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Old 02 March 2014, 08:18   #1
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Threaded spark plug repair

Routine gear oil change on my Tohatsu M60C engine yesterday. Took a little longer as the fill cross head screw was seized. Heated it up and it came out fine.

Then onto the sparkplugs. The screw threads have never been quite right, and at some point someone probably cross-threaded them. Part of the reason I always thread them on by hand, then use a ratchet. They tightened up okay last season, but wouldn't tighten sufficiently yesterday.

On removing one of the sparkplugs, there were little metal shavings. Just great.

Screw thread is goosed on top cylinder, and middle one doesn't look great, so will need to fix both. I thought twice about removing the cylinder head, as I was wary of shearing bolts.This engine only ever gets used in the sea, so that was a distinct possibility. Pressed on and removed the cylinder head. The bolts were tight and had powdery residue when extracting. The gasket is in good shape and didn't stick to the cylinder face so can probably be reused. I cleaned both faces up. Cylinder bores are clean, no scoring. Water passages as you can imagine had salt crystals, so again, all cleaned up.

To repair the spark plug holes, what would you recommend? I've used Helicoils before (on a car) and has worked okay, however that was on a suspension strut (secured by another 2 bolts) not on a cylinder head. The depth of the hole on the cylinder head isn't huge, possibly 8-10mm. Should I be looking for a solid insert instead, such as V-Coil? Only trouble with V-Coil is then removing the excess material inside the head (smallest they supply is 12.7mm). Probably need to use a Dremel with cutting disc.

Engine is 2006.
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Old 02 March 2014, 08:25   #2
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Try recutting with a tap first. Then Helicoil, then solid type insert unless it's really obvious you need to go straight to the solid type. I don't like any of the options much, but Helicoil is far preferable. Solid inserts are a recipe for white deposits round them. I've had bike spark plug threads helicoiled before and they lasted for a very long time..

Or find another head.

Don't resuse the gasket. For want of a nail, the kingdom was lost, etc.
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Old 02 March 2014, 08:32   #3
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Pictures. If you look closely there is cracking on the inside of the cylinder head bore. The spark plug thread could have been weakened before, as the outside of the head also had a little deterioration on the face of the plug thread.
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Old 02 March 2014, 08:34   #4
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No scoring on the cylinder sleeves.
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Old 02 March 2014, 08:40   #5
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Face of cylinder head. Note the top hole face deterioration, (left hand side).
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Old 02 March 2014, 08:41   #6
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New head. I wouldn't trust that now if there's cracks appearing.
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Old 02 March 2014, 08:48   #7
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Quote:
Originally Posted by Nos4r2 View Post
Try recutting with a tap first. Then Helicoil, then solid type insert unless it's really obvious you need to go straight to the solid type. I don't like any of the options much, but Helicoil is far preferable. Solid inserts are a recipe for white deposits round them. I've had bike spark plug threads helicoiled before and they lasted for a very long time..

Or find another head.

Don't resuse the gasket. For want of a nail, the kingdom was lost, etc.
Cheers Nos. Just knew you'd mention the gasket. Honestly it looks fine. I was going to use a smear of Hylomar Blue gasket sealant, little marine grease on the cylinder bolts, tighten in sequence, etc.

It's all money just now. I've got a Helicoil kit on order, so will try that. Might know where I can source another head, but will try to repair first.
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Old 02 March 2014, 08:57   #8
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Helicoiled one of my opti plug threads. (I paid someone as it involves fuel rails and stuff) No problems...:-)

Defo a new gasket....it might look ok, but it will have been compressed so won't ever be the same.

As for the head...if you can see cracks...it's worse than it looks. Find another would be my choice.
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Old 02 March 2014, 09:03   #9
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Originally Posted by spartacus View Post
Cheers Nos. Just knew you'd mention the gasket. Honestly it looks fine. I was going to use a smear of Hylomar Blue gasket sealant, little marine grease on the cylinder bolts, tighten in sequence, etc.

It's all money just now. I've got a Helicoil kit on order, so will try that. Might know where I can source another head, but will try to repair first.
It's been compressed once. Don't do it-sea water in bores won't be good.
Blue hylomar on a head gasket is a really REALLY stupid thing to do. (sorry, had to fix too many motors where it'd been done)
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Old 02 March 2014, 09:14   #10
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I used a spark plug helicoil kit before. Worked perfectly, just make sure you use the correct heat resistant loctite.
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