Diesal lock confusion

[Poppin]

How would you go about locking this type of cap!?

[Alan de Enfield]

 

 

Is yours a Reeves boat !

Edited January 4 by Alan de Enfield

[Troyboy]

Drill straight through the cap and fit a long shackle padlock. 

[Poppin]

35 minutes ago, Alan de Enfield said:

 

 

Is yours a Reeves boat !

Colecraft 

24 minutes ago, Troyboy said:

Drill straight through the cap and fit a long shackle padlock. 

I thought that, but water ingress? Surely rain would get in

[MtB]

3 minutes ago, Poppin said:

Colecraft 

I thought that, but water ingress? Surely rain would get in

 

I doubt any diesel tank is totally free of water under the fuel. A little bit of rain added to the litres of condensation in there won't hurt much.

 

 

[pearley]

I had a chain welded to the cap and padlocked to the fender chain.

[Troyboy]

Provided you ensure the padlock is a tight fit in the hole you drill no rain water will get in . As Mtb has replied there will already be condensation in your diesel tank and I suspect you also have a water trap in your fuel line . There will be some swarf when you drill the hole so some grease on the drill bit will help catch some of that or a magnet placed on the upstand pipe . Any you don't catch will settle at the bottom of your tank and won't be worth bothering about. 

[Tony Brooks]

Drill and tap the cap, tighten it down, drill a dimple through the hole and use a security screw to prevent the cap being easily turned.

[PeterF]

Here is the set up on mine, holes in the welded on lugs for a padlock.

 

[MtB]

On 04/01/2026 at 20:55, PeterF said:

Here is the set up on mine, holes in the welded on lugs for a padlock.

 

 

 

The usual way a thief uses to defeat this type of lock, or so I've heard, is to unscrew the air vent fitting nearby and put his syphon tube into your tank through that instead.

 

 

[magnetman]

You could get a hex cap and do it up really tight with a stillson or large adjustable spanner.

 

That way the person considering nicking the fuel would need to have a large tool with them. They might have one but they may equally have come with the assumption that they can remove the cap without heavy tools or knock a padlock off with a hammer.

 

 

 

 

Sometime will correct me if I am wrong but I think these are 1.25" BSP caps. 

 

Stainless steel hex cap £12.

 

https://www.ebay.co.uk/itm/125786757186

Check the exact across flats size then get a big spanner.

 

If you routinely keep a 50mm spanner hanging inside the back door this is a valid self defence tool as it is there for another reason. 

 

This one is tasty 

https://www.ebay.co.uk/itm/157595951524

 

Or a British Rail brake pad spanner

 

https://www.ebay.co.uk/itm/205334908058

[Chris John]

How often is fuel being stolen an actual thing ?

IMO if there are a couple of boats together and one has at least some sort of lock in it and the other doesn’t they’ll go to the other one 

Edited January 9 by Chris John

[magnetman]

I suspect it comes up on facebook now and then and their algorithms promote the story like mad for hand wringing and people who have aintitawfulitis and thus need to winge about the state of society. These algorithms will be suggesting it's happening all the time when in reality it is isolated incidents.

 

 

[David Mack]

7 hours ago, MtB said:

The usual way a thief uses to defeat this type of lock, or so I've heard, is to unscrew the air vent fitting nearby and put his syphon tube into your tank through that instead.

 

 

The joy of having a historic boat, where both filler and vent are inside the engine room!

[Tony Brooks]

30 minutes ago, David Mack said:

The joy of having a historic boat, where both filler and vent are inside the engine room!

 

Or one with a right angle bend in the filler and breather pipes that enter/exit the tank horizontally.

[magnetman]

I once had a arrow with twin floor mounted tanks which had fillers inside the side doors and vents rotted out the side of the hull. A good design. Also had another vessel with the right angle pipe. 

 

It would be a lot of cutting and welding work but another good one would be a baffle plate below the filler and vent like a weed hatch. 

 

 

[Alan de Enfield]

Or just buy an anti-syphon device - 100s of variations, sizes and prices.

 

 

Shop Fuel Defend 49mm Ultraplus Internal Fit Anti Siphon Device

 

Or, simple and cheap

 

Amazon.com: Stanco GBB-01 Gas Bandit Blocker - Fuel Anti-Siphon Device 2-Pack : Automotive

 

 

 

[magnetman]

Looks easy to break. 

 

 

[Alan de Enfield]

2 minutes ago, magnetman said:

Looks easy to break. 

 

 

 

Not once you have 'screwed it' deep down in your hose (using a long shanked flat blade screwdriver)

[magnetman]

18 minutes ago, Alan de Enfield said:

 

Not once you have 'screwed it' deep down in your hose (using a long shanked flat blade screwdriver)

Generally speaking a narrowboat tank as shown the OP has no hose is just a small BSP nipple welded on the deck with a screw cap on it. 

 

 

Beneath this is the tank so one of those screw devices could simply be pushed into the tank with a stick. 

[MtB]

30 minutes ago, Alan de Enfield said:

 

Not once you have 'screwed it' deep down in your hose (using a long shanked flat blade screwdriver)

 

Not when the stern block IS the fuel tank. It would just fall straight through the 2" long filler tube and down into the bottom of the tank! 

[pearley]

7 hours ago, magnetman said:

Looks easy to break. 

 

 

And would stop you using a dipstick or sucking out any water.

[blackrose]

On 04/01/2026 at 18:16, Troyboy said:

Drill straight through the cap and fit a long shackle padlock. 

 

Sound like that might allow small amounts of water that hit the padlock shackle to in go through the holes.

[MtB]

18 hours ago, blackrose said:

 

Sound like that might allow small amounts of water that hit the padlock shackle to in go through the holes.

 

Point aready discussed, upthread.

 

 

[Llamedos]

I put a post up with my solution to this one last year.  Here it is… Locking Fuel Cap