Wet Sumping - Why It Happens and How to Stop It
- Andy Marks
- Mar 23
- 5 min read
Wet sumping is a common issue on classic motorcycles where engine oil drains from the oil tank into the sump while the engine is not running. This guide explains why it happens, how different types of oil pumps affect it, and the practical solutions available to prevent it, from maintenance and riding habits to anti wet sumping valves and oil line taps.
What Is Wet Sumping and Why Do Classic Motorcycles Do It?
Wet sumping is when engine oil slowly drains from the oil tank into the sump of an engine, which is meant to be dry, while the engine is not running. As well as the inevitable oil leaks, it can make the engine 'heavy' to turn over leading to difficulty starting. Once you get the engine running, it will cause excessive smoking until the oil has been pumped back into the oil tank and can even result in oil being blown out of engine breathers.
Wet sumping tends to be a slow process, happening over several days or even weeks. When our bikes were new and ridden to and from work every day, wet sumping wasn't really a problem because every time the engine is run for a few minutes, you effectively reset the clock on it. Now that our bikes are often parked for weeks, or even months, at a time we're suffering the effects of gravity.
Why Does Your Bike Wet Sump?
In order for the oil to make its way into the sump, it must pass the oil pump. The rate at which this happens is dependant on the type and condition of the oil pump.
Plunger Oil Pumps
If your bike uses a plunger type pump you are unlikely to suffer with wet sumping as long as the pump is in good condition. If you are, we would advise you to overhaul the pump before looking at other options.
Gear Oil Pumps
If you have a bike with a gear pump (Norton oil pumps are one example) you are likely to have trouble regardless of the condition of the pump. There has to be a clearance, no matter how small, between the two gears, and between the gears and the side walls - this makes room for the oil to pass. You will still find that the more worn your pump is, the worse the problem will be and we would always recommend that you ensure your pump is working properly before addressing wet sumping issues.
Wet Sumping Solutions
As you'd expect with such a prolific problem, there are many solutions out there - let's take a look at a few anti wet sumping devices which are currently available.
Built in solutions
Some bikes already have them built in. Good examples of this are Triumph and Ariel who fitted oil pumps, from new, with a spring loaded ball valve which prevents wet sumping. If you're having wet sumping problems with a bike which has one of these fitted, the pump needs some attention.
Velocette used a different method. They fitted a spring loaded anti drain valve to the bottom of the oil tank. When fitting one of these it is important to prime the oil system properly as the valve relies on the oil pump to suck the oil through and any air bubbles in the system can cause havoc.
Start the engine regularly
As we mentioned earlier, wet sumping was rarely an issue 'back in the day' when the bikes were used every day. If you start and ride your bike most days, or even every weekend, you might not have a problem. There are downsides to running engines for short periods of time so we wouldn't recommend this purely for anti wet sumping purposes.
Drain the oil when you're parking the bike
We're getting into 'Captain Obvious' territory now. Drain the oil and store it in a sealed container until next time you use the bike so you don't end up with it leaking out all over the floor, or filling an uncovered drip tray up. OK for something like a race bike, or a vintage bike that is rarely ridden, but a nuisance if you want to be able to jump on the bike and go for a ride without any hassle. It's also another job to do every time you get home.
Fitting a spring loaded ball valve
We have already mentioned the anti drain valves fitted to Velocettes. They can be made to fit other bikes but be very careful! They rely on the pump being able to suck oil. A lot of pumps, Norton is a prime example, don't do this very well. You also run the risk of air locks which will result in no oil going round the engine.
The timing gear in the photo was taken from a Velocette engine after a faulty anti drain valve wasn't detected in time.
Fitting an on/off tap in the oil line
Firstly - please don't use a petrol tap for this! They significantly reduce the oil flow which is the last thing we want to do. A plumbers type ball valve offers far better flow for the size of tap and as such is the most common aftermarket fix we see. It's very simple and very effective. When the ball valve is switched off you won't have any wet sumping problems at all. The only thing you need to do is remember to turn the tap on before every ride. I think most of us know someone who has forgotten to do this so we don't need to go into what happens - safe to say it's a costly mistake!
You can write 'OIL' on the speedo glass or put a tag on the handlebars. But you only need someone to ask you a question at just the wrong time and you're in trouble. We've seen a lot of reminders used but once people get used to them the effectiveness somewhat disappears!
Kingpin Component's Anti Wet Sumping Solution
This brings us onto our solution. To overcome the risk of forgetting to turn an oil tap on, we developed a ball valve with a built in cutout switch. It offers all the advantages of the aforementioned ball valves, it's compact, offers good oil flow and can be used on any bike or pump type. Where it differs from a straightforward ball valve is with the addition of a microswitch which can be wired into the bikes ignition system. If the oil is switched off, there will be no sparks and the bike will not start. You might look a bit foolish kicking like mad and getting nothing, but you'll soon realise what you have done, turn the tap on and away you go - feeling very relieved that the bike didn't start and the tap did its job.
For more info on our oil taps take a look at the following pages:
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