Cylinder block valve seat cracks and valve inserts.

A major fault in a block casting is the dread of every enthusiast, but after some years of service, faults can arise within castings and especially take place in areas where differential expansion occurs. Some of the faults mentioned later had started to show up during testing and were well known, others have arisen after years of service. Certainly the availability of good and consistent quality foundry castings was a thorn in the side of most engine manufacturers during the 20 years after the war. All the foundries capable of producing large cylinder block and head castings in either aluminium or cast iron, experienced similar troubles. To a large degree, sustaining casting quality at the time, whilst being under the pressure of retaining production volumes, even stretched Rolls-Royce staff to the limit. These casting are the main foundation of any engine and it can be seen throughout the history of these engines that, in many cases, they were the source of many of the troubles that may be experienced. That is not to suggest in every case that the foundry was at fault, in the case of these six cylinders, no doubt the particular design also played a major part.

With the exception of the exhaust valve face and its seat, the hottest part of this engine is the bridge piece of the top cylinder block deck between the exhaust valve seat and the parent cylinder barrel. During the exhaust stroke the hot gases must follow an inverted ‘U’ shaped path from the cylinder to the exhaust port. One of the obstructions is the edge of the liner and also the resultant throat restriction between the top of the combustion chamber and the edge of the liner, on the exhaust valve side. Any obstruction will naturally cause power losses and localised hot spots; in this case they occur in an area that already suffers the highest heat retention. These particular issues normally limit the compression ratio to a maximum of 8.5:1 on this type of design, when natural aspiration is used. The exhaust valve will shed heat directly to the seat and, to a lesser degree, to the valve guide. The heat actually passed through the seat can only be transferred to the top deck bridge, this entire area relies heavily on reasonable quantities of water impinging on the bridge, exhaust port and seat areas for final dissipation to the cooling system. These engines have little or no valve overlap whereby both valves are open together for a short time and some of the incoming cool charge can pass through the exhaust and assist in valve and seat cooling. The nature of the design of the side exhaust valve and port area does cause extreme temperature gradients to occur across the top deck bridge and the valve seat and valve face have to contend with the 550 / 600 lbs maximum pressure combustion loadings. The bridge temperature, I would assess, would be in the order of 400-500C and as high as 600C in severe cases of overheating.

Fig 3 shows the result of very hard driving on a 4.5 Ltr, the crack rising in the valve port core has crossed the seat area and continued across the top deck bridge piece. Very often smaller cracks appear, which are more over to the left or right hand side than that shown in Fig 3. All these cracks have one thing in common; they commence in the exhaust port under the seat and continue in the direction of the cylinder. Very rarely will a crack appear in the seat area on the opposite side to the cylinder bore. The company initially installed exhaust valve seats, on production engines, to combat this defect.

The later engine sizes are right on the edge of requiring valve seat inserts and it may be of interest to follow the history of these inserts from conception. Initial trials started in 1939 with the fitting of inserts to B80 engine no 5, this engine was subsequently fitted into chassis no 30 G V11, which covered 160,000 miles during war time. These particular inserts were made from Brichrome, the same 30% chrome cast iron used for the later top cuff cylinder liners.

As production gained pace after the war a few of the earlier 4.25 engines were fitted with inserts for production salvage reasons, usually due to incorrect initial machining. For historic interest, by August 1951, the following chassis had their engines originally fitted with exhaust valve seat inserts under this PSS scheme and many more were to follow.

Bentley MKVI

B198 AK. B258 BH B312 BH B346 BH B337 CD. B103 GT
B218 AK. B260 BH B326 BH B348 BH B405 CD.
B222 AK. B266 BH B328 BH B352 BH B413 CD.
B226 AK. B268 BH B330 BH B354 BH B439 CD
B5 AJ B274 BH B332 BH B362 BH B477 CD.
B7 AJ. B278 BH B334 BH B366 BH. B142 DA
B11 AJ. B280 BH B336 BH B368 BH B302 FV
B21 AJ B298 BH B340 BH B378 BH B354 FV
B35 AJ. B308 BH B344 BH B384 BH B476 FV

Silver Wraith

WYA 1. WYA 27. WYA 33. WAB 58.
WYA 7. WYA 28. WCB 5. LWGC 88
WYA 25. WYA 32. WGC 71. WME 38

The material used under the PSS scheme was Brichromium, a much lower chrome content iron than that first used in 1939. All these inserts were fitted by heating up the blocks and pressing in the inserts with a very high 0.0065 – 0.0085 interference fit. The company machined all these early PSS inserts from cast sticks. In 1951 service records for all the above chassis indicated that no problems had been experienced with the seat inserts. It is perhaps also significant to note that all these chassis were fitted with relatively low output 4.25Ltr engines, furthermore, with the exception of Silver Wraith WME 38, they would have been cooled by the high speed water pump and fan.

