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Discussion Starter · #1 ·
I’ve a feeling this topic might develop into something more involved but for now I need to investigate a couple of issues which have come to light.

Since my bike was laid up for the winter I’ve been freeing the clutch plates and kicking the engine over periodically. Yesterday I noticed that the left cylinder compression had reduced significantly compared to the right, it wasn’t like this when I stopped using the bike, it’s such an obvious difference I would’ve noticed it. Valve clearances are present, perhaps a little too loose on the exhaust valves but not by much. A dribble of oil in the bores improves both cylinders slightly but does nothing for the differential. I’ve a compression tester on the way so I’ll measure things properly when that arrives but the cylinder head needs to come off for a look.

When adding oil to the cylinders I could see a small patch of shiny alloy on the right piston crown, it looks as if it’s started to melt. I hope the picture is clear enough, is this a sign of detonation do you think?
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Well, its dead in line to the spark plug so might be. I changed pistons in a tiger 90 a few years back and there was a mark like that plus a crack right down to the gudgeon pin. The timing had been too far advanced after an expert fitting of a Boyer. The new pistons were lower compression 3ta ones. the bike still ran nearly as good with lower power.
 

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Hi Rusty,
Generally detonation witness marks are not left in the centre of the piston, they tend (note:I did not say always) to show up at the edge of the piston. Light detonation can most times be tolerated by a piston, heavy detonation often collapses the piston ring lands.
Did you notice any ‘pinging’ on acceleration.

Marks in the centre of the piston are more likely to be the result of Pre-Ignition, a much more malevolent problem.
This problem tends blows holes in your piston crown, it destroys in seconds.
(Pre ignition can be brought about by the heat generated by detonation).

Are the spark plug values correct, a glowing plug can cause detonation.
It looks like you have a helicoil fitted, are you certain there is not the end of the coil hanging down in the combustion chamber, so that it will glow red and cause pre-ignition.

Regards
Peg
 

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Discussion Starter · #4 ·
Hi Peg,

The plugs are NGK B7ES and yes the right cylinder has a helicoil, I can’t see whether it protrudes into the head. I lowered the float levels a few months ago which cured an off throttle stutter but was careful to check the plug colour afterwards, a tan electrode with a black outer didn’t ring any alarm bells, I thought they looked just right.

I haven’t heard any obvious pinging but it is quite a loud bike. I know what to listen for having experienced it on previous old cars I’ve had, if it’s happening at speed I probably wouldn’t hear it. I’m confident the ignition timing is correct, I set that with a timing light.

Things will become clearer when I get the head off. I don’t really know the history of the engine so I don’t mind getting into it but the pre-ignition issue was unexpected.

Chris
 

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Hi, I don’t know why, but all the holes I saw in Triumph pistons were on top of piston not necessarily the exact center, but it down the side.
I’ve seen a few detonated pistons. Much of the piston was gone, part of the top & part of the side. All these were 650 or T140. Both hole & detonated. Is a hole detonation or something else? I don’t really know.

I know this, kicking bike over without starting & running can make rings do odd things. Give false compression readings. I’ve seen this several times.

Before I’d take head off, I’d start bike & ride it until warm. Then test compression.

On a side note at kicking speeds the oil pump will feed rod bearings. However much of rest of motor depends on splash lubrication. The motor pretty much needs to be running for that to happen.
If your clutch sticks maybe tying lever back or run rod adjuster screw in to separate plates is a better plan.

Squirting a tablespoon of oil in plug holes, a few hard kicks to spread oil before long storage is good plan. Best to do on warm motor as all is lubed from the recent running. Run gas out of carbs too before oiling cylinders. Reinstall plugs using oil on threads.

Next spring, charge battery, fill tank, tickle carbs your ready to go.

