Why is there rust in oil tank? and how do I clean it?

[DerekAJS30]

I washed out my oil tank with water and detergent to get rid any grot. Emptying into a white sink I noted a lot of “rust” and “grit”. I persisted with vigorous washes, but it kept coming. I decided to try Vinegar with some salt added to remove any rust. I filled the tank and left it for two days. I emptied it and a mountain of grot came out. I’ve washed it again many times with detergent and water with vigorous agitation but still it comes. So now it's on its second vinegar soak.
BUT my point is: why would a tank that has spent its life almost full of oil have rust in it ? Is it water vapour in the oil from engine operation? and how do I guarantee the tank is clean and grot-free before I reconnect to my now hugely expensive rebuilt engine?

[DerekAJS30]

FYI - AJS model 30 - tank actually looks in good condition except for the isses indicated.

[SPRIDDLER]

BUT my point is: why would a tank that has spent its life almost full of oil have rust in it ?

In an earlier thread you mentioned that the bike is new to you so anything could have happened to the tank.
Condensation will form in the tank when the oil cools which will collect at the bottom of the tank, below the oil.

Cheap Cola and a string of nuts shaken around in petrol tanks is pretty good at shifting rust.

[g80csp11]

Generally oil tanks are full of sediment that has settled over the years from the oil , always a challenge to clean out . just don bead blast as the beads are even more difficult to remove

[DerekAJS30]

As an update, the second 3 day soak in vinegar with a tablespoon of salt removed a lot more grot. There was initially a lot of rust etc coming out but after 15 vigorous washes it looks pretty clean. That said, that's what happened with the first wash. Since time is not an issue i'm going to try a third soak and see if we can see a significant reduction. I'm also lookong into potentially inserting a stainless threaded bar into the tank with Samarium Cobalt ring magnets spaced along it to see if that can remove any residuals when the bike is running. I might even try flat bar Neodium magnets attached to the outside to magnetise the lower portion of the tank. Neodinium not so great at high temperatures though. Work in progress and still researching and thinking it through.

[jackstringer]

Why not just braise in a collar that can take the thread similar to that of a magnetised sump bolt that can be bought for a few quid?

Be careful of going mad with Salt and Vinegar as that is a acid at the end of the day and could cause a lot of rot. I would be using some gunk to remove grease and some Vaporust to turn the metal into something more stable and then try some tank sealer on it. If the tank is as rough inside the crap will only build up again and you will be back to a tank full of crap.

btw buy one of those inspection camera things so you can see inside the tank etc.

[Invicta]

I had a similar issue with rust in the oil tank on my 1950 model 18. Whikle cleaning it out it sprang a leak. It had rotted right through from the inside. I bought a replacement tank from someone in the Jam Pot , but when I cleaned that out it was even worse it was totally rotted from the inside. If your oil tank has that much rust and crud in it I would certainly go down the tank sealer route once you have it cleaned to bright metal inside. There are actually microbes that can live in oil. Maybe it was these that somehow caused the corrosion.

[Brian 54G80S]

After cleaning using most of the methods already described above, I went the extra step and lined the tank with one of the liquid tank liners readily available. I used a product called KBS Gold Standard Fuel Tank Sealer but I'm not sure that product is available in the UK ( I'm based in Australia) I know for sure you have similar products available. The kits generally have cleaning, rust removal and sealing products included and have comprehensive instructions. I actually had pin holes rusted through my tank and this process sealed them completely and the tank is 100% OIL tight.
The one tip I would provide is that you need to protect any threaded bungs from the coating. It's almost impossible to remove once dry.