Red hot exhaust valves

Just taken the top end apart after 13000 miles, on my 1980 TR7. The exhaust valves have clearly been running red hot, judging by their dark blue colour. They were still seating well (the seats had also gone quite blue). Valves were good quality (G&S). So, that's my first question really - is it normal for exhaust valves to run as hot as this and if not, what conditions might be the cause?

The mixture shouldn't be weak - carb jetting is way richer than standard (straight through 2-into-1, no silencer). Ignition timing was OK when I fitted the third Trispark unit about 3000 miles ago, and nothing has moved since (though I thought it was sounding a bit retarded the last few times I rode it - one of the symptoms of a Trispark about to pack up!).

The head has some kind of aftermarket exhaust stubs, meant to convert push-in to push-over pipes. They are fairly thick aluminium - more than double the thickness of the genuine screwed-in steel stubs. I wonder whether these could have been restricting exhaust gas flow to the point where heat was building up excessively?

The combustion chamber squish bands were in a strange state, looking as f they'd been roughly shot blasted. They looked as if the aluminium had become so hot it had been close to melting, leaving a 'ridge' around the upper edge of the squish band of roughly .5mm (maybe 1mm in the worst places) that you can feel very easily with a fingertip.

The pistons seem ok though, and if this kind of heat had been accumulating, wouldn't the expectation be for the pistons to suffer? Any thoughts would be welcome. I have some photos but not sure how to put them on here?

Without pics, we’re stabbing in the dark here...

But, how long have you had the bike? A lot of questions lie in it’s history...

It sounds like it’s got bloody hot. The squish bands that look like they’ve been roughly shot blasted may have been!? Alternatively, that sounds like the kind of damage that detonation can cause. And / or the debris from a grenaded piston can cause.

If it was heat / detonation, as you’ve already said, one would expect the pistons to have melted. But perhaps they did? Is it possible they have been replaced and the damage scared head bolted back on?

No, it’s not normal for ex valves to come out blue, or the seats. That engine has suffered from very excessive heat at some point for some reason.

I had a friend who’s oil light came on on his almost new T140 on the motorway, so he ‘slowed down to 50mph until he got to the next services’. When I explained, with horror, that the oil light was not an oil level warning, it was an oil pressure warning, and it meant LITTLE OR NO oil was getting pumped into the engine he looked gutted! That engine got so hot the head warped. It smoked a bit afterward but was surprisingly OK otherwise. It got stolen shortly after this little hiccup though, so we never got chance to do a proper post-mortem.

Can only guess it’s weak or retarded.

"The combustion chamber squish bands were in a strange state, looking as f they'd been roughly shot blasted. They looked as if the aluminium had become so hot it had been close to melting, leaving a 'ridge' around the upper edge of the squish band of roughly .5mm (maybe 1mm in the worst places) that you can feel very easily with a fingertip."
That kind of look is typical of detonation that beats up the pistons around the circumference..High speed detonation can go unoticed for a time if it's not too severe..
Of course what TT says above can also be considered..

Weak mixture can start the detonation!

Thanks everyone for your input on this. I realise that wothout photos it is pretty well impossible to say anything definite. I have got photos, but I can't figure ut how to post them on here - is it something only paid-up members can do?

I can add a little more info since my original post, ie: the perimeter of the piston crowns are actually similarly beaten up, though nothing like as bad as the squish band. Excessive heat seems the cause, but how the pistons got away with it seems a bit of a mystery, and also, how this excessive heat got generated in the first place?

The carb is set up miles richer than standard (330 main jet instead of 270, needle on richest setting, 2 1/2 cutaway slides). The ignition timing was spot-on according to my strobe light 3000 miles ago (when I fitted the last of 3 Trispark ignition modules). It was also spot-on by the strobe (rotor timing mark verified by flywheel notch and Triumph locating tool) when the previous Trispark module was fitted, and when the first complete system was fitted. So I don;t think the timing was retarded (though having said that, it did SOUND retarded during the last few rides before taking the top end apart - this seems to be a first sign of the Trispark systems being on the way out, so maybe it was?)

I will post pics if anyone can tell me how! Or I can email pics to anyone, if they can then post them?

Tigernut said:

Thanks everyone for your input on this. I realise that wothout photos it is pretty well impossible to say anything definite. I have got photos, but I can't figure ut how to post them on here - is it something only paid-up members can do?

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L.A.B. said:

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Brilliant, thanks LAB - here are two pics, one showing what I mean about the squish band, the other a close-up of a piston. The head shot also shows the 'blue' exhaust valve seat. As I mentioned previously, the engine was running nicely with no loss of compression when kicking it over, and only a slight loss of power noticed in the last 500 miles.

The head did not look at all like this when I fitted it 13000 miles ago, and the pistons were brand new. Mixture unlikely to be weak (see previous post on this). Timing not out when set, and no reason to believe it has slipped (the Trispark module hasn't moved - I made marks on it and the housing when I first timed it).

