Unleaded fuel
AS I mentioned above, most Series petrol engines are not built to run on unleaded fuel and will eventually cause problems unless an additive is used. A question I am quite often asked is, “will my 1962 Series IIA (or whatever) run on unleaded?” The answer is not entirely simple.
Firstly, valve seats tend to become work-hardened to some extent with age. Valve seat recession is worst on engines which are worked hard at high speeds. So if you have an old Series vehicle which is just used for pottering around the local lanes, with an engine that hasn’t been apart since Harold Wilson was Prime Minister, there is a good chance that it will keep running happily on straight unleaded for very many years.
Secondly, a fair few engines will by now have been fitted with hardened valve seats when the head has needed overhauling. I would guess that most heads rebuilt in the last 20 years will have been made unleaded-compatible. The problem is that unless you have a receipt for the work, the only way to be sure is to remove the head, extract one of the exhaust valves and inspect the seat. If it has been fitted with a hardened insert you should see a ring in the metal around the valve, and a change in the graining of the metal either side of the ring. It might be possible to inspect the valve seat using a small inspection camera down the spark plug hole but I have not tried this.
2.5 petrol engines (code 17H) had hardened valve seats from new, and the head casting was otherwise identical to the late metric-threaded 2.25. Some of these heads will have found their way onto older engines. There should be a part number stamped into the flat square area adjacent to the middle head bolt on the manifold side: this will tell you whether you have a 2.5 head.
Is it worth converting to unleaded? The cost including labour is going to be maybe £600-£800 which buys a lot of lead replacement additive. My advice is that if the engine is running well, keep using the additive until the head has to come off for some other reason, at which point it makes sense to have valve seat inserts fitted. Apart from anything else, when you lift the head you might find a badly worn set of bores, and ignorance is bliss, as they say.