Valve Seat Recession and Unleaded Fuel
What is the problem?
Lead additive in petrol prevented wear (recession) of the exhaust valve seat under the combined effects of heat (due to combustion) and valve closure (hammering on the seat). Lead acted by inhibiting impact welding of the valve on to the valve seat. Such welding, even on a microscopic scale, eventually leads to a significant loss of soft metal from the valve seat in cast iron heads, and allows the valve to sink further and further into the head.
The withdrawal of lead from fuel removes the protection your engine has enjoyed for all its life.
What exacerbates the problem?
Will this problem affect my Healey?
It may, depending on the state of your cooling system and the manner in which you drive.
Note that the ĎA seriesí engine was selected for fuel additive tests in the UK specifically because of the cylinder headís known susceptibility to valve seat recession. The ĎA seriesí engine powers all Sprites and its basic design is very closely related to all Big Healey engines.
The use of motorways (sustained high RPM) and towing (engine working hard) are the most likely promoters of valve seat wear. Aged and fouled cooling systems are culprits too.
It is difficult to generalise, but wear can be quite rapid under adverse conditions (a few thousand miles life), yet barely measureable under gentle conditions.
The initial symptom is a quieter running engine as the valve clearances gradually disappear and the associated clatter ceases. The next (and final!) symptom will be severe misfiring due to valve burning and failure.
What can I do to reduce or prevent valve seat recession?
There are several options relating to choice of fuel and engine modifications. Your particular choice will depend on pattern of car usage and the state of your finances.
Use plain unleaded fuel
Probably OK for low mileage drivers who take it easy, and whose cooling system is in good shape. Problems may not arise for some years.
Lead replacement petrol (LRP) contains alternative additives (potassium based) to combat valve seat wear. The efficacy of these additives is unquestionably inferior to lead, but they will offer limited protection (read the disclaimers!). They are widely available but moderately expensive. The view is that branded additives used with unleaded fuel (see below) will provide a higher level of protection, but this is unproven since LRP was not submitted for comparative testing.
Use Unleaded Fuel with Branded Additives
Do-it-yourself liquid additives are available at Halfords, motor factors and by mail order. The Federation of British Historic Vehicle Clubs carried out engine tests in association with MIRA and, as a result, gave its seal of approval to several brands including Superblend 2000, Nitrox, Redline, Castrol Valvemaster, and Millers VSP all of which performed acceptably well. These additives have different chemistries and the advice is not to mix them; choose one and stick with it. Rumour has it that Superblend 2000 performed best under test, and has the advantage that its chemistry is compatible with LRP in case you run out.
Note that some of these additives are also available with an octane booster component.
Aggregate cost of fuel and additive is comparable to LRP, perhaps a little cheaper.
Continue Using Leaded Fuel
Itís back on the market at selected filling stations (now over 100 in the UK). The classic car press often carry listings and/or updates.
Beware of the price, expect between 94p and 104p per litre, but enjoy the best protection once more.
Engine and Other Modifications
Install Fuel Catalyst Pellets
So-called fuel catalyst pellets (often tin based) can be installed in the fuel tank or in-line feeding the carbs. Rely on them at your peril. Theyíre inert and wonít do harm, but neither will they provide any protection.
Install Hardened Valve Seat Inserts
Specialist engine machine shops can machine out your cast iron valve seats and press fit hardened valve seat inserts which will resist wear and enable the use of unleaded fuel without additives. Only the exhaust valve seats need to be replaced. Cost is moderate and performance should be reliable if the job is done properly. If it isnít and the inserts drop out, youíll probably need a new head and an engine rebuild.
Install an Alloy Head
The ultimate modification. Aluminium alloy heads come ready fitted with special valves and seats. You can run unleaded fuel indefinitely, obtain better engine performance due to the improved head design and carry a lot less weight! However, theyíre expensive (well over £1000) and currently only available for Big Healeys.
Nigel Unsworth. CEng MIChemE
NOTE: The above advice is offered in good faith, but please note that the Austin Healey Club accepts no responsibility for the consequences of any actions taken based on this advice.