Spark plugs anomaly

I have recently acquired an '67 mk2 which requires some work - not too much. Replace brake calipers, redo the veneer etc.

I thought it would be an idea to refresh the plugs and discovered that 5 / 6 are 14mm diameter. However, 1/6 was 18mm!? Never seen this before - surely that cannot be the original intention?

Also need to source a new battery if anyone has any tips for a reasonably priced one.

Am in UK

Short answer - no. All my Jaguars have the same sized spark plugs. Are you talking of the actual thread itself or the hexagonal nut? I have heard of cylinder heads being damaged by cross threading spark plugs - requiring an insert - but always back to standard size. I have a 500CCA battery in my Mk2 - no air conditioner and runs the original dynamo. Works just fine. Paul.

Yes - the thread is different - very strange!!

I expect that a cross threading has occurred sometime in your car’s past and the repairer has “gone out” to the next size. Provided that both plug types are of the same heat range and depth towards the cylinder, all should be well. Paul.

I have a Platinum prestige , went to my local battery outlet , and got the biggest that would fit on tray and clamp , think it was around £90 but that was 5 odd years back , never had to recharge it , I do have a kill switch , and run a Dynamo 20180609_044109

Costco ‘Group 9’ batteries fit and work just fine in my '67 Mark 2, don’t know if Costco sells batteries in the UK, however.

Are you sure an insert hasn’t come out of the head stuck to your plug? What is the make and code of your “18 mm” plug and the other five?

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Well, that’s another way to look at it. Paul.

So what was the outcome???

Shall check at the weekend, though am fairly certain there is no insert stuck to the plug!