Although the production scheme had used Brichromium material for the inserts, as early as autumn 1948, Wellworthy Valmet, a 15% chromium iron, had been specified for the C60 single cylinder laboratory engine. This latter material was eventually to become the normal standard many years later. It was found to be very resistant to valve seat pocketing, particularly when rotators were used on the valves in B81 engines.

Later inserts were supplied ready made to the company in Brichromium, under part number EB 4027, to a BHN of 220/260. Normal 4.25Ltr engine cylinder blocks at the time were in the range of 202/240 BHN. Company interest was awakened to exhaust valve seat inserts once again, initially to salvage exhaust seats on higher power test engines.

Although a few valve seat cracks on 4.25 engines were experienced from very late in 1951, these had all occurred due to the presence of residue sand and flash in the water passages. However the situation changed and had become quite disturbing by the end of 1953. One early 4.5 Ltr engine in Bentley B461 NY, suffered in particular, even though the block was unusually clean.

The 4.25 and 4.5 cylinder blocks are hard enough around the exhaust valve seats to cope with unleaded fuel, providing the engine speed is restricted. One shortcoming is that the seats are still subjected to distortion and cracking if the bridge piece heat transfer is interrupted. Distorted seats will quickly bring about failed exhaust valves and it is advisable to fit seat inserts when engines are removed for overhaul, for peace of mind if nothing else. This suggestion is not directly related to the use of unleaded fuel but to towards valve seat cracking, which if left unchecked, means that, the cylinder block will be scrap.

Fig 4 shows the underside of the top deck, or cylinder block ceiling, when the crankcase top has been sawn off. The smaller holes along the top are the valve guide and inlet push rod locations. The area where dangerous silt build up can take place, between the parent barrels and exhaust valve ports, is clearly shown. In practice it is difficult to clean this area, especially when the silt has compacted and hardened because access is very limited. Comparisons between fig 3 and this photograph will show that the cracked top deck bridge area runs directly above the cooling tube. One of the most important functions of the cooling tube is to sufficiently scour the areas around and between the exhaust ports, to prevent or displace any steam pockets forming. Later S1 crankcases had bulged exhaust ports to increase the area available to the coolant. The siamesed barrels and the difficulty of providing even cooling of the cylinder barrel areas can also be seen in this view.

The 4.5 ltr engine in particular is known to be on the limit of cracking in the vital top deck bridge area. Only during the course of the last few days have I seen, once again, two different 4.5 Ltr engines with exhaust seat cracking. On a few engines, problems of residual casting sand and flash caused some exhaust valve seat recession and cracking to occur, due to the interference of heat dissipation to the coolant. Cast iron, when it is subject to stress and high temperature cyclic gradients, will only experience a limited number of cycles before failure happens. A prime object of any owner wishing to retain the integrity of the cylinder block should be to look for ways to reduce any potential localised heat spots.

Differential expansion continued to be a problem. Even with exhaust valve seat inserts fitted, a number were reported loose in service. Most of these loose inserts on SI engines were traced to a batch of inserts made from the wrong material by the supplier. During 1956 the old exhaust valve seat inserts EB 4027 were finally superseded by the Valmet inserts RE 23855. Extensive engine overheating will, however, still distort the top bridge area together with the valve seat block counterbore. In cases where the inserts had been loosened and the block needed counterboring again, a 0.010 oversize Valmet insert RE 23983 was introduced. These final Wellworthy Valmet seat inserts were installed with a lower interference fit of 0.0025- 0.004. Valmet material is especially durable and, after hardening and tempering, has a hardness of 472- 547 BHN and a low expansion rate.

Considering that during the first year of production of the S series, the hardness of the cylinder blocks had reduced to 175- 185 BHN, the presence of these inserts was well timed. Present S1 owners should not have problems with block bridges cracking and the valve seat is very capable of running on unleaded fuels. That of course is providing, and only providing, the correct exhaust valves are fitted and the coolant passages are kept clean. It is worth noting that a reduction of only 10 points BHN can make a drastic difference to valve and seat life. In the presence of unleaded fuels any exhaust valve seat area much below 200 BHN is not going to survive for long.

