Maintenance - Minor Service

6. Check and adjust the spark plugs

Either perform this task when the engine is cold, or take great care to prevent personal injury through burning one's hands on the hot cylinder head. Remove the three retaining Allen screws and withdraw the spark plug cover plate from the middle of the engine left-band outer (cylinder head) cover. Noting exactly how the leads are routed, pull off each spark plug suppressor cap using a heavy pair of pliers applied to the tab protruding from each cap. Clean off any particles of dirt or other foreign matter from the spark plug channel, then unscrew the spark plugs, keeping them clearly identified by cylinder number. First use feeler gauges (see spark plug maintenance), preferably of the wire type for accuracy, to measure the gap between the spark plug electrodes; BMW state that the ignition system is so sensitive to spark plug condition and electrode gap that the plugs must all be renewed if the electrodes of any one spark plug have been eroded to a gap of 0.8 mm (0.032 in) or more. lf this is found to be the case, or if the plugs are in any way suspect, new spark plugs of the specified make and type must be purchased and fitted. Note that if this particular make is difficult to obtain locally, the advice of an authorized BMW dealer should be sought; provided that exactly the equivalent heat range and type is obtained from a good quality brand, spark plugs of alternative makes may be used eg NGK07EA. Fit them as described below.

lf the spark plugs are still serviceable, carefully compare the appearance of their electrodes with the accompanying color section (see spark plug conditions 1 or spark plug conditions 2) and note any information which can be obtained from this. lf any plug appears to show a fault, seek expert advice as soon as possible; do not forget to take the old plugs with the machine to an authorized BMW dealer. The standard grade of spark plug should prove adequate in all normal use and a change of specification (such as fitting a hotter or colder grade of plug) should not be made without export advice. Clean the electrodes by carefully scraping away the accumulated carbon deposits using a small knife blade or small files end abrasive paper; take care not to bend the center electrode or to chip or damage the ceramic insulator. The cleaning of spark plugs on commercial sand-blasting equipment is not recommended due to the risk of abrasive particles being jammed in the gap between the insulator and the plug metal body, only to fall clear later and drop into the engine; any plug that is so heavily fouled should be renewed.

Once clean, file the opposing faces of the electrodes flat using a small fine file. A magneto file or even a nail file can be used for this purpose. Whichever method is chosen, make sure that every trace of abrasive and loose carbon is removed before the plug is refitted. If this is not done, the debris will enter the engine and can cause damage or rapid wear.
Whether a cleaned or new plug is to be fitted, always check the electrode gap before it is installed. Use a spark plug adjusting tool or feeler gauges to measure the gap, end if adjustment is required, bend the outer, earth electrode only. Never bend the center electrode or the porcelain insulator nose will be damaged. Before the plug is fitted, apply a fine coat of PBC or molybdenum disulphide grease to the threads. This will help prevent thread wear and damage. Fit the plug finger-tight, than tighten it by a further 1/4 turn only, to ensure a gas-tight seal. Beware of over tightening, and always use a plug wrench or socket of the correct size; tighten all plugs to the specified torque wrench setting, where possible. Never over tighten a spark plug otherwise there is risk of stripping the thread from the cylinder head, especially as it is cast in light alloy. A stripped thread can be repaired without having to scrap the cylinder head by using a 'Helicoil' thread insert. This is a low-cost service, operated by a number of dealers. When refitting the spark plug suppressor caps, be careful to ensure that the HT leads are correctly routed; note that the leads are numbered as a further aid to identification.


Remove spark plug cover plate from cylinder head cover.....


..... and use pliers to pull of suppressor caps, noting exactly how HT leads are routed


Spark plugs must be renewed as a set if any gap has worn to beyond the service limit


HT leads are numbered to assist identification on refitting