How tight are your spark plugs ?

[The Gurnard]

I wonder if anyone can assist with working out why one of my spark plugs appears to be “rusting” rather quickly.

Its the bottom plug in this photo. Both plugs were brand new at the start of the season and although I go long multi day trips with the OB , I doubt they have done 1500 miles so far. I check everything out after every trip and can’t say I noticed the plug colour last time. I do unscew them and check the electrode colours after every trip too and both are still a nice tan brown with no soot as expected.

Is it because I perhaps didn’t tighten the plug enough? It certainly wasn’t loose when I unscrewed it but I don’t overtighten them when I put them in ..possibly finger tight then a half spanner turn after that, as I seem to recall the washers should not be compressed with over tightening ?

Is it a sgn of something more sinister ..or just a “Friday afternoon” plug ? I have swapped them over for the next trip to see what happens and I always carry a spare set anyhoo.

The engine runs faultlessly and starts easily ..so I don’t think its a problem.
Any ideas why the plug is different ? Its never happened before



A closer view of the offending plug



How tight do you screw up your plugs ? I don’t own a torque wrench so hopefully a verbal description will help me. Thanks for any thoughts on the matter.

[spartacus]

I don't think it's anything to do with tightening the plugs Gurnard. I get the same on my Tohatsu, but maybe towards the end of the season. Generally the salinity of the environment finds a way to corrode spark plugs. Check the rubber trim on the cowl, and clean with a little methylated spirit.

You have a bit of corrosion on the cylinder head, so check the thermostat gasket.

In terms of tightening plugs, on outboard alloy heads its very easy to strip the threads if you overtorque, so I tend to tighten them up, then quarter turn. Don't use grease on the threads.

You can use a little white grease to keep corrosion at bay. I spray the linkages while I'm there.

[TmMorris]

I'd agree. It won't be related to tightening. And with regards to that I just spanner them until you feel the squishy washer taking the load and then give a bit more for luck.

My first thought when reading the thread was just that you had got salt on the lower one, transferred from your hands when checking them.

Out of interest, I would swap them over just to see if the clean one is hen goes rusty in the same position. If it doesn't then I'd say it was simply plug related and a bit random. If it does go the same way then I'd be wondering how salt water is getting to it?

If the inside of the plugs are equal and looking good then it doesn't strike me as being anything wrong with the engine.

[Fenlander]

I've never know any cowl that is sealed to a degree it would prevent heavy splash entering the cowl a little (or a lot in some cases). As it is the lower plug I'd assume the one most likely to suffer such splash.

Of interest but not a worry... and like TM I'd probably swap them just to see if the other possibility might be the case... that some batches have poorer material at resisting corrosion.

I've certainly seen one rusted and one not on used outboards in the past but always change them for new without really noting if it's top of bottom that's worse.

[The Gurnard]

Thanks for your thoughts guys, much appreciate.

It did take me by surprise seeing it as Im almost certain it happened during the course of my recent 4 day trip. I checked them after the journey with Jeff the other week and would have noticed the difference then if it had started a while ago. I generally keep the same plugs for two seasons and have never seen one so rusty.. perhaps its just been luck.

At least your replies give me piece of mind its not something that could prove a disaster if it was self sacrificing because of some electrical issue. Although I always have the auxiliary ..it would not help too much if I broke down in the Corryvreckan tide race or somewhere similar.

The thermostat gasket should be tight as its a piece of my old Seago PVC SIB. I check it regularly and re grease both it and the bolts, as the original ones broke a few years ago, when I first went to check the thermostat. I had to drill and re-tap and don’t want to do that again ..so they are unscrewed, re greased and tightened several times a season.

I do get some salt water into the cowl .. because when I take the cover off to check things..moisture can quickly appear on the base plate and I see slight traces of salt. I generally flush the base plate with fresh water while I flush out the outboard in a bucket and that helps keep it in reasonable condition. Its a 2005 Mariner 2 stroke and its had a lot of use so its not too shabby considering.

Anyhoo..thanks for your thoughts..I can sleep easy but keep an eye on the plug swap and see what happens.

[TmMorris]

I think the key is that if it were something sinister then you'd see something on the thread and around the internals. If the plug just looks identical to the other then I really can't see why it would be anything to do with how the engine is running.

Is the 2005 Mariner a Yamaha or had the change over happened by then? If it's a Yam then don't you have the telltale hose right by the lower plug? There are also the drain holes at that point also?

I imagine from reading your fantastic threads that your engine is possibly no stranger to wash hitting the back of the motor?

That's why I'm wondering if swapping the plugs over might replicate or suggest it's a one off due to the plug itself?

[Fenlander]

Something that does come to mind... heat. Could that lower cylinder be running hot? With paint such as a the lightish blue of the Yamahas you can see overheat as browning on the paint and it's something I look for on a used OB and avoid if excessive. But on that black engine you'd not notice.

