Spark plug stripped threads in alloy cylinder head

skyflyer

skyflyer

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Not strictly boaty, but easily could be and I value the advice of other forumites.
I started to remove the spark plug from a (4 stroke) engine today and initially all was normal. Then it started to bind. I couldnt really work out what was happening as it couldnt be a crossed thread (having gone in by hand easily a few weeks ago). I tried to screw it back in and then out again but nothing helped. Eventually I could see no option but to use more force.
So out it came - the last three threads of the plug (i.e. the part deepest into the cylinder) are clogged with alloy - so badly that I cannot clean it out of the threads.
The equivalent part of the cylinder head threads are stripped. i.e the three or four threads nearest the top of the hole.
Thread spalling seems to be the issue and I know this has long been an issue for sailors with stainless screws in aluminium and alloy boat parts.
Anyway, we are where we are - when i repair it I will use some form of anti seize compound. I will make a reapir with a helicoil type kit, reaming out the hole, cutting a new thread and putting in an insert.
My question is this. Having researched the use of these thread inserts some people say dont worry about the small alloy chips going into the cylinder as they will be blown out or melt as soon as the engine is started. I'm not entirely convinced.
I will try to stop as much swarf as I can by greasing the tapping tool but some is bound to get in.
I dont think blowing out the cylinder with air is any guarantee either.
The 'perfect' answer is to take off the head, but its a brand new engine just a month old, so I am reluctant to do that with all the dismantling that will be required.
Has anyone done this sort of job before, and if so how did you deal with the swarf issue?
Thanks
 
Dunno if this can help ... but an old trick for a blind hole and retrieving swarf ... was to get a piece of rag with grease .... pass it through hole ... do the job and then carefully extract the greasy rag with the swarf stuck in the grease ...

Whether it could work here ?????

I have to say that when I've had Helicoils fitted - both times - the head has been taken off to do it ...

I don;t think I would like to have swarf inside the cylinder ...

If the engine is only a month old ... I would go back to dealer and discuss with him - because this should never happen ... it may be a g'tee issue ... nothing lost by trying ....
 
I did think about a warranty claim but as its isnt cut and dried that would almost certainly involve the engine (and attached machine) going back to the manufacturers which would in turn involve substantial disassembly and packing, and take it out of use for several weeks. I can do the helicoil type repair on Monday morning and have it back working in the afternoon.

Greasy rag would help thanks
 
I'd keep the cylinder as clean as possible. Turn the engine over until the piston's at the bottom and about to start the compression stroke - valves shut to stop swarf getting onto the seats, piston out of the way to avoid drilling and tapping a hole in it... Do the job a bit at a time with a little grease on the drill and then the tap so that as much comes out on them as possible, but not enough to build up and drop off into the cylinder. The 'Recoil' branded helicoil kits come with a magnetic tool to break off the tang at the end of the coil, so you shouldn't lose that. Get an airline with a long nozzle blow nozzle and use it a lot. If you bent it to get a swirl going inside than so much the better.
 
I have done this before. As others have said, grease the reamer and tap that come with the helicoil kit. Once the helicoil is in you snap off the bar with a pair of needle-nose pliers so it does not drop inside. Then find a piece of 8 or 10mm diameter flexible plastic tube and attach this to a good vacuum cleaner and go in through the sparkplug hole and have a good suck around with the piston at the bottom of its stroke. Put the new sparkplug in and fire up.

Www.solocoastalsailing.co.uk
 
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Galling is probably more likely that spalling but even that seems unlikely with mild steel against aluminium alloy. Copper or alu grease on the threads is definitely the cure although new engines are usually not so treated and I've never seen this problem before.

I have also never helicoiled without access to the other side other than in blind holes but if I was doing it I would probably use plastic tubing taped to the end of a vacuum cleaner to suck out all the swarf after fitting.

Richard
 
Done this hundreds of times, never removed the head. Never had a problem with bits under a valve seat either.

Young men and motorbikes plus high leverage tools meant it was an almost routine job.

Set the engine at TDC ensuring both valves are closed.

Follow the instructions for the thread repair kit, greasing the tap to stick the chips in the flutes.

Ensure the end of the insert if of helicoil/recoil type is held tightly when snapped off - you dont want an 8mm length of SS wire left in the cylinder!

Use a length of small bore tube inserted into the combustion chamber and a compressor to blast in the combustion chamber for a few seconds. This will clear any loose chips. Wear eye protection! They come out of the repaired plug hole.

Simples........................................
 
Plum said:
I have done this before. As others have said, grease the reamer and tap that come with the helicoil kit. Once the helicoil is in you snap off the bar with a pair of needle-nose pliers so it does not drop inside. Then find a piece of 8 or 10mm diameter flexible plastic tube and attach this to a good vacuum cleaner and go in through the sparkplug hole and have a good suck around with the piston at the bottom of its stroke. Put the new sparkplug in and fire up.

Www.solocoastalsailing.co.uk
Click to expand...
Why on earth would you want the piston at the bottom?

Best practice is at top dead centre on the compression stroke - IE both valves closed, smallest space below.

On some engines, high performance types, the piston might need to be a little way down so the tap end clears the piston. In practice the tap just pushes the piston over whatever side of TDC is less resistant to movement, always before a valve opens.

