What's the difference between the S3 and S5 sender?

How to describe a sender?

The senders are normally described by the type of head / - / resistance value / length.
For example; an S3-E400 has an S3 head, 'E' for European resistance.  'A' would be for American resistance (S3-A400) and 400mm long body (from top of thread to end of stem). An S5-A250 would have an S5 head, US spec, 250mm long.

What are the differences between the Wema S3 and S5 senders?

The S3 & S5 senders are the same except for the fitment on the head.  

Both are made from 316 stainless steel and both are suitable for use in Petrol (Gasoline), Diesel, Water, Grey Water & Black Water. 

The S3 has an 1" ¼ BSP thread and screws into the top of the tank.

 

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If the tank doesn't have a thread then a fitting flange can be bolted to an existing hole (holes match most standard drill patterns).

The S3 is supplied with two O-rings that locate in the underside of the sender head.  

 

• 
  The sender is simply screwed in and nipped tight.  The O-rings will make a liquid-tight seal.  Be cautious not to over-tighten the sender.

 The S5 has 5 holes to bolt the sender to the tank.

 

• The S5 is supplied with one large flat nitrile gasket to ensure a liquid tight seal.

• The S5 sender is simply placed through the hole in the tank and 5 bolts are screwed through the holes in the sender head and into the corresponding holes in the tank.  The drill pattern is a standard SAE5-hole.