At 04:32 PM 1/26/05 +1100, Tony Clarke (in Australia) wrote:
"I have just bought a new petrol cap [Moss Motors 470-190] and noted that there are two washers supplied. The instructions state discard the 1-9/16 dished washer and use the smaller 1-7/16 one."

Fast forward four weeks. I have clipped out a lot of interim discussion and will get right to the historical facts and current conclusions. This investigation represents an interesting bit of history, and the problem is something for restorers to watch for, but it should by now be a non-problem for most of the world.

This particular part (with two splash washers and a note) was in supply around year 2000. Apparently there was some "perceived" problem with the larger splash washer (perhaps too thick) which caused some concern for lack of proper sealing for the spring loaded seal cup on the cork seal. For a "cure" at that time a smaller splash washer was added to the package along with a note to use the smaller washer in place of the larger one. There were only 180 pieces supplied in this configuration in year 2000. Today it is quite surprising that this part was still in inventory to end up in the hands of a customer.

As an additional curiosity, I have a 1986 vintage replacement part on my car which has the smaller splash washer. My best guess is that around 2000 some attempt was made to improve the design with a larger splash washer, and maybe it had some bad reviews. There is some indication that original factory issue at time of production of the cars was a larger diameter splash washer. The current issue part has a larger splash washer that is not as tall.

".... (mind you I don t go 100mph every day!)"

Well I don't make a regular habit of it (chuckle). The bigger problem is fuel sloshing around during spirited autocross driving when it will slosh out if the tank is more than half full.

"The cork gasket is in good order. What are your thoughts on the dished washer size I should use?"

It appears to me that the intention is for the dished washer to fit closely in the neck to obscure the opening as much as possible to prevent splash out. With a larger gap all around it might be nearly worthless as a splash guard. Fuel splashing past the first washer can escape through the center hole of the spring loaded cup washer around the center guide pin. The larger closer fitting washer should be a fairly effective splash gaurd, allowing only minimal liquid to get past with a sudden splash, and still allowing it to drain back down after the splash (never a perfect air tight seal). As long as the larger one fits and works I'd say use it. Just be sure it's not too tall so as to keep the spring loaded cup from contacting the cork seal.

I will mention this issue to the folks at Moss Motors, US, to see if they have any input.

At 01:33 PM 1/27/05 +1100, Tony Clarke wrote:
"I ordered it from Moss UK Bristol branch. .... I have sent an email to the Moss manager there who .... asked me to return the goods or some pix for inspection."

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Response from Moss Motors Ltd, US, Feb 14, 2005
From: "Grant, Michael" <mgrant@mossmotors.com>
      I am perplexed by the description of the MGA cap that may have come from Moss. We make these, and have for years. The caps come fully assembled. As far as I can tell they always have.
      We supply Moss Europe, and have since at least as far back as Nov 1999. I will ask Moss Europe if they have ever filled in from anyone else. In the meantime the Moss Europe Invoice number might be helpful.
      I am sending you a new cap and a cork seal for your evaluation. Put it on the car and see what happens.
      I have begun a product report and will share the findings when I'm done.
      I have checked and in a period of time where we sold over 1300, I show 3 returns that are related to the product. [Nothing about this problem].
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Continue, Feb 15, 2005

I have received the larger splash washer from Tony Clarke. It does indeed fit closely in the filler neck, and it sits solidly on top of the inside ridge so it could not fall through into the tank if dropped. When the spring loaded sealing cup is placed on top of the large splash washer there is ample space beteween the two parts, so the splash washer is definitely not keeping the cup from sealing against the cork. So far so good.

But now I have discovered another problem with my installation, perhaps not a design problem but some service damage and misalignment. I noticed that the hinge tab securing the cap to the neck was bent inward to the point where it interfers with the edge of the spring loaded sealing cup. It appears that the cap has at some time been forced too far open, which has resulted in the hinge tab being bent also forward, toward and even beyond the plain of the underside of the cap. When the cap is closed the hinge side of the cap then sits too high, and the sealing cup may not contact the cork seal. This would have allowed fuel to easily spill out, especially with the smaller splash washer inside.

So I removed the internal parts and bent the hinge tab down more securely, much closer to the outside rim of the cap, and also downward to be completely inside of the plain of the bottom edge of the cap. Having done this, the hinge tab will no longer touch the spring loaded cup, and the hinge side of the cap will close much lower and closer to the cork seal. The cap now opens only about 90 degrees, and I will have to get used to the new open position. Forcing it open any farther would only bend the hinge tab up again to cause the original misalignment. So be careful with the fuel filler nozzel at the gas station, as 90 degrees opening is all you should have.

With the hinge more properly aligned, the spring loaded cup comes into contact with the cork seal (at the hinge side) before the latch tab touches the edge of the neck. When I hold the latch tab up and press on the cap it is obvious that the spring loaded cup is pushed securely into contact with the cork seal all around. The larger splash washer now installed also moves clearly inside of the filler neck, so it appears that it will be nicely effective at reducing fuel splash.

Now I should check the calendar for the next autocross date to give it a good workout. This would be delightful if I can now hot rod around with a mostly full tank without slobbering fuel on the body paint.
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Epilog, Feb 22, 2005

I have received the new MGA fuel cap from Moss, US. I have installed it, and it seems to fit and work fine as is. The splash washer is larger diameter, very close to the dimensions of the one Tony Clarke sent me from Australia, the one originally fit to his MGA from many years past, possibly original factory issue. There is no small washer with the new Moss fuel cap, and rightly so. Apparently the instruction Tony Clarke got with his replacement part from the UK is in error. Or perhaps the larger splash washer supplied at that time may have been too tall. I do not yet have the exact thickness of the larger washer from the "dual washer" fuel cap kit.

The larger (old) center disk Tony Clark sent me is .245" thick and 1.615" diameter. The new center disk with the new Moss cap is only .180" thick and 1.57" diameter. I suppose either one will work about as well, since they both sit below the rim of the filler tube, both fit inside of the entrance to the filler tube, neither one falls through the ridge apperature, and neither of them extends to touch the inner ridge.

The only potential problem I see is if the hinge tab is not properly crimped down. If it was very loose (not crimped at all) it could interfere with the seal cup. If the heal of the lid is left a little too high it might not bring the seal cup completely into contact with the cork seal, but that doesn't seem to be a problem when the hinge tab is properly crimped.

Users should exercise care in handling, not to force the cap open too far when loading fuel. If the cap is forced open beyond about 90 degrees the hinge tab can be bent up out of the plain of the cap bottom edge. This would allow the heel (hinge side) of the cap to sit too high, and the seal cup may not sit down firmly on the cork seal.