How screwed am I?

I have been working pretty hard on getting my 850 commando back on the road after sitting for almost a decade. I just finished rebuilding the carb and making a throttle cable for it.

After this work, it fired right up and ran better than I ever remember it running. Went for a short ride today and everything seemed to be fine once again, which gave me hope I can actually ride it further than a mile without getting nervous.

I get back and figure I need to change the plugs, as I skipped that because I was too excited to ride it. Upon removing the plugs however, I noticed some shiny speckles on the otherwise carbon fouled plug — my stomach sank.

Anyway, this is the picture from the piston with the worst of it. Sorry the picture isn’t great. As the title suggests, how screwed am I?

Borescopes are a gift from the heavens, but that picture is not the best. Is that a hole in the piston or carbon buildup? If it's a hole and the pistons are at +.040", and there's no lower end damage you could be $106.21+ cost of the gasket set screwed.

it is carbon build up, not a hole. The last time I looked in there the whole piston was carbon fouled, so whatever made it get hot started cleaning it off before the piston edge got too hot it appears.

Calibrate the degree plate in the primary chain case and carefully check you ignition timing.

So long as your carburation is half way right as per the book it's hard to destroy a piston from bad carburation. But it's very easy if you have your ignition too far advanced.

Unfortunately I proved this to myself. Twice.

johnm said:

Calibrate the degree plate in the primary chain case and carefully check you ignition timing.

So long as your carburation is half way right as per the book it's hard to destroy a piston from bad carburation. But it's very easy if you have your ignition too far advanced.

Unfortunately I proved this to myself. Twice.

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I'd say he's not screwed at all, and definitely needs to check the ignition timing. Do you think those close to a decade old spark plugs might be too hot?

Would the damage on the edge of my piston necessitate a tear down, or do you guys think it’s minor enough to not be a huge issue?

Not sure if I should start by checking ignition timing, or just pull the head.

The plugs were changed recently before my carb failed, which is why I had to rebuild it. The plugs were NGK BR7ES.

Running a Trispark ignition with single 12v coil if that matters.

Try running some NGK BR8ES. My Commandos ran fine on them and they are a cooler plug than the 7's so less chance of detonation. But by all means check the timing. Piston doesn't look that damaged to me but picture is not that great. I would try timing check and cooler plugs and see if you continue to see specs on the plug.

Put new plugs in and ride it, you said it was running fine just enjoy it.

Was it running fine before it was parked? Are there specks on both plugs? If the engine was fogged before storage, the specks on the plug(s) could just be a combustion by-product of the fog

ashman said:

Put new plugs in and ride it, you said it was running fine just enjoy it.

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He said "Shiny specks on the plugs !"

If that's aluminium from detonation then that advice will do a perfect job of destroying the pistons. Perhaps I should explain further. I would get a good magnifying glass and look at those shiny specks. If they are tiny round balls of aluminium do not use the bike any further.

I definitely would check ignition timing. And if it were a magneto on BOTH sides.

As far as plugs are concerned I ran BP7ES plugs in a 500 twin Norton race bike for years on petrol and methanol with no issues. Once the timing was correct and carbs tuned. Developed 51 bhp at rear wheel on petrol and won a lot of races.

Drain der juice .

Might be clogged car burators .

Try 100 Octane / avgass . ( while you sort it - better anti knock . keeps the rats away . )

Unless its gnawed the rings , thatll soon carburise over . Carbonise over .

LOOKS like yr ignitions to far fwd . Or plugs to soft . Whada yer PLUGS look like .
Try a fuel flow test too ,See it gets sufficent TO carbs . Theres undersize bore taps about .

I am fairly confident it is aluminum from the piston. This is the other side. The bike is running a VM34, so I would expect both cylinders to look somewhat uniform, but they seem to be quite different.

I am getting sufficient flow from the taps for sure. Definitely leaning towards timing being out, but the difference between cylinders is odd.

