Spark plug stuck-corroded-broken.

[K2-K6]

As title, ultimately a spark plug with corrosion stuck in cylinder head. All attempts to "encourage" it to free up failed, leading to the plug casing fracturing just below the hex section to leave the portion below that inside the head. Porcelain insulator lifts out with the hex section of steel completely free.

A friend used to run a little jig setup on his milling machine specifically for 400 four centre plugs, when the bikes were contemporary, with local bike shops sending heads to him for attention when this happened.  Relatively easy to bore out with that arrangement.

Many years (so many I can't remember) when I had to last extract a spark plug like this. With experience in engineering tool room, drill, mill, jig bore, spark erosion  .... all giving me the feeling of not wanting to tackle this one in situ and freehand too. I ran out of excuses though 

Same plug as the SOHC but on early eighties DOHC and deeper down into head casting, harder to reach in there.

Method used:- hole size for tapping 12mm x 1.25 metric fine thread (all the 12mm plugs) comes at 10.8mm, could only find a long drill bit of 184mm @ 10.5mm dia.

From experience, going up in size of drill incrementally maybe OK in a clamped workpiece, with assured spindle (as in a drill or mill) but "snagging" on the outer cut edges of drill bit flutes when hand holding gets frenetic and often leads off out of centre. So went with one hit straight to full bore size. There was a taper lead in to help as the centre bore in remainder of plug is such to accommodate the porcelain section of plug.

Drilled into first section with the next part of the plug "collar" coming away as drill machined that part. Checked for still concentric, OK, further down as is cut the fully threaded section in got about 5mm more depth. Then just a feeling that the remainder was turning as the drill was pushed to cut it. Vacuumed worksite (had grease down in plug electrode hole to catch any swarf from dropping through) inspected to see that it looked like it had cleared the corrosion at top part.

Tried a T30 Torx impact bit (3/8 socket drive) but too small. Went to T40 and found that it was just about perfect while exploratorily tapping into the bore. Plausibly encouraging as could feel progressing in "broaching" the form into the bore of the plug  going down to electrode (this is where you'd see the insulation on centre electrode on a complete plug) to good secure effect and full engagement.

Put a ratchet handle onto extension and last section of plug wound out easily on existing threads.

Then a thread chaser "Sealey Model No VS724" to clean through the wholly intact threads in head casting https://www.ebay.co.uk/itm/174196484643?shprz=EBAY_GB_131090&_ul=GB&_trkparms=ispr%3D1&amdata=enc%3A1V4Wy6h6zQJqtR50e5UUKOg37&mkevt=1&mkcid=1&toolid=20006&mkrid=710-53481-19255-0&campid=5338986375&loc_interest_ms=&loc_physical_ms=9217597&adtype=pla&customid=Cj0KCQjw9ZLSBhCcARIsAEhGKgOdU848GzduCUvbq4eJlaq6MaXP8gZdj8MH5trOkwyxuxPxJagjO2caAgngEALw_wcB|null|null&gad_source=1&gad_campaignid=20235103899&gbraid=0AAAAADcddnZer3KtdOQMpkk2PhDTSyfVt&gclid=Cj0KCQjw9ZLSBhCcARIsAEhGKgOdU848GzduCUvbq4eJlaq6MaXP8gZdj8MH5trOkwyxuxPxJagjO2caAgngEALw_wcB

I prefer a thread chaser rather than tap as a decent tap will cut fresh threads or diminish original structure with ease, if not aligned or "indexed" correctly with original thread cut. Chaser is kinder in preservation of original as it has thread form, plus clearing flutes to withdraw corrosion etc all without specifically cutting, accidentally, fresh head casting material.

The drill bit, bought from "UK Drills" is excellent for this. The bore just taking out the thread core to leave the thread spiral like a helicoil and, showing that although there's usually some "wander" in this type of application, the difference between specification  clearance of 10.8mm to the nominal drill size of 10.5mm gave enough protection to prevent the drill from removing any material from head casting.

As noted above, I really don't like doing this as its often fraught with complications if there's any wander away from centre to let the drill cut out the side and into surrounding metal (same for any steel in soft casting really) but went carefully to, happily, produce a non compromised outcome.

[K2-K6]

A significant contribution to the above operation was the drill.

Borrowed from my son " Makita 18v DHP 481" with particular attribute of high torque, 141nm in low gear (isn't that about 100 ft/lb ? ) also with a very fine trigger control that really does let you use very high torque at virtually 0 rpm with excellent modulation to help in this restricted scenario. 

A little like having a good workshop pillar drill or mill with very low gear to cut at very low rpm while leaning on it with enough weight to get that consistent cutting needed in steel materials, without fear of just clattering around to detriment of potential accuracy. 

Not a current product, but there are models superseding it. A pretty potent device, but with trigger control Oh So Soft  made easy work of this tricky job.

[Bryanj]

Last 400 i had to do the threads were all worn out so i used a kit as shown, only problem is you get 4 different length inserts so had to buy 4 kits!

[K2-K6]

I saw those kits, but was delaying buying until I found out what state it was in after the operation. And fortunately not needed.

Forgot to put in description above, but ran engine until hot in attempting to first undo the plug originally as that usually can persuade one that's stuck.

Fortunately, more planning I suppose, I've avoided this for years by maintaining them with anti-seize products on anything I work on.

This one caught me out though, being first time I'd taken plugs out.

Did a nervous one recently on son's car, at initial try to change plugs they felt reluctant (right down inside long tubes too) made sure that it was really up to temp and they all came out cleanly. Those are little 10mm plugs too, quite a nervous time before all were done.

That thread chaser is a good plan for maintenance though, if someone has got any that are a bit grimey with potential to give trouble later on. It's a good 3/8 drive do can be mounted on a long extension to give good alignment, turn it backwards to find lead thread, then carefully screw in to clean existing threads.