Some technical help wanted here, regarding the overheating on my BEA coach MLL738. The engine came from a provincial Regal IV and has an oil filter fitted, no problem. It also has a "conventional" wax type thermostat fitted. that has seized up and with that removed the engine runs at 70c. I replaced it with a "standard" LT wax thermostat as fitted to RTs, RMs etc and that causes the engine to run at 98c. Comparing the 2, the original stat has a much wider aperture than the LT type, say 2 1/4" diameter compared to the RM type at 1 3/4" diameter. So the extra internal diameter would allow a better water flow better suited to longer distance coach operation. Maybe this is why so many RFs have overheating problems these days? According to my AEC Regal IV parts catalogue the thermostat is part no. 81-1545. Maybe the larger thermostat was fitted in the larger 11.3 engine or those fitted in BR railcars?

RFs don't have overheating problems these days or in the past. Unless the radiator is blocked, of course.

The main reason they appear to boil is that they are over-filled with water. It is only expansion that chucks water out and drops the level to the correct point. Of course in LT days, drivers just filled them up again until the water spilled down the front. And then the whole thing happened again down the road, and they cursed them (unfairly) for overheating. When cold, the water level should literally only be just visible in the header tank.

We have three RFs - the first did originally boil on long shallow hills (but not elsewhere) and when the radiator was stripped down there were only three tubes that weren't blocked! After fitting a reconditioned radiator that one has never boiled since, and the other two never have. And we are talking of a period of over thirty years!

I don't see any reason why an LT thermostat should give you any problems. Many RFs ran without a thermostat at all in post-LT use, although that isn't an option I would choose.

John

An engine which has poor compression will run hot if it is subjected to full load for a sustained period. One of the common reasons for overhauling AEC engines was because of a lack of power caused by low compression. Add on a radiator in poor internal condition from years of running with either plain water or a weak antifreeze mixture and overheating is a predictable consequence. Underfloor engine buses and particularly some AEC models also suffered from a lack of adequate airflow through the radiator noticeable at higher speeds. So given a tired engine stuggling with its load trying to maintain higher road speeds and hindered by a partially blocked radiator it is not surprising that it should run hot. Should the coolant level in the header tank drop significantly below the level of the saloon heaters then air locks in the system are likely to develop leading to initially localised overheating.

I will check later this week, but IIRC the RF thermostat housing requires a bellows type thermostat capable of blocking off the by pass port. If this port is not covered at operating temperature then the coolant flow will take the path of least resistance; which is not via the radiator.

RFs do indeed overheat. I've just had a NEW radiator made for mine as the original was beyond repair. I've also fitted a temperature sender on the engine side of the thermostat housing. With new rad it runs at 98c; without the thermostat it runs at 70c. The engine is low on compression but not too bad. I haven't done a compression test on it as I'm building another engine for it that has had problems.

The engine in the bus is from a "provincial" Regal IV and has the larger diameter thermostat fitted. What you say above about the bellows stat may solve the problem, but I don't want to risk a "new" engine being cooked.

I've just filled it with a strong anti-freeze mix and it's out for a jaunt in Rochester on 30th December, so i'll find out if that makes a difference.

An update on the Regal IV overheating, the trip to Rochester last mentioned went well with the engine running at 90c in the middle of winter. I left the LT RT/RM stat in place for the start of the next season and on summer's days it ran at 98c at the block. I wasn't happy with this as it rose to just over 100c on the drag up long hills such at Wrotham Hill on the M20.

I subsequently took the stat out and it now runs at about 68c in winter which is a bit on the cold side for saloon heating, but rises to about 85 on a hot summer day. the new radiator uses a modern honeycomb arrangement with new ali tanks and sides as the originals were badly pitted inside. That cost about £1800 but seems to have sorted any doubt about being blocked with much improved water flow. I know RFW14 and I believe RF644 have similar radiators fitted. It's a pity nobody seems to have a definitive answer on the thermostats. I have a few different bellows stats that I could try, but they may not fit round the temperature sender I've fitted.

An update on my Regal IV, it hasn't been out at all this year, but during the previous year attended a couple of rallies and done about 6 private hires. the engine performed will but smokes quite a bit on over-run and down hills. I suspect either worn valves and guides or worn ring. the amazing thing is that even in the depth of last winter it started literally first turn!

During the previous season it hadn't used a drop of water, and to think that engine cost me a small fortune back in 1977, £150, collected from an old fella in Newcastle.

I'm still running it without the thermostat, it gets a bit chiily in winter and still gets to 98c uphills in summer!