Everybody has their own opinion on this subject. The most experienced people will tell you that the best way to maintain your engine performance is to accept the fact that valve jobs need to be done 2 or 3 times more often than rings and bearings! And this data base was concluded when leaded gas was the norm! Now that unleaded is all you can get, valve jobs are needed even more often!
The purpose of a valve job is two fold; first the valve seats. The impurities in the air and gas often get trapped between the valve and the seat in the head; under worst conditions stopping the valve from closing, but more commonly being smashed enough to allow just a little seepage of hot gases. The valve is dependent upon intimate and total contact with the valve seat to dissipate the heat; anything that prevents this contact will allow the valve to heat up and eventually burn!

The second reason for a valve job is the valve guides. The problem with unleaded gas not only affects the valve seats, but just as significantly the guides; the lack of lubrication in the unleaded gas causes excessive wear. Most original and early cars have steel guides, which might last longer but can wear the valve stem. Bronze guides will not last as long, but stem wear will be greatly reduced.

Another factor in the need for more frequent valve jobs is the increasingly common use of high performance camshafts. High performance cams achieve their objective by increasing valve lift, among other factors, This is great, but the stock rocker actually pushes the valve sideways as well as down and the higher the lift the greater the side loads on the stem and guide. Every performance gain has a price, but this high performance penalty can be offset with the use of modern roller tipped rockers! While on the subject of rockers it is important to note the condition of your existing rocker shaft and arms. When these parts are worn, setting the valve clearance is very difficult and if you run the engine with poorly adjusted valves not only will you lose horsepower but you can also burn a valve. The very early rocker arms do not actually have any sort of bush in them and must be replaced when worn; these are typically the pressed steel type which are not very strong in the first place. The most common rocker used to be the forged version; these have bushes in them and can be used on any and all size motors. The latest design is a cast sintered iron that is very strong and now easy to find as complete assemblies.

Casting #Inlet sizeExhaust sizeChamber capacityNormally fitted to:Also fits
2A628 1 1/16" 1" 24,6cc 850 Mini, A35,A40 998,1100 and Cooper
12A1456       Minor, SpriteMkI  
2A629         round port on intake
12A1458 1 1/16" 1" 24,6cc late 850 &998 as above but a hole in the front for temp gauge
12G1851 1 1/16" 1" 24,6cc 997 Cooper fits as big valve head for 850 & 998
12G202 1 5/32" 1" 26,1cc Austin/Morris 1100 As above but square formed inlets
12G206 1 7/32" 1" 28,3cc Early MG 1100 upgraded 12G202, square intake ports with location rings
12G295 1 7/32" 1" 28,3cc 998 Cooper,MG 1100, Sprite MkIII Fits on 998 when bigger vales as needed
12G940 1 5/16" 1 5/32" 21,4cc 1275GT,A/M1300 Can be modified to the specifications below
9 Bolts 33,3mm 29,53mm   Sprite MK IV, Marina 1,3  
12G940 1 13/32" 1 5/32" 21,4cc Austin 1300 GT Fits on 1275 Gt when bigger valves are needed
11 Bolts 35,6mm     MG 1300 MK II, late Cooper S  
AEG165 1 13/32" 1 7/32" 21,4cc Cooper S Can be used instead of the 11 bolted 12G940
12A185         12A185 was early casting coded yellow AEG165 is later colour coded brown or blue
12G940 1 5/16" 1 5/32" 21,4cc Turbo MG Sodium cooled exh. Vlaves & improved cooling but thinner deck
CAM4180 1.0625" 1" 25.5cc 998 Mini A+ and lead free