Spark plug failures

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Does anybody have experience of buying a plug from an apparently reputable supplier, only to have it fail within a few running hours?

I bought a couple of NGK B7HS plugs from Am*z*n, which certainly look genuine, and were a sensible price (not ridiculously cheap, in other words), but I've had two of them fail within only a few miles.

They are fine from cold in my '74 350 widecase, and the engine runs very well now I've sorted out the carburettor jetting etc., but trying to restart it when hot is a nightmare.

I thought it must have been my fiddling with the jets, but I found that I was getting little or no spark at kick-starting speed with the hot plug. Change the plug, and bingo! Loads of spark, even with the plug that had previously failed this test and been left on the bench for a day or two.

So it looks as if the "NGK" plugs are either being built to a less than adequate standard, or else the prevalence of fakes has suckered even the canniest retailer on the planet. What I have seems to break down when hot, although they also recover after cooling down!

Tomorrow I shall do two things. First I shall go to my local motor factor and try to buy a Bosch equivalent (not quite so popular, so less likely to be faked...?), and then I shall talk to F2 Motorcycles in Wisbech about Brisk plugs, which are made in the Czech republic, and get a very good press.

And I shall let you know how I get on. In the meantime, I know that the sort of aerobic exercise one gets in trying to start a hot and reluctant engine is probably very good for the elderly.

Onwards and upwards,

Pete.