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The lead that was in petrol acts as a cushion on the valve seats to stop recession of the valves. That is why when the lead was removed there was a great demand for hardened valve seats. Hardened valve seats are not really necessary on our old bikes as why tend not to do the mileage anymore.
[..'Tetraethyl lead [TEL] is a petro-fuel additive, first being mixed with gasoline (petrol) beginning in the 1920s as a patented octane rating booster (antiknock agent) that allowed engine compression to be raised substantially. This in turn caused increased vehicle performance and fuel economy. TEL had been identified chemically in the mid-19th century, but its antiknock effectiveness was discovered in 1921 by the General Motors research laboratory, which had spent several years attempting to find an additive that was both highly effective and inexpensive...']
Reduction in wear of the valve/seats was an added bonus of lead use...I have found with the cooler running post war OHV B Series BSA engines the removal of lead has no discernible effect on the valves/seats under normal and varied use...
I have had problems with the exhaust side of my M20 but only after using it at sustained cruising speeds on motorways over a period of time...I experienced deterioration of the valve and seat sooner than anticipated and I put it down to that...I have read that continuous running of that type is the worst case scenario for hotter running engines...
I fitted a 'lead free seat' a number of years back and used a modified exhaust valve from an Iveco turbo diesel...Since then I've not had a problem...Many people who do not use their M20s in that way have reported no problems at all with using lead free fuel...Ian
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Valve seat recession was real and identified in extensive laboratory testing
OF AIR CRAFT ENGINES when running in lean burn conditions as is done at high altitude.
It was never ever found to be a problem in auto engines at sea level.
But the technically illiterate who write 99% of all motoring magazines got hold of it and ran hard.
Did wonders for sales as owners desperately needed to know if their precious possesions were going to self destruct.
Naturally the snake oil salesmen followed hard in the footsteps .
For carburettor engines there never was a problem.
Some blown engines could be adversely affected.
I used to do thousands of miles on my M20 and the only valve problems has been a single burnt exhaust valve.
Done enough miles to do a full series of rebores on one barrel out to +80 and no vlave problems.
The people who had problems were the clots running 11:1 to 12:1 which is right on the limits so they either had to detune or run
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