Moose3869 said:

I am fairly confident it is aluminum from the piston. This is the other side. The bike is running a VM34, so I would expect both cylinders to look somewhat uniform, but they seem to be quite different.

I am getting sufficient flow from the taps for sure. Definitely leaning towards timing being out, but the difference between cylinders is odd.

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Is this bike still on points? If it is then you can get different timing either side. Just like a badly built mag.

johnm said:

Is this bike still on points? If it is then you can get different timing either side. Just like a badly built mag.

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No, it is not. Trispark with single 12v coil.

Ok, here's your answer..... You spit out a circlip on one piston and it's eating the side of the piston as it goes up and down and giving you a lovely coating of fairy dust in that cylinder....... It's also giving you that special metallic oil that looks so cool in direct sunlight, but not so cool inside your engine....

So, pull the head off, roll the pistons down in the barrels and look for the tell tale sign of a loose circlip, a scratched up cylinder wall inline with the wrist pin. I've even seen people ride their bike with a loose circlip to where it broke up, traveled down the wrist pin tunnel and scarred the other side of the barrels too. It's not uncommon for this to happen to nortons, and for them to run fine until it's discovered usually during an oil change

Thats disa

o0norton0o said:

Ok, here's your answer..... You spit out a circlip on one piston and it's eating the side of the piston as it goes up and down and giving you a lovely coating of fairy dust in that cylinder....... It's also giving you that special metallic oil that looks so cool in direct sunlight, but not so cool inside your engine....

So, pull the head off, roll the pistons down in the barrels and look for the tell tale sign of a loose circlip, a scratched up cylinder wall inline with the wrist pin. I've even seen people ride their bike with a loose circlip to where it broke up, traveled down the wrist pin tunnel and scarred the other side of the barrels too. It's not uncommon for this to happen to nortons, and for them to run fine until it's discovered usually during an oil change

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Disappointing. But at least a clear result.

If it is that maybe youd spot it with them at B D C , with your picthure McHine . the mark would only ? go to wrist pin hight .
a big no - no . Anyway .

Circlip Rant .
We All
INSPECT THE CIRCLIP GROOVES for no forign matter . Check the Circlip Size - relitive ( Bigger Than , unfitted ) the receptical .
So it IS under tension fitted . and we dont use cheap ones . AND the Hard edge Outward . ( Radiused edge to pin . )

With Circlip plier nose , after its in , it will usually rotate . UNTILL YOU ' SEAT ' it . So if you want em set relitive - arefull -
draging an end opens it . Pushing an end closes it . So - when where you want it ( put it tht way going in ! ) YOU THEN

" SEAT IT "

By pushing outward on both ends with circlip pliers that open outward when handles are closeing . ( prefeably - you get pliers both ways - opening & closeing types )
and ones that are a Correct Fit . Circlip dislodement is lack of attention to these . Little 4 m.m. wrist pins with self made . 016 inch wire circlips teach you precision , in these matters .

A load a vauugely relephat tripe . ( THESE are 'wire ' round section things . Like I wouldnt use in a Norton . but some motorcycle pistons DO USE Wire Circlips .

Many have commented about ST G4500 circlip failures, damaging the piston and cylinder. They have decided that breaking off the 90 degree tab on the clip is the solution. This should not be necessary. The primary cause of the failure appears to be among engines whereby the factory installed the clip with the tab horizontal, perpendicular to the piston travel, which is WRONG! It should be installed with the tab vertical, inline with the piston travel.

Apparently not all G4500 clips have been installed this way and why all engines do not experience this problem. The tab should be in the vertical position, either up or down, inline with the piston travel, as stated above.

I have found on my own NIB ST G4500, the clip was installed with the tab horizontal. The reason for the failures is primarily due to the flexing of the tab as the piston stops and reverses direction at TDC and BDC. The piston has significant insertia and each time it stops and changes direction, the circlip tab will flex. At the maximum recommended 8,000 rpm, that equals 16,000 times the tab will flex "per minute". It doesn't take long for the steel wire clip to fatigue and the tab breaking off, damaging your engine.

