question about damaged sparkplug thread
 

hi guys

This isn't about a M-B but it's more about cars in general. I was wondering what happens to the aluminum pieces that fall into the sparkplug hole when a thread gets damaged? This happened on my Volvo (240) to one of the plugs. I have heard that the Al gets shot out of the exhaust and there is no need to worry, and I also heard that the Al and any gunk may fall into the engine and get mixed with the oil and there is need to worry. I did a search and it seems that it is evenly split between "no need to worry" and "WORRY!"

I took my Volvo to a shop where the mech. said the thread was not majorly damaged and he was able to salvage it by "tapping" it. He said it did not need a heli-coil. Is it possible to fix a sparkplug thread without using any replacement parts? Exactly what did he do by "tapping" the thread? The car runs fine and does not sputter, but I am concerned about driving it on the freeway or working the engine hard for fear of shooting the sparkplug (the one in the repaired thread hole) out of the engine head. I heard that can happen. He said it's as good as new and that I should not worry. What about the Al pieces that fell into the sparkplug hole? What are some of your experiences concerning damaged plug threads? Thanks guys!

A tap is a tool that cuts threads, but if something had existing threads it can help clean up the threads if they are a little messed up because of someone crossthreading a hole.

If you have already driven it, its probably fine. If the threads were pretty good, and just needed some cleaning I highly doubt there was much Aluminum debris in your cylinder. However putting in a helicoil requires drilling out the threads, then taping, then putting in the helicoil insert, which I am certain puts a lot of debris in your engine. Some mechanics will try to blow the chips out or vacuum them out with an air vacuum.

I hope everything is ok. Maybe change your oil soon and check. It probably got pushed through the exhaust system already if there were any chips.

I used to have a bunch of British cars -- TR's mostly.

They were easy to work on -- cheap and fun - by the time we got them in the late 70's they were tired -- cross threading was a common problem (does any one remember how often one had to pull plugs in cars from the early 60's)

Most of them would have the first few threads crossed or sometime it would just be a check in the first thread -- A tap would straighted it all out and clean it all up. Not much shavings involved you are not cutting -- just pushing the thread bact in its proper place

It is not normally needed to hell coil - even in Al you really need to put the plug in wrong and then be foolish enough to believe that it requires that much force to put it in - of

And to keep the metal particles from falling into the cylinder just smear a little grease on the tap adjacent to the threaded areas.

Droppen der bits in der cyl
 

Once had to do a helicoil on my old 250/8. Rolled it over to get a little clearance in the cyl in question. Pushed some cloth material in and after the thread cutting to accept the helicoil, I was able to pull out the material the cuttings with it. No problems installing the coil and ran ok thereafter.

Was precipitated by using antiseize and overtorqing, because the antiseize reduced torque requirement. Long ago and far away in the past...

spark plug rethread tool
 

Both Snap-ON and KD sell the "BACK-TAP" internal spark plug internal rethread tool. "Patented design allows for chasing plug threads from inside the cylinder head and working outward...." Slim profile slips inside the spark plug port. Adjusting knob expands the thread chaser to exact thread size. Debris clings to tool...Eliminates chance of cross threading.

Snap-On: 12mm=SPHR3
" : 14mm=SPHR1
" : 18mm=SPHR4

KD: 14mm=KD3545

5280BENZ

Scoring up of cylinder walls is the biggest risk.
 

Pieces of metal can get caught between the piston and the cylinder and make some marks which may or may not cause a significant increase in oil consumption and often a measurable loss of compression. Back in my earliest days of working on cars (I don't think I was even a teenager yet), my dad showed me how one could handle the problem with Fords and their then new huge 18mm spark plug holes in cast iron heads: bring the piston near top center on the firing stroke, stuff a coil of clothesline coated with sticky grease into the cylinder, cut the threads (or do the helicoil installation) and then use a tweezers to get the clothesline out. There is still some risk of a few metal fragments left behind. Probing around with a magnet also helps on iron heads.

Many years after that, a neighbor trying to learn to do his own tuneups radically overtorqued the plugs on a late '60s 200 Fintail and stripped out everything, so I told him his options, and since the rings in the motor were pretty tired anyway, he decided I should risk doing the helicoil with the head on. I started looking around for a clothesline piece to try to stuff in the long 14mm hole in such a way that I could be confident of getting it out with some tweezers, when just then my wife came along and asked what I was looking for -- after my explanation, she said she had just what I needed, left, and came back with a couple of tampons. They worked perfectly.

But I think these newfangled back-tap tools are the clear answer -- in fact, I think that the genuine Helicoil kits have long had bits and taps designed to maximize the amount of shavings that are thrown out of the hole rather than allowed to drop in, but I also suspect that many times when people report no problems, it's actually because the shavings embedded themselves in the pistons without doing any damage to the rings or cylinder walls. I base this on the appearance of a piston or two in a few motors that I've torn down.

Good evening,

Standard way to tap a hole and not drop the chips into the hole is simple: fill the tap flutes with grease. Tap a few turns, backing regularly to break the chips... then remove, clean and regrease.


Michael