Could the exhaust stubs (aftermarket aluminium things, supposedly to be 'pounded in' but I had them welded in) have restricted exhaust gas flow to the point where excessive heat has built up, enough to do this? Or am I missing something more obvious? I would very much like to understand what has happened here so as to make sure it doesn't happen again.

That looks like RATS . = Detonation .

Re Valves . Id always figured they ran White Hot , injun fully wound out .

Blue / White being 1100 degrees . Exhaust Flame .
Which is why their racing bikes and some production jobs ran NIMONIC VALVES . Hollow Sodium Fulled .
Of not fulled actually . Half Fulled . - Hollow Stem .

ANYWAY , most Triumph chaps have seen the Odd Exhaust go Dull Red or better , at night .
Pulling up from a hard run . Red Hot on a dyno is normal ( If the engines any good )
WHITE HOT Exhaust System on a Dyno , if its got any real balls .

but again , the erosion would be detonation . Checking timing Marks Vs top dead center
on any engine thats due for a workout , when built is mandatory .
Not to do so is negligent .

PLUS there were two Triumph Twin timing Index set ups on the Crankshaft .
A combination of the two would give a MISSMATCH .

Also , most honda boys with their C V carbs dont have the aptitude to modulate the throttle correctly on a Bonneville .
Some d*ckwa*k sticking it on the stop at below 1/2 revs would get pinging , and rats eating bits - so its not that unusual
to see a pock marked squish band , on a alloy head .
Tho could also be no air filters on gravel roads . If theres airborne fine debri where its ridden .

Could the coke in the pits mean that the damage was done a long time ago and the engine hasn’t had detonation recently?

Looks like it may have been burning oil and that valve seat looks a bit pocketed in the photo.

Matt Spencer said:

PLUS there were two Triumph Twin timing Index set ups on the Crankshaft .
A combination of the two would give a MISSMATCH .

Click to expand...

Please explain?

Triton Thrasher said:

Could the coke in the pits mean that the damage was done a long time ago and the engine hasn’t had detonation recently?

Looks like it may have been burning oil and that valve seat looks a bit pocketed in the photo.

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Good detective work! Yes, the engine has been burning oil (that's why I took the top end apart). I hadn;t thought of the fact that the pits have black in them meaning it happened some time ago. But thinking about it, I don;t suppose it would take very long for freshly eroded pits to become blackened?

The valve does look pocketed but it is actually a build up of carbon around the seat.

I'm wondering when this damage got caused. I have taken it for a couple or three full throttle top gear runs, just to check performance (and found it to have dropped off a little). I don;t normally use full throttle at all, so maybe it was this? I experimented with closing the throttle slightly to see whether the engine ran better, but it just slowed slightly as you'd expect, so the main jet mixture isn't rich. And with a 330 main jet I can't see that it would be weak.

Any idea how long (in terms of time or miles) it can take to get this level of damage?

Are you sure this isn’t due to insufficient clearance between piston and head?

Fast Eddie said:

Are you sure this isn’t due to insufficient clearance between piston and head?

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Do you mean direct physical contact? I'm sure that hasn't been happening. But I'm guessing you don;t mean this - in which case, how would I know, or work out, whether there is too little clearance? I don;t know what too little would be - I have never seen any reference to figures. The head was only lightly skimmed (about .008" I seem to remember). It doesn't look to have been skimmed before I got the bike. The gasket I used was the composite type, which I regretted as I had to re-torque repeatedly. It had crushed down to .041" (from at least .051", going by the thickness where it had been pushed into the annular grooves around the cylinder tops).

Fast Eddie said:

Are you sure this isn’t due to insufficient clearance between piston and head?

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Stock clearance on 650-750's is around .060 or a bit more...I have reduced the clearance on my 750 to .032 and it required milling the cylinder . 016 and using no cylinder base gasket, a total of .036....Of course who knows what was done to the engine ..and piston to head contact can look like that. And contact is quite noisy.. But detonation damage can look just like that also

Tigernut said:

Do you mean direct physical contact? I'm sure that hasn't been happening. But I'm guessing you don;t mean this - in which case, how would I know, or work out, whether there is too little clearance?

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Check clearance by turning the crank over TDC with pieces of soft solder between the piston and head and then measure the thickness of the squashed solder.

http://www.nrhsperformance.com/tech_squish.shtml

Edit: What compression ratio are the pistons? What octane petrol have you been using?

Excuse the digression , but you can regard it as a treat .





Picture in a old Autocar mag. of a 4.7 / 1970 Ghibli engine , exhaust white hot ( Theres a rotisary on u toob same )
Factory test then had Max Output held , often for several HOURS .

Temperature Stability obviously approaches ' normalised ' for those rpm's after several minutes .
Thus the ten minutes running before a large high output engine is HELD on W F O .