With the tab facing up or down in the "vertical" position, the flex is minimized, if not completely eliminated.

Remove your piston and check how the clip was inserted and if wrong, correct it. This just might save your engine!

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you can see Tool Marks ! on this . ( naughty ) see above ! use yr stamped sheet type . Please .

talk about rough . WHO ' finished 'the ends . mustve been at smoko .

Yr S Tigre are hard bend ONE ARM . Not your open radius dual 'grabbers 'So tweezers to fit / remoove . !


These re J E . ( Whatever ) "Wrist Pin Locks "

The LEFT one - obviously shown " back upward " as in the 'hard edge . Which goes OUTWARD . The underside off it should be rounded . And DEFINATELY DOES NOT go ' outward '
So it 'eases 'in . And 'sticks ' if pushed by the wristpin .

Just to over emphisise this .


this appears to be the INNER FACE of a B M W one ! the Hard Edge not being visable ? . Tho THAT is a female circlip .

A quality wrist pin one , CAREFULLY in your circlip liers , will twang / give SLIGHTLY . ( Dont abuse it )
Sat against a piston / wrist pin . OBVIOUSLY its bigger . but set oon opening , will 'give 'enough to DROP IN - Square - not fed one leg at a time ! . IF youve got good pliers . Not wobbly ones .
Elasticity and other tecnical terms .

Fitted , a light twist , and youll fell it 'seat 'and ' lock ' , installed . you Do Not "just drop it in '' it must be FITTED caefully & correctly . & not by four people simulteaneously useing a quater of their brain .

Good Light . Clean surfaces .

Ha Ha . little 4 m.m. OS Max . 40 FSR circlips FLY , you find them , in a few days . usually . But Big Oones can fly , too . Some People use a big plastic bag , removeing stuck ones . Unseating thems a lark .

THESE are little 5 m.m. ? fit , model aircraft ones .

probably 5 m.m. across - 16 thou wire . COOL CALM COLLECTED . Seady Hand , snake eyed & focused . To Fit . ( and remove ) With NO DISTRACTIONS .

They CAN be carefully fitted exactly . If you get n a comfortable clear sightline direct access position . First . Not a half harted throw em in , she'll be right .

A Precision Fit . One of the more critical . Spotlessly clean . inspected . groove . That Tang . some remove . But Id think its from wobbling / stressing it ,
usually bend em round round nose pliers with 'hard 'crease to leg . Not had one go . But consider wire second best to ' C ' clips . IF of a superior grade !

johnm said:

He said "Shiny specks on the plugs !"

If that's aluminium from detonation then that advice will do a perfect job of destroying the pistons. Perhaps I should explain further. I would get a good magnifying glass and look at those shiny specks. If they are tiny round balls of aluminium do not use the bike any further.

I definitely would check ignition timing. And if it were a magneto on BOTH sides.

As far as plugs are concerned I ran BP7ES plugs in a 500 twin Norton race bike for years on petrol and methanol with no issues. Once the timing was correct and carbs tuned. Developed 51 bhp at rear wheel on petrol and won a lot of races.

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I managed to destroy a set of Jim Schmidt's lightweight pistons once. One of the carb manifold cap screws came loose and the resulting very lean mix produced a very interesting burnt channel down the edge og the piston to the first ring.

Might be an idea to check for air leaks on yours.

The colour on the porcelain inside the plugs should tell you if you have an air leak in the suspect side, With a double-ended coil, it is unlikely to be electrical. I would pull the motor apart and have look. Neglect can be very expensive. You are the boss of the motorcycle. There is not much effort in taking the top end off the motor. It will probably be a leak in the manifold.

Assuming your spark plug wires are not bad, the only possible issues with your Tri-Spark setup is timing off or a bad plug. there are threads he about knockoff NGK plug that are not in spec.

Any air leaks on that side can cause too lean.

I'm not positive you have a problem. Check/correct timing, check/correct air leaks, new plugs, ride a while and see if the plugs look the same.