[41Sqn_Banks]

Originally Posted by Crumpp

They did rush it!!'

If we take January 1942 as the time the conversion for Spitfire Mk I's was complete that represents about two years and four months between initial flight test and 100% ground operational adoption.

Well, if conversion was completed by January 1942 it wasn't done by converting engines but by phasing out the remaining aircraft (if there were any, what I seriously doubt) from operational service ...


The Merlin V was 100 octane only (according to AP1590B, A.L. 4 November 1940). Compare the differences between Merlin II, III and V here), there is no essential difference. This is also confirmed by AP1590B, A.L.4 November 1940 and the fact that only amendments to AP1590B were required to cover the new engine by the same manual (AP1590B without amendments only covers II and III).


Merlin XII, XX and 45 were all 100 octane only. Compare the cylinder block of these engines, which show the largest differences between those marks:

Merlin II: D.8985 and D.8986. One-piece with narrow inlet ports. Ribbed liner with 0.15 in. spigot. Radiused top joint ring 0.08 in. thick. Unshrouded. May be modified to receive rectangular-sectioned top joint ring 0.100 in. thick. Partially shrouded.

Merlin III: Early type engine as Merlin II. Later type engine as Merlin X; still later type engines as Merlin XII.

Merlin X: Early type engines fitted with D.11521 and D.11522 blocks. One-piece, with wide inlet ports. Alternative types ribbed liner with 0.15 in. spigot. Rectangular section top joint ring 0.100 in. thick. Partially shrouded. Later type engines fitted with Merlin XII type block.

Merlin XII: D.13256-1 and D.13257-1. One-piece, with narrow inlet ports. Ribless liner with 0.20 in. spigot. Radiused top joint ring 0.080 in. thick. Fully shrouded.

Merlin XX: As Merlin XII.

Merlin 45: As Merlin XII, but later engines have front and intermediate camshaft bushes deleted.

It's obvious that the early engines matured into the XII and the later engines were based on the XII design and most parts were interchangeable.
All these engines were in service by January 1942 and all of them used 100 octane only.

According to Crumpp RAF hesitated to "rush" the introduction of 100 octane fuel for an engine that they started to replace in autumn 1940 with similar engines that were using 100 octane fuel.

Of course Crumpp will now claim that only late production Merlin III which were very similar to Merlin XII were cleared for 100 octane fuel and will take this as a proof for his January 1942 theory. However this will ignore the fact that the Merlin V used 100 octane only by November 1940 and that it was identical to late production Merlin II and early production Merlin III.