Overview of Unleaded Conversion
Because of CNC machining commitments we are now unable to undertake any engine work, these pages remain for information purposes only. But – we will be happy to find someone to help with your job, please phone or e-mail if you need a recommendation.
Unleaded fuel only contains a tiny amount of lead that occurs naturally during its manufacturing process (so to say lead-free is incorrect), whilst 4 Star fuel has quite large amounts of tetra-ethyl and tetra methyl lead added during its manufacture. Lead was added to petrol as early as 1923 because it was an easy way to raise the octane rating and help prevent detonation (pinking). Its lubricant qualities were secondary as it was only discovered later that lead based deposits were protecting valve seats. Nowadays petrol is manufactured much more precisely and octane rating can be controlled in other ways.
Most older engines were not designed to run on unleaded fuel, especially those with cast iron cylinder heads which will generally have a valve seat that is simply a machined part of the iron casting, so every time the hot exhaust valve closes it will try to micro-weld itself to the soft iron seat, eventually causing the seat to wear away. Once valve seat recession starts it rapidly progresses, compression and tappet clearances will reduce, with poor running, loss of power and increased fuel consumption being the results.See our best multi tool in action here.
Converting an older design of engine to run on unleaded fuel is generally straight forward and often only involves fitting exhaust valve seats of the correct hardness. Once fitted, most engines will run perfectly on modern unleaded or super unleaded fuel. Specifications of exhaust valve material must be carefully checked though most valves are already made of sufficiently tough materials, but if not, correct grade valves can usually be found or made.
With aluminium cylinder heads the original steel valve seat inserts are sometimes already hard enough to cope without added lead, (although only careful (laboratory) examination will determine exact material specification). The vehicle manufacturer, however, sometimes states that unleaded fuel must not be used. If this is the case the owner should approach the manufacturer and ask the question “Is my car unable to run on unleaded fuel –
a) Because the valve seat material is too soft? or,
b) Because of the risk of pinking? (Bear in mind that Super Unleaded has a similar octane rating to 4 Star)”?
If neither pinking or valve seat material is the problem then try to pin the manufacturer down to another reason.
Of all the older engine designs that will need converting for use with unleaded, only a tiny percentage will cause problems during conversion.
Since January 1st 2000 4 Star fuel has only been available from a small amount of petrol station forecourts.
The car owner must confirm their engine’s compatibility with unleaded fuel and take the appropriate action. When there is any doubt due to lack of positive information the only course to take is that of fitting the correct grade of valve seat inserts. We have now absorbed enough feedback to provide a “guesstimate” of Unleaded suitability, please contact us for an opinion on your cylinder head.
To soften the blow bear in mind that this conversion will also allow the owner to rectify any other cylinder head related problems.