High humidity in air if it gets into combustion chamber can make carbon peel on top of pistons such you can see alloy. Oddly not evenly. Seen this dozens of times at work doing head gaskets on rainy days. A few days in humidity it peels.
Don
 

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Discussion Starter · #6 ·
Well, either I’m losing the plot or whatever was causing lower compression on the left cylinder has cured itself. I was nervous about running the engine so the compression readings are cold but with a little oil dribbled into the bores 5 days ago. Both cylinders measured 170 psi, about the same as they were two years ago, with the plugs re-fitted they feel equal turning the engine over by hand. This definitely was not the case last week.

I’m not sure what to do now, carry on and remove the head to investigate the possible pre-ignition (curing rocker box and PRT seeps at the same time) or leave it alone. I’ve got most what I need to do the job.

What would you do?
 

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If it was really pre-ignition (as opposed to detonation), it can wreck an engine in minutes, not worth risking. Ditto a sticking valve. I really would have a look for what it takes - of course my opinion!
 

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Hi Chris, if you have all the bits then I'd take the head off for a look, and sort the weeps at the same time. If you have a copper head gasket (as do I) then they can be annealed and re-used. I use Permatex copper spray too.

After all, we are not going out at the moment so we might as well get ready for when we can. If....

By taking the head off you can see what's wrong, if anything, and at least then have the confidence that you've seen what's what and know what's needed, or when some action might be required. When I first got my TR7RV to run and on the road, I'd no idea what the engine condition was, but it ran. I spent that first summer listening for nasty noises, looking for leaks and smoke, and finding stripped threads etc - so when I came to rebuild the engine last winter I already had a good idea of what had to be done. Now it's been rebuilt, I know every nut, bolt and bearing, and I know what condition it's in. Gives you great confidence that it's not going to go bang unexpectedly.

We hope....

It's only a good day's work to lift the head and put it back, plus time for fixing things. Now's the time.....
 

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Hi Rusty1, So yet again we see how Triumphs do with rings. Unlikely valve is sticking while riding if it was running good. Carbon from removing plus is is real & it can/will hold valve open until it blows out. Since I gather you didn’t start motor my hunch is oil had drained off rings. I’m always skeptical of cold compression readings. I’ve seen several motors torn down needlessly from the above.
That said, if you see actual damage to piston top, that should be addressed. From your photo I can’t tell.

As important is if you have piston damage is to correct the cause. If not corrected damage will reoccur.

Actually fairly common to repair damage & not address root cause. Soon motor is apart again. Loud mufflers prevents hearing ping. Makes it a lot harder to know if it’s happening. I sure can’t feel it when it’s pinging. Ping has already started before you can hear it. If you hear it your in the danger zone.
Your kind of between a rock & hard spot.
Don
 

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Discussion Starter · #10 ·
Ok, the cylinder head is off and the good news is that Don was correct, what I thought might be signs of pre-ignition is where some carbon had lifted from the piston, shining a torch through the plug hole made it look as if the piston was melting. I don’t think that there is any really bad news, the aftermarket allen head valve adjusters need replacing and there are some wear patterns which I’m not sure about so I’ll post some more pics for advice if that’s ok.

Carbon build up on the pistons is more than I expected and there’s quite a distinct ridge at the top of each bore, I think it’s carbon but haven’t cleaned it yet. The bores themselves are at +20 and are free from any scratching, in fact they’re a bit too smooth perhaps with no indication of any cross hatching, I’ll try and get some better photos. The bike did use a little oil but not enough to warrant taking it apart and it didn’t smoke…but now it’s apart.

I filled the combustion chambers with petrol and 45 minutes later most of it was still there, a little had drained out through the exhaust valves but they were doing a pretty good job of sealing. I’ll remove the valves, check for tight or worn guides and give them a light lap seeing as I’ve gone this far.

I’m no closer to finding out why compression on the left cylinder reduced significantly and then came back. Don might be right on both counts, maybe oil drained from the rings on that cylinder while it wasn’t being used. Adding a bit of oil and