I had mentioned the possibility of different tanks.
Here is why.
Yes, that’s quite an interesting picture. The chassis looks like a 1937 car but the engine and bellhousing look to be 3.5 litre. A drawing of an early 3.5 litre prototype???
Here’s the 1936 2.5 litre chassis for comparison.
Peter
Ah, yes, even though the title says 2-1/2 Litre the engine has the water manifold of the 3-1/2 Litre. I see it has different body mounts than the earlier chassis, and Dunlop “Fort” tires rather than "90"s. So it would seem to be the prototype chassis for the 1938 2-1/2 and 3-1/2 cars, but still with the '37 tank. Perhaps they hadn’t quite worked out the final design of the new fuel tank when this picture was taken, if they were having all sorts of problems with fitting the body panels together.
This picture with the 3-1/2 engine and '37 tank but 2-1/2 Litre title and descriptive paragraph was carried over into the '39, '40 and '46 brochures.
I looked through my spares locker and found this 1938 petrol tank that had been cut open to expose the interior (baffles, etc).-- don’t think it’s a resto candidate. 
Ron
Hi Ron,
Thanks for posting the photos. It looks as if there is quite a large dip in the middle of the reserve baffle. Does that look original to you, or has it been damaged in some way?
Peter
Very interesting, thanks for posting them.
From what I can see inside mine looking into the fill spout with a dentist’s mirror and flashlight, I have three holes about 3/4" diameter at the top center of the reserve-to-main wall.
I’m considering petrol tank Plan B –
Based on the cost of fabricating a new tank for my '39 DHC, I thought I would see if anyone has, or knows about, a '38 or '39 tank (in good order) for sale.
Would a Mk IV tank work? The part numbers in the SPC are different –
'38-'40 – Part No. 3358/RI
'40 - Early Mk IV – Code No. C.70
Later Mk IV – Code No. C1558
although I realize the renumbering doesn’t necessarily connote a structural difference.
Thanks,
Ron
Dear All
Our 1946 3.5 litre having sat idle for some years had sludge in the tank that was difficult to fully remove. As Ed mentioned, we split the seem of the tank, fully glass bead blasted the tank and coated the internals with solder. Not cheap. Then re-soldered the two halves together. That was 23 years ago, touch wood will last at least another 23.
Thanks again for the help with the head gaskets, on their way from Cordell Newby.
Regards
David
David
Ron the MK IV tank is an entirely different method of construction
If I was dismanting it to repair, I would look at taking the original joints apart. If you have them sandblasted you will need to be geared up to have some protective coating on immediately as the blasted surface will rust extremely quickly Either coated with solder as David did or cynide copper plated. But something.
And then thoroughly coat inside with slosh tank sealant*
