Thread: Fuel tank differences between GEMS and Thor

Well, after fitting two long range tanks (GEMS and Thor) this weekend (P38 Central today) we know a bit more about the tank and fuel pump design, but not all. When I download some more pics from my camera, I'll update this post.



In the above pic, the GEMS tank is on the left. Note the Thor tank (right) has a notch at the front of the tank (smaller end). That's to clear the charcoal canister (not fitted in that location on a GEMS).

On the Thor fuel pump (below), the breather is over hard to the RHS of the vehicle, not leaving much room for the breather hose on the long range tank, hence the need to slope the tank upward on that side, rather than the vertical side of the GEMS tank. The GEMS pump is also higher (for what reason we have no real idea, only conjecture) and has the breather further away, thus giving the hose a better exit angle. The GEMS tank also has a short stub pipe welded on for the breather hose.



GEMS pump:



We had to modify the Thor pump and seal to make it fit. I recall Brown Davis told me that when I queried it many years ago. I just can't find their emailed instructions to confirm it. The Thor pump is bigger in diameter than the GEMS pump. That and the different seal design might explain why Brown Davis used a different locking ring design. I made up tools to tighten both design locking rings.

Here are the Thor (left) and GEMS pumps oriented as they would be in the tank. You can see the breather on the Thor is much further left and, if you refer to the earlier pic of both tanks, that the Thor pump sits much lower in the tank. Hence the need for an angled wall on the left side of the tank (as viewed above) to allow the breather hose to be fitted.

We have no idea why the Thor pump sits so much lower in the tank than the pump does in the GEMS tank.

Note the different float arm lengths. Why? Is the lower mounting of the pump to make the fuel gauge more accurate?

We had to remove the anti-rotation lug from the Thor pump. See it just below the breather. With it in place the pump won't sit down on the tank sealing surface.

Here are the standard Thor (on left) and GEMS tanks. They are similar but not the same. See, in particular, the pump retaining ring.

If you've ever wondered why it is difficult (impossible) to siphon from a standard tank, Scouse showed me why. The filler inlet goes into the tank then down near the bottom. At the bottom there is a flap, much like one puts on plumbing drains to stop vermin going up into the drain.

Here's a question about something that has been puzzling me. Look at the photo below and see how the Thor pump is mounted quite low in comparison to the GEMS.



Noting that the breather is on the fuel pump, how can the tank be completely filled once the breather is fully covered by fuel? Surely filling will slow or stop?

I can see we are going to have to experiment next weekend.

The small breather at the top rear is the roll-over valve. I disconnected the hose connected to it and tried blowing through the hose back toward the filler. It takes a fair bit of pressure to open the vent at the filler end so I'm not sure how well it would vent air when filling the tank. However, I cannot recall if I had the petrol cap off when I tried that.

I'll remove the petrol cap tomorrow and try it again.

FYI, below is the cover over the pump access hole on my P38A. As you can see, I used a bit of floor out of another (wrecked) P38A. I stripped back some of the underbody sealant on that panel and glued on a 25mm (approx) wide strip of closed cell foam around the edge to act as a seal on the joint.

Originally Posted by p38arover

The small breather at the top rear is the roll-over valve. I disconnected the hose connected to it and tried blowing through the hose back toward the filler. It takes a fair bit of pressure to open the vent at the filler end so I'm not sure how well it would vent air when filling the tank. However, I cannot recall if I had the petrol cap off when I tried that.

I'll remove the petrol cap tomorrow and try it again.

I tried it and there seemed to be no difference to the pressure required to blow through the line with the cap on or off.

The tank was under quite a bit of pressure when I removed the petrol cap

The hole in the floor and the access.

I've been thinking about how well it is sealed. I do the occasional river crossing and I don't want water flowing into the car.

I might redo the sealing tomorrow.

Originally Posted by p38arover

Noting that the breather is on the fuel pump, how can the tank be completely filled once the breather is fully covered by fuel? Surely filling will slow or stop?

The breather on top of the pump is to speed up fuel delivery by venting air from the tank to the top of the filler neck. Once this vent is closed off by fuel, air will need to escape along the filler neck itself, competing with the incoming fuel resulting in an early 'click off'.
The last 10-20 litres in the Thor tank will be slow as a result of the breather being lower down & blocking off earlier than the GEMS.

Well, we found out the tank can't be filled. Pete tried. So Pete and Scouse removed it today to have it modified. Why would an aftermarket tank manufacturer design a tank that can't be filled?

Scouse noticed in an earlier post in this thread that another tank (below) that was for sale on eBay had been modded. Note the blocked off breather on the pump and the extra breather pipe welded on near the filler.

Originally Posted by Scouse

That eBay tank has had a breather welded onto the side by the looks of it - there's weld residue around the fitting (to my untrained eye).