Edit: Because it interested me I Googled a little more to see several mentions of a over-low tightening torque preventing proper heat transfer and causing a plug to overheat. It seems that the plating will reduce in its rust resistance when heated much more than 10degC over an average thermostat rating for an outboard. Could be a factor?

[neil.mccrirrick]

I’d say it’s fine Donny. Likely got some water in the engine bay. That with the hot weather we have had means it would be baking inside the cowl. Hot air water and salt equals rust. As long as it’s not on the threads should all be good.

Swapping the plugs out is a good plan. As fenlander says it’s a possible, but unlikely, the head gasket has gone which is allowing hot water to land on the plug but super unlikely

If it has gone and you need a hand give me a shout not a big job to take the head off

[The Gurnard]

Yup..thats the tell tale hose in the first photo TM. I extended it through the drain hole with a small length of thin brass pipe so its def not coming in from it. Yes there are several drain holes around the base plate though so understandable some salt water could get in but not a lot. Im not sure on Yamaha or mariner but its made in Belgium and not the US is that means anything ?

Now I would be worried if its heat Fenlander.

Would I be able to tell from the plug electrodes ? They both look exact same. Ok a little darker than golden tan colour but Im running it on 50:1 premix instead of the 100:1 recommended for leisure use. Reason I do that is commercial use is recommended 50:1 and I do use it more than an average leisure user.



It runs as sweet as a nut and doesn’t cut out even when run for hours.. would it still do that if it was overheating ? Plenty water coming out the tell tale too so impeller is not a issue ?

However it is the first time a single plug has rusted so quickly..and its def rust.

Hi Neil ..good to hear from you too.. I guess I will have to keep a close eye on it with some local trips before going further afield again. Were you on the Seasafari boats on Tuesday when the mist was down ? They must have thought I was mad messing around in the Grey Dogs in the mist. I lost them just at the entrance but "found " them again on the western side of Scarba..I presume they went through the Corry. We had to dodge each other but with a cheery wave and not a honk of horns

[TmMorris]

It's the crust on that plug that is niggling me.

Let's say the heat managed to break down the protection on the metal and then repeated heat cycles plus salt water went to work, I'm not sure you'd have a crust like that.

Obviously, the bulk is rust but there looks to be a chemical accelerant at play?

I wonder if the fact that the corrosion has extended under the sealing washer is the clue? Being flush to the engine block it should be free of the corrosion that we can see on the exposed metalwork.

What springs to mind is that if that plug were only finger right then blow back could be the cause and the combustion mix would certainly be the corrosive accelerant.

[The Gurnard]

That makes sense to me TM... and it does feel a bit crusty as well as rusty.

I didn’t notice until you mentioned it but yes..the contact face of the washer is also rust / crusty which could happen it it wasn’t tight enough.

I presume it would still spark and run well as there would be contact through the threads ?

In truth .. like your thoughts better than Fenlanders as I can easily tighten the plugs a bit more. I suspect removing the head would be a nightmare as that is two of the bolts that hold the thermostat cover on and they both sheared ..so suspect more drilling and tapping if it has to come off. However I don’t see any water seeping out when its running in the bucket..so hope its not the gasket.

[jeffstevens763@g]

It’s salt splash had it on a 40 mariner Donny I just sprayed WD40, the washer on the plug is a type of spring washer just need plugs nipping up.

[neil.mccrirrick]

Wasn’t on seafari Donny I’m only doing the odd favour. Was planning on loch Aline but called it due to the mist. Went out later to loch buie [emoji106]

[The Gurnard]

Ah..I hope to camp on Loch Buie soon as I always enjoy a night or two there. I was going to camp at the secret old slate harbour not far from you two nights ago but a JCB with a jack hammer was pounding rocks near by so I just came home again. Will give you a text next time Im passing and see if you are around

[The Gurnard]

Hi Jeff..just noticed you slip in there so thanks for that..Im looking on the bright side that now I have swapped positions..I will ignore it unless the other one rusts/crusts as quick ... now its in that cylinder. Hope all is well with your and the missus.. just yell next time you want to come to Oban way

[Fenlander]

Those extra images in post #9 makes it look very much like a loose plug as TM thinks. Rusty/crusty due to some blow by plus extra corrosion as an over hot plug due to poor heat transfer.

Nip up a bit more and you should be sorted.

[jeffstevens763@g]

Originally Posted by The Gurnard

Hi Jeff..just noticed you slip in there so thanks for that..Im looking on the bright side that now I have swapped positions..I will ignore it unless the other one rusts/crusts as quick ... now its in that cylinder. Hope all is well with your and the missus.. just yell next time you want to come to Oban way

Wednesday is opp day for her fingers crossed thanks for thinking of us. Definitely will give you a shout

For info bottom pic you can see salt deposits on the plug cap