"When I set up Model Enthusiasts in 2004 the idea was to find out
what people with a Mamod engine needed to get the
engine running as it should.
That's still true today and you can see our catalogue here.
Trevor. 

 MAMOD SIGHT GLASS ASSEMBLY

 WITH RIVETS

Just a bit of advice:

You want to avoid the rivets spinning as you are drilling them out. When removing the old rivets the aim is to skim off the top of the rivets and to then punch (gently as it's essential the boiler end is not bent) what remains into the boiler.

Rivet size is 3.2mm 

 Use a sharp drill bit and light pressure. You may need to place a thin screwdriver under the rivet to keep it from spinning. Stop drilling when the rivet top is removed.

Now you have reduced the rivet down, pick off the brass sight glass cover and lightly punch what is left of the rivet into the boiler for subsequent removal by tipping out. 

When drilling out the rivets, put the absolute minimum downwards pressure in an attempt to stop the rivet from revolving. 

The good news is that normally everything goes OK. Phew...

This Mamod Sight Glass Assembly is produced by Mamod Ltd and is the type introduced when they changed from the Water Level Plug method. 

When running your engine it is good practice to keep an eye on the water level - so you are not caught out by a dry boiler and a melted sight glass window.