Dynos being notorious for underjetting would be evident with their ' flash ' readings .
The apparatus would not be Temperature Stable . Also pressure cyles on a allready at highly wound up
I.C.E. will not stabilise untill 10 to 15 seconds throttle ' on the stops ' ( or carb slides in reality )
perhaps and probably twice that . To get accurate mixture for say a salt lake machine .

Besides that , was recond to be 40 thou. for a fresh built race / hot street Triumph .
Piston to head . To allow for bearing take up and rod stretch . The Alloy Rods having greater elasticity that steel .
( Think about held Max Output Vs Flash RPM on that )

Past 20.000 miles ( or half that . or less . If Raced ) cumulative clearances will get 50 or 60 needed . Produced rather .
Beyond that it probly to loose to get up there . Maintanace pre detergant oils was a annual De Coke on a every day motor .

But these are MINIMUM . 65 clearance noting'd hit ever. Unless it broke. Which the hammering resultant from excess cleances would produce .

L.A.B. wouldnt stake my life on it , but if the recolections not to abysmal , around 18 months after the screw in crank timing rod was introduced ( or thereabouts . Was going to say 2 years . Approximate . If Im not misstaken !) The Position of the receptical for the rod was put in a less inaccessable posion .
Thus differat degrees in relation to T D C .
Thus requireing the appropriate / matching crank ( flywheel ) to case . for the thing to work . As Required .

The Alternator Rotor Timing Marks are notorious for being ' about right ' . If the keywas in order and the magnets / alumnium hasnt wrenched ( Like a tyre on a rim . )

Thus at any overhaul are verified . Easiest to mark T D C at T D C actual . on the rotor , as a verification check . If your persnikity on your ignition point .
Which Pays .
If your in the habit of opening it up on occasion . Theres a nice Video on board a Rob North Triple at Spa where He isnt shy of the throttle .

Back to the issue at hand , somewhere well inside 10 thou sqish , piston to head , the shock wave goes supersonic , or whatever it is .
This means the gaseous becomes aqueous . So in effect it is hitting . Even if it isnt . Which throws the whole shooting box askew .
Id think a cleance of less than 6 thou. ( anywhere )there would perhaps explain the gnawings present .
Particularly at the inner edge of the band ( diameter ) as effectivly it would ' trap ' the outer matter over T D C .

Looks like wear / milage is disquising hammered outer piston crowns. If indeed they are , where the welt has worn away as it has formed .

Keeping to logic , the nasty little 2 stroke racing model engines - from the 60s , The pistons grow with thermal expansion.
Thus running in required untill they stop growing . Like a dustbin inverted. Red Hot . The large area ofd the disc - adjacent to the combustion
has greater thermal expansion . On a 15 mm bore a taper of maybe 10 thou , 20 thou. deep . ( a chamfer ) will allow it to grow thermnally running
without catching in the sleeve ports , or siezing through expanding past the clearance . Keeping the explanation simple .

Thus the thermal expanision of a piston Is Not Uniform . Which amoung other things is why theyre made oval , so theyre about round at operating temperture .
Then there the heat transferance from the piston . Past that its infinite minuatae , My thought is the hot racing glow motor tuners put the clearances at what a
' on peak ' ( after several hours of carefull operation ) one would be . So it was effectively pre clapped out . Good for 20 flights. No more than 50 .

Thus careful care and maintance and prolonged caution in approaching hard out use , will undoubtably prolong the operating life . At cutting edge performance.

Anyway , looks to me the shock wave theres getting overly ingrown . Tho this is only a opion observing a bit of a ridge perhaps , clearnce wise , maybe 1/4 in.in.

As in it appears the piston is not entirely flat to the squish band , across the width of the squish band . Then theres Flat & Coned Squish .

Off course it could be something else altogether . Like the fuel octane .

Hillbilly Bike said:

Stock clearance on 650-750's is around .060 or a bit more...I have reduced the clearance on my 750 to .032 and it required milling the cylinder . 016 and using no cylinder base gasket, a total of .036....Of course who knows what was done to the engine ..and piston to head contact can look like that. And contact is quite noisy.. But detonation damage can look just like that also

Click to expand...

This engine has always been quiet, so I can't imagine any metal-to-metal contact has taken place. The perimeter of the piston crowns don't protrude above the top edge of the block, and the head gasket is about .040", so no way can there have been actual contact.

L.A.B. said:

Check clearance by turning the crank over TDC with pieces of soft solder between the piston and head and then measure the thickness of the squashed solder.

http://www.nrhsperformance.com/tech_squish.shtml

Edit: What compression ratio are the pistons? What octane petrol have you been using?

Click to expand...

They are 8.6:1 pistons, but the head was skimmed (by about .008") without me asking for it to be, when I first had machine shop work done on it before I built the engine.

According to my compression tester gauge the compression is very high, at 210psi (within a few psi side to side). The exhaust cam is a 70-9989, set using the stock timing marks so no idea what the actual timing is.