Air Outlet Types

by C. Shillito

A characteristic that is often referred to when describing/classifying Churchill Tanks is the size of the air outlet at the rear of the tank. This is classified as either 5 inch or 8 inch a change that was introduced as part of the rework scheme and thought necessary to improve engine cooling by increasing the size of the outlet. But how was the air outlet increased and how can we recognise if a vehicle has a 5 inch or 8 inch outlet?

 

5 inch Outlet

The Churchill as originally designed had an air outlet as shown above. The shade areas represent armour plates at the rear of the vehicle. The air exited the engine compartment through two ducts one directed upwards and one downwards. The ducts themselves were each secured to the rear armour plate by three pairs of bolts
This excellent photo by Bob Grimster shows the 5 inch air outlet of the MkI at Borden. You can clearly see how narrow the air outlet duct is and can see clearly two of the three brackets that hold it to the outer armour plate. You can also make out the angle iron to which the outer plate is riveted.
To the right we see how the mounting for the 5inch outlet appear on the rear plate of the tank. The bolts circled in red are those that hold the inner ducting in place. The rivets circled in blue are worthy of note for later comparison with athe 8 inch outlet.

 

8 inch Outlet

The modification form 5 inch to 8 inch outlet consisted of removing the upper inner armoured plate and reposition the rear armoured plate an inch or so further back. The ducting was removed altogether and instead metal louvers were attached to the rear late and lower inner plate. The upper louvres being held in place by 5 bolts
To the right is a top view of an 8 inch air outlet showing clearly the louvres and some of their securing bolts
As this view from the engine compartment shows, there is very little to restrict the flow of air to the outlet and this must have represented a vast improvement over the ducted 5 inch outlet.
This photograph shows how the rear plate has been moved backwards and actually overlaps the end of the side plates.
This photo by Bob Grimster of Aberdeen Prooving Ground's MkIII shows the rear view of an 8 inch outlet. Circled in red are the bolts the secure the upper louvres. If you look closely you can also make out the corresponding bolts for the lower louvers (just below the rear plate). Circled in blue are the rivets that hold on the rear plate. Note how they differ from the arrangement used on the 5 inch outlet.
Indeed there does seem to be some variation on the number of rivets used on the rear plate. This MkII ARV has five compared to the three used above
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Unless otherwise attributed, all photos and diagrams Chris Shillito