Is it something like this that you are after:

https://www.ebay.co.uk/i/122930544038?chn=ps&dispItem=1&adgroupid=54682298551&rlsatarget=pla-466931251647&abcId=1133906&adtype=pla&merchantid=7463084&poi=&googleloc=1006490&device=c&campaignid=1058607774&crdt=0

Phil.  This was measured on a C15 - I'll measure my B40 tomorrow which might be different. The 1/2" is what I would consider to be the minimum height for the bung although you might get away with 1/4". I assume the mountings for your B40 have the correct rubber buffers?

Phil. If I was you, I would work out what the minimum length is for the long spacer to project at least 1/2" above the bottom of the central tank tube. This is the minimum height you need to achieve for the bottom of the rubber bung. This measurement will then tell you how much shorter you can make the long spacer and whether the bolt is actually long enough. I'll also have a go and check the bits for my C15 are ok.

I've cleaned up a lot of tanks and the most difficult have been when someone has used a sealant on top of rust - this seems to happen a lot and some of the older types of sealant can be difficult to spot. It's always best to start by trying to remove as much of the rust or crud as you can by giving the tank a good shake with pebbles or nuts inside followed by a good wash out with soapy water or other solvent to clean and degrease it, followed by a good rinse with a hose pipe.

Since you can't always spot the presence of sealant, you sometimes need to try a rust removing liquid first to see how effective it is. With the liquid I use, I normally have to leave it in the tank for up to a week. If there is no sealant present, the liquid will remove all the rust. If it doesn't, then there is probably sealant present.

I remove sealant with paint stripper after protecting the tank if necessary with cling film. Depending on what sealant has been used, this can take a number of days to dissolve and it can be difficult to tell when it's all gone. Sometimes you can only tell when the sealant has gone after re-applying the rust removing liquid and seeing whether it completely removes the rust. If not, more paint stripper might have to be used followed by another stage of rust removal.

Once all the rust has gone then the use of POR-15 sealant is very effective.

Normally, rust removal from inside the tank is straightforward unless sealant has been used. If it has then it has to be removed before de-rusting. I've got a similar tank to do and the best way to protect the tank during the process is to cover it in cling film. I will then use strong paint remover poured and swilled around the inside the tank to dissolve the sealant. Sometimes it can take quite a bit of effort to remove the old sealant but has to be done if you want to properly de-rust it. Swill out when necessary with plenty of water using a hose pipe.

To simplify the understanding of the way the wiring works, pretend that the ammeter has been removed and all 3 brown and white wires that connected to the ammeter now connect straight to the battery -ve terminal. This means that there are now 4 wires connected to the -ve terminal if you include the brown and blue wire to the horn and stop light.

Of these wires, current always flows from the rectifier to the battery (it can't flow the other way) but it then flows out of the battery along all the other 3 wires to power the lights, horn, coil, etc depending on what is switched on. In each case, the amount of current flowing in each wire depends on the battery voltage. The power for the lights goes to lighting switch T3 and the power to the coil goes to ignition switch T12A.

The amount of current that has to be provided by the battery is then the difference between the amount of current flowing out of the battery in 3 of the wires minus the amount of current flowing in from the rectifier. If the battery receives more current than it is being asked to provide then this excess current will 'charge' it. When the battery is being charged, you can then view the current being drawn to power the lights, coil, horn etc as coming primarily from the alternator/rectifier.

Unfortunately, there is no regulator built into the wiring harness and, even if the battery is fully charged, the excess current from the rectifier will continue to flow into the battery; in the worst case, cooking it. In order to match the alternator/rectifier output current with the amount of current that needs to be provided to the lights, horn, coil etc, the lighting switch also controls how the alternator coils are used. Thus, when the headlights are on, the full output (3 pairs of coils) of the alternator is used whereas, with no lights on, only 1 pair of coils is used.

All the ammeter does is to show the combined current flow in the 3 brown and white wires so that you can tell when the battery is being charged and by how much.

The rectified (DC) output voltage from the alternator and the wiring harness for the headlights, horn, stop light, coil, etc are all effectively connected together at the battery -ve terminal. The way the system works depends on the instantaneous battery voltage.

If the engines revs are high enough, and the rectifier output voltage is greater than the battery voltage, current will flow into the battery from the rectifier. At the same time, current will flow out of the battery to supply the wiring harness load demanded at that time depending on what is switched on. The actual current flowing out of the battery will therefore be the difference between that demanded by the load and the current being provided by the alternator/rectifier.

At lower revs, more current may flow out of the battery than the rectifier is providing but, hopefully, over a period of many minutes riding, there will be a net average flow into the battery and it will maintain its charge.

Well done in managing to extract the valve. I've never had the need to remove one before but I also ordered one earlier just to get a better idea of what it actually looks like.

In terms of the wheels, it's almost impossible to smarten them up once they start to rust. However, they are expensive to replace. I recently paid £375 to Central Wheel Components for a set of S/S rims and spokes for a C15 - getting them to do the rebuilds would probably have added another £180.