Valve guides - Materials and usage 

Material choices for guides are down to two distinct types - cast iron and bronze. Yes, cast iron. NOT steel as described in many adverts and by vendors. Never have been, never will be. These are as fitted to the various A-series cylinder heads as standard in all applications. Cast iron is used because it is a very dissimilar metal from any used in valve manufacture - important to eliminate galling that causes seizure of the valves in the guides - is softer, yet resilient enough to wear well. Bronze, on the other hand, is a very general description as there are a variety of 'bronzes' used by various folk. In days gone by, the bronze base type used was 'PB1' (also known as 'Navy Bronze') - a phosphor-bronze alloy that was very orange in colour, and very soft, used primarily in race engines. Unfortunately they wore out very quickly. Silicone-brass content bronze is probably the most popular now - often with a high aluminium or manganese content - is very yellow/gold in colour; modern technology facilitating very controlled manufacture of these specialist alloys. 
Valve materials are very varied, but essentially there are three groups. The first is the old-style standard road spec, second up-rated standard road spec, and lastly race spec. The material types for both road spec valves are both complicated and irrelevant. All you need to know is that the early types had plain finished stems and seats because leaded fuel was very kind to them, the up-rated (or modern) ones have chrome-plated stems with triple-material heads giving a super-hard seat area - both to improve longevity, especially in later years where unleaded fuel is used. 
Race spec valves are almost universally EN214N stainless steel. Despite what folk have been lead to believe, they are generally not as hardwearing as the standard type valves. This material is used because it is easy to form and machine yet resilient enough to perform reliably in the combustion chamber's extremely harsh environment. Used in their 'raw' form, they're OK when used with leaded fuel, effective lead-substitute treated fuel, or proper race fuels but wear very quickly when unleaded fuel is used. Paul Ivey's 'Specialised Valves' company have been chrome-plating the stems for many years - instigated on his Rimflo valves so folk could use them in the standard cast iron guides. However, chrome plating is an expensive process and difficult to apply correctly without causing weakening of the valve stem - something Paul Ivey found out early on - so up-to-date designs use Tuftriding or nito-carburising instead. The latter are easily identified by the grey/black finish it leaves. 
Unleaded fuel is the bugbear as can be seen from the former text; largely because of the 'high-drying' solvents used in it. These are extremely abrasive, cleaning away any traces of normal lubrication - such as engine oil. Material mis-match between valves and guides will cause galling leading to seizure of the valve in the guide, exaggerated by the unleaded fuel thing. So which valves do you use with which guides? 
Cast iron guides; use either standard road spec valves - preferably the up-rated, chrome-stemmed items for longevity - or race spec valves that have been chrome-stemmed. DO NOT use raw EN214N race valves - they will seize in the guides. It is possible to run the Tuftrided/nitro-carburised type valves in iron guides - but I have had mixed success with this, so cannot heartily recommend it. 
Bronze guides; easy this as you can use any type of valve material you like. The earlier standard road spec valves that have no surface-treatment on the stems will wear out quite quickly though - especially on the more modern manganese/silicone-brass alloys as they are very hard. 
As to where to use which - the controlling factor is more about rpm than anything else. I have already stated that the road spec valves are predominantly multi-piece - not only the way the heads are formed from different materials, but by the way they are made. A manufacturing process called 'fusion welding' is used - the stem and head are two different pieces. Each are spun up to high speed then pressed together. The friction developed creates heat so immense it physically welds the two pieces together - hence 'fusion welding'. Convenient for mass-manufacture of valves having the same stem diameter with a multitude of optional head diameters it may be, but it adds a weak point. This process negates the more expensive process of forging the valve heads where only a limited number of diameters can be turned out of one 'blank', but the joint is a weak link. For road use, they are not a problem. But when higher rpm is used along with the necessary higher strength valve springs to stave off valve bounce, the valve head can break off. Serious engine damage ensues. 
So where is the limit? Difficult to give a definitive answer that won't have someone beating a path to my door to sue me over engine damaged accrued because of my ruling. Generally though, most road-going engines will be fine on the road spec valves - particularly the later spec ones. It's constant high rpm use that's the killer. The odd visit to 7,500rpm won't hurt them IF the valve springs used are up to the job and correctly fitted, and missed gear changes are a rarity. The English racing series 'Mighty Minis' for the 1.3i single point injection cars use the standard MG Metro road spec valves (very good quality) where the engines are held against the rev limiters constantly (6,500rpm); as far as I'm aware there hasn't been a dropped valve yet. Continual and protracted visits to 7,500rpm and above will have the heads popping off their stems after not too long. Therefore ANYTHING that is going to see any kind of race use - autotesting, pylon racing, autocross, sprinting, hill-climbing, rallying, circuit racing, etc. - on a regular basis and where sportier profiled cams are used really requires the race spec valves if long faces are to be avoided. 
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