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MG Midget and Sprite Technical - Austin A40/ Frogeye

Good morning all. I have an opportunity to purchase a very good condition Austin A40 948 engine and my enquiry is what do I need to do to it to enable it to be the same spec as a 948 Frogeye engine. I am possibly aware that the cylinder head had double valve springs and different valves, as regards the block I am not sure if the camshaft was different but have seen somewhere that the oil filter bowl assy was slightly different being at a different angle. I'll await to be enlightened. Thanks.
D Sinclair

As far as I understand it, the cylinder head itself is different. It also has a larger inlet valve, as well as the double valve springs.

I can't find my A40 manual, so I've only been able to look at Morris Minor valve timing figures, which are likely the same:-

The camshaft is different, as the exhaust valve has an extra 22° of duration (9.6%).

The A40 engine may just have one camshaft bearing, on the front journal. The other two running directly in the block. I think some of the later A40 engines did have three bearings.
Dave O'Neill 2

Afaik it will be no different from the Frogeye engine (9CU)
Double valve springs and three camshaft bearings were introduced with the 9CG engine which also saw the intro of the 630 camshaft.
The Frogeye engine has externally a spacer between the engine & filter that angles the filter toward the opening in the chassis

The earlier engines can be line bored for the fitting of cam bearings
A duplex camgear is another useful upgrade.
If you bore the block it can accommodate later 998 mini pistons with the latter conrods..

The Morris Minor 948 shared many parts and these are often much cheaper from MM suppliers than from Spridget suppliers.

I run a 948 (1046) in my Frogeye that I have now owned since 69.

A useful upgrade to the Frogeye braking system is to convert to the A40 farina front 8” drums using the A40 backplates and slave cylinders.



Alan Anstead

Alan has given a comprehesive reply.!!

As regards engine numbers the early Mk1 A40 had engine prefix 9A or 9D the Mk11 A40 had engine prefix 9DB. They all had single valve springs the same as the Frogeye (9C engine) The Frogeye had stronger single valve springs to allow higher revving, with stellite faced exhaust valves and harder copper/lead main bearing shells. Valve timing 5-45 40-10 with .285" lift was the same for all the engines.
Bob Beaumont

Following what Alan said, East Sussex Minors carry a good stock of Morris Minor parts which may be applicable to Spridgets. Prices are often lower than the other well-known suppliers of BMC parts.

http://www.morrisminorspares.com/

Everything I've ordered from them has arrived quickly without paying extra for a speedy delivery. The only drawback is that they have their own parts numbering system, so searching on part numbers is only partially successful.
C Mee

Lots of sales businesses seem to use their own parts numbering systems. I have always thought it an attempt to discourage using their site as a part number index and then going to a competitor for the purchase. Maybe its just more straightforward than that!
GuyW

This thread was discussed between 30/03/2025 and 31/03/2025

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