This is from a pro user of helicoil type kits for nearly 50 years.
 
I drive a kit car based on a Citroen 2CV, horizontally opposed cylinders with aluminium heads. The spark plug thread of one head has been repaired by a PO using a thread insert. This has the unfortunate habit of coming out attached to the spark plug. I assume it might be possible to prevent this mechanically but it was not done.

On another engine the threads failed as described by the OP. I helicoiled them both with the heads bolted down. Grease on the tap provided with the kit was not very effective and quite a bit of swarf fell inside the cylinder. This was easily removed using a length of thin rod with blobs of grease. Finally I blew air into the cylinder in case some small pieces remained.

Of the two methods the Helicoil seems to be the more suitable.
 
vyv_cox said:
I drive a kit car based on a Citroen 2CV, horizontally opposed cylinders with aluminium heads. The spark plug thread of one head has been repaired by a PO using a thread insert. This has the unfortunate habit of coming out attached to the spark plug. I assume it might be possible to prevent this mechanically but it was not done.

On another engine the threads failed as described by the OP. I helicoiled them both with the heads bolted down. Grease on the tap provided with the kit was not very effective and quite a bit of swarf fell inside the cylinder. This was easily removed using a length of thin rod with blobs of grease. Finally I blew air into the cylinder in case some small pieces remained.

Of the two methods the Helicoil seems to be the more suitable.
Click to expand...
 
rotrax said:
Follow the instructions for the thread repair kit, greasing the tap to stick the chips in the flutes.

Ensure the end of the insert if of helicoil/recoil type is held tightly when snapped off - you dont want an 8mm length of SS wire left in the cylinder!

Use a length of small bore tube inserted into the combustion chamber and a compressor to blast in the combustion chamber for a few seconds. This will clear any loose chips. Wear eye protection! They come out of the repaired plug hole.

Simples........................................
Click to expand...

Surely it's the drilling out of the old thread that is the most problematic part of the exercise with regard to swarf?

However, as you say, an air or vacuum line is the solution.

Richard
 
RichardS said:
Surely it's the drilling out of the old thread that is the most problematic part of the exercise with regard to swarf?

However, as you say, an air or vacuum line is the solution.

Richard
Click to expand...
You don't drill it out. Use the tapping tool in hole as is. Not sufficiently bigger that it requires drilling. Hence starting the process with the piston at the top is not an issue.
 
Paulg25 said:
You don't drill it out. Use the tapping tool in hole as is. Not sufficiently bigger that it requires drilling. Hence starting the process with the piston at the top is not an issue.
Click to expand...

All the helicoils I've inserted needed drilling out first as tapping into a partially stripped thread sufficiently to be able to insert the helicoil needed to restore the thread back to its original size would have been impossible, although I'm working with steel.

Richard
 
rotrax said:
Why on earth would you want the piston at the bottom?
Click to expand...
Because if you are going into the sparkplug hole with a vacuum tube you can wriggle the tube around to reach most of the piston crow to suck up any swarf. If the piston is near the top you can only reach a small area of the piston crown. If you are going to use compressed air to blow the swarf out of the plug hole then I agree with TDC and valves closed.

Www.solocoastalsailing.co.uk
 
As to drilling, the link to the kit showed a stepped tap. So I presume one uses the first bit to follow the original thread, with the next bit cutting the new one for the insert?

Slight interest.. I have just been given a 1950s little tractor. Not used for over 20 yrs. Engine is a little ally flat head . Bit worried about the plug. Google Energic 511
 
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RichardS said:
All the helicoils I've inserted needed drilling out first as tapping into a partially stripped thread sufficiently to be able to insert the helicoil needed to restore the thread back to its original size would have been impossible, although I'm working with steel.

Richard
Click to expand...


The first Helicoil spark plug tap was a combined tap/cutter. The first - lower part - followed the stripped hole, the next increased the diameter enough for the second part to make the thread for the insert.

The Recoil kits required drilling out to the required size.

I still have many of my thread repair kits, mostly in cycle thread and imperial.

To Plum - both methods obviously work. My pro motorcycle workshop did not have a vacuum cleaner among its kit. We used a small combustion space and 175 PSI compressed air and blew the remains out.

Always worked, never a problem.
 
So in the end I chickened out and removed the head which took all of thirty minutes! Because the engine is quite new all the gaskets separated cleanly and easily.
i used a thread insert rather than helicoil and have used threadlock on it (and anti seize on the plug) so hopefully future plug removal will not undo the insert.
The tricky part, which I slightly cocked up, is getting the tap correctly aligned with the hole (which is angled relative to the base of the head, so not as simple as putting it in a drill press)
because the thread has stripped there is nothing for the tap to ”follow” so it is prone to wobbling about until the thread cutters start to bite.
so my insert does not sit quite flat and flush onto the head.
i have yet to reassemble and test everything but I’m hoping this doesn’t mean combustion gases leaking past the insert having found their way through the small gap in the threads. I guess threadlock and carbon build up in any such microscopic gap will help.
Longer term solution is a new head for about £35. But hopefully this fix gets me back running today as I’m up against a deadline!
 
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