Blown plug hole!

I had a slight ticking the other day, then Friday coming home it sounded like a blew an exhaust pipe off! I made it the 1/4 mile home and found out my #4 cylinder(? - rear left) blew the plug out. I thought maybe it had worked it's way out. So I bought new plugs, removed plastic to get a clear shot at the hole and it turns out the threads are toast! I'm now pulling the head to get an insert in and to inspect it all.

At this point, I just need to remove the timing cover to get the belt off, then can remove the head itself I do believe. Trying to find my PDF copy of the Service Manual. So far no broken bolts on anything at least.

What would cause that? A loose plug? Corrosion eating away at the threads? No doubt this has happened to someone somehow somewhere maybe with super high compression rations but I've never heard of this happening to anyone. Wow.

ST Gui said:

What would cause that? A loose plug? Corrosion eating away at the threads?

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Possibly a cross threaded plug insertion at some point weakened the threads, but this is a good place to recommend the use of a dab of anti-seize lube on the plug threads whenever they are replaced.

Bush said:

but this is a good place to recommend the use of a dab of anti-seize lube on the plug threads whenever they are replaced.

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Yep learned that when a friend was working on his Yammie TX500. He was lucky and a plug was finally removed with without damage but it was a struggle and we were— concerned.

dkruitz said:

At this point, I just need to remove the timing cover to get the belt off, then can remove the head itself I do believe. Trying to find my PDF copy of the Service Manual. So far no broken bolts on anything at least.

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the carbs are attached to the intake ports, and the exhaust to the exhaust ports, so both of those need to be removed also.

I suspect a PO wasn't careful with the limited access on that rear spark plug and cross threaded it badly. I had c removed plastic to get a clear shot at it, so I don't think it was me.

Success, it took about 5 hours with a couple of breaks. Left head is off and cylinder looks good and now to take it in to get a Time-sert installed.

I'll replace the timing belt (Irish has a stash) and I've already got the carb boots at the ready. Oh and I'll do the radiator hoses too. Fingers crossed it seems to have gone well so far. I'll check the other plug holes too while it's apart.

Bush said:

Possibly a cross threaded plug insertion at some point weakened the threads, but this is a good place to recommend the use of a dab of anti-seize lube on the plug threads whenever they are replaced.

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Uh oh.... Didn't I recently read a thread about this? To anti-seize or not to anti-seize, that is the question. IIRC, if you do goop the threads, it becomes easy to over torque the plug so be careful when tightening it down. For the record, I've always gooped my plugs (not that I change them all that often.)

Many moons ago the instructions were to tighten spark plugs just enough to compress (collapse) the crush washer that came on the plug. Always wondered how I could see that washer crush while tightening.....

Actually there should be a torque value for the plugs in the service manual. Torque them to spec and forget about it.


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I had the same problem with my old Moto Guzzi. The plug was never cross threaded, but when I removed the plug one time, all the threads were stuck to the plug. It was as all the threads just stress fractured.

If I was you, while you have the head off, install inserts in both holes.

Most likely plug was loose causing it to motion with the cylinder piston.. very common on engines. aluminum threads don't last long in a vibration situation. I don't use anything on the threads, found that proper torque is all that is needed.
I used antisieze in the past can't tell the difference as plugs without come out easily also that have been in for ages.
I'd be curious as to what thread insert they'll use? I prefer the ones that use the drive pins to lock it into place, versus the standard thread over thread type.

Spark plug torque is 11 N-m or 8 ft-lb, which is not very much.

OhioDeere said:

I prefer the ones that use the drive pins to lock it into place, versus the standard thread over thread type.

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Never heard of these. What is the brand? All I'm familiar with is helicoil type - which is a thread over thread type - i.e. screw the helicoil in and then the plug (or bolt) into the helicoil. On second thought, it is probably a very good thing that I'm not more familiar with thread replacement inserts - just like I'm not familiar with the metal plates used to repair broken bones.....

Here's the photos as of last night. I've got the head off looking to get the insert done today or tomorrow.

STring looking like a sad, wet cat!
https://goo.gl/photos/kHqEDm2v1TUL6cdN9

Timing belt and pulleys.
https://goo.gl/photos/oU36y3Ub8BYKbeh87

Closeup of timing marks - one each on cam pulley and lower crankshaft pulley.
https://goo.gl/photos/mwNCXsXEZhoEyiwo9

Cylinder head ready to come off.
https://goo.gl/photos/NfCuc61wjtynJXkBA

Bad plug hole.
https://goo.gl/photos/977BewyNcFmpSaU78

Good plug hole.
https://goo.gl/photos/o4Sk5LJoFQESum4N6

At least the walls are OK. (right cylinder blew the plug, note that's a reflection in the middle of the wall)
https://goo.gl/photos/FcPx6zssGzzwhLke8

Just curious... how did the threads on the plug look, assuming it was still attached to the ignition wire?

SMSW said:

Never heard of these. What is the brand? All I'm familiar with is helicoil type - which is a thread over thread type - i.e. screw the helicoil in and then the plug (or bolt) into the helicoil. On second thought, it is probably a very good thing that I'm not more familiar with thread replacement inserts - just like I'm not familiar with the metal plates used to repair broken bones.....

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I've used helicoil in the past on some antique outboard heads. I've had a 50/50 success rate with them. I like the concept of the solid metal insert of the Time-sert better, so I'll give that a shot.

They looked OK, but a bit dirty. In the pics above you can see the difference between the good and bad threads.

The bottom of the sparkplug holes are just right to catch dirt/grit......also getting the plug in just the right position to start , takes a bit extra attention . for both reasons., I found that for peice of mind I was best off just refraining this work until much of the plastics were off and I had an unobstructed view of the plug. Vacuuming the area around the plug first

If you are running the engine for any time with a loose plug it will destroy the aluminium head threads very quickly.
The combustion pressures are very high, probably 1000psi plus, couple this with suck, blow cycling and the steel plug will make mincemeat of the aluminium.
In steel heads the plugs usually just get blown out or you would notice the fault earlier and rectify before serious damage occurred.
Good luck with repair.
Upt'North.

Blown out spark plug is pretty common on Ford Trident engines, and the Time-sert is the go-to fix for them. I was riding in a friends V10 powered van when it decided to spit out the right-rear plug... was like a gun going off under the hood!

Let us know how this turns out!

These----> http://www.noblefix.com/noblefixcom/key-locking-threaded-inserts.htm

I hope you are not pulling the head off , there is no need to