Taper seat spark plugs

[J44EAG]

Don`t we all just love these plugs!

Just been round to my pals to do a quick service on his GM based Mercruiser inboard boat engine fitted with taper seat plugs.

Two came out easily, the third with difficulty and the fourth snapped off below the socket flats leaving the taper seat and the threaded portion in the head! So off it all had to come followed by a trip to the engine works to remove. What started out as a quick service has now developed into a minor top end engine overhaul!

The problem with these plugs is that they are very easy to mash into an iron cylinder head if a torque wrench is not used when fitting. With an aluminium head, over tightening is even easier!! So that is one point which affects us all as Jaguar owners.

The second is the issue of tightness, age, heat, oxidisation and corrosion. Given that intervals between plug changes seem to be ever increasing as plug technology develops, the longer the period between removal, then perhaps the more difficult plugs will be to remove at service time.

What can be done to help? Correct fitting torque is essential and use of Copper Slip will certainly do no harm at all. One might even apply a little Duralac (Zinc chromate paste) to the seats to help prevent corrosion in service. Do use protective gloves if you use this product as it contains barium which is radio active and can contribute to cancerous problems. Most good marine chandlers stock it in a tooth paste sized tube. It is not a common item within the motor trade.

Mike