Even before 1954, repeated recommendations from tests and experimental records make the previously mentioned points very clear. I quote some extracts from one such report discussing exhaust seat cracks, which, I generally consider, contained very sound advice.

“ If failures are to be avoided the following recommendations must be observed:-
Every effort should be made to ensure crankcase cleanliness in water passages around exhaust valve seats, ports and bridges”

“Exhaust valves with Brightray coated heads and Stellite faces should be used on 3.75 inch bore engines and should be seriously considered for 3.625 inch, particularly Continental.”

“ It would be desirable to have exhaust valve seat inserts on all engines but they are essential on 3.75 inch bore engines”

“ The water rail does assist in cooling bridge pieces. Providing it directs coolant to them, as it does currently, the quantity is relatively unimportant above a certain level”

Brightray was developed by the company and used to coat the exhaust valve seats of aircraft engines. The brightray coated exhaust valves were, of course, eventually instructed for production on the 3.625 inch bore Continental R type engine and the later 3.75 inch bore engines, under part number UE786. Valve heads, which have been coated in brightray, are extremely resistant to corrosion. In addition, they are well known for their ability to delay the onset of pre-ignition. If that condition is allowed to go unchecked it results in fantastically high and uncontrolled combustion chamber temperatures.

The last report statement is a little ambiguous, as it depends what that “certain level“ may be, in terms of water volume. High volume flows along the water rail are important, not just for top deck bridge area cooling but also for those components down stream of the flow. These down stream areas receive the coolant after it has been directed at, and gains heat from, the bridge area. Most of the archive reports, quite naturally, were directed at the immediate problem then in hand and not particularly directed at other areas.

In our engines the main means of heat dissipation is by passing water through the radiator, this depends upon volume. As a general comment, the more volume of coolant that is passed through the radiator, the lower will be the coolant temperature. During the life of the Bentley Mk VI, as we shall see later, the fan speed was reduced. When bulletin BB141 dated 25/2/52 was issued, it stated that the reduced fan speed had “slightly reduced the cooling efficiency, but not sufficiently to cause overheating unless the safety margin is low for other reasons” These are not the words I would have used, the fan speed was reduced by 16.31%, but more significantly the water pump speed dropped by no less that 29%. It is also significant that the fan and water pump speed were reinstated in later years, for cases of overheating complaint. In addition, the first cases of reported valve seat cracking were reported in a November 1951 production car and less than two years after the water pump speed had been reduced. The matter of water flow is discussed later in more detail and owners can make up their own minds on the issue.

Unfortunately these original troubles highlight the weakness in this top deck area. The original foundry problem of leaving sand and flash in the cylinder block has now been superseded by corrosion and silt accumulation in later years. Whilst block cleanliness is essential for long engine life, this cooling system needs as much assistance as possible to disperse the heat. This becomes more evident when the engine is operated in a harsh environment, traffic jams or at altitudes when the coolant boiling temperatures are lower.

The engine cooling is not helped when the enormous exhaust backpressure is increased by exhaust silencer blockages. This is more common than may be first thought. Excessive exhaust backpressure is enough to overheat the exhaust valves and, just as important, the top cylinder block deck. If excessive backpressure is present, the engine will not fully exhaust the residue gases on the exhaust stroke and the incoming inlet charge is diluted. The result is poor performance in any case and intermittent misfiring down the exhaust system, at engine idle speeds.

Another drawback of the exhaust restriction is the resultant severe heat and pressure that prevails around the exhaust valve stem. The exhaust valve guides are well known to take up a bell mouth shape at their upper ends and exhaust pressure and carbon is directed in that case straight down the guides. The build up of carbon on the top section of the valve stems does nothing for their ability to transfer heat to the guides. In the worst cases the valve to stem clearance increases alarmingly and sticking exhaust valves are a likely probability. The carbon works its way down the valve stem, creating havoc in the tappet chest and finally coming to rest in the crankshaft sludge traps. It is not surprising that when exhaust valve guides are renewed in these engines that noise reduction and cleanliness of the engine oil takes on other meanings. In extreme cases it is possible to detect the excess backpressure, from a rise in cooling temperature and repeated failure of exhaust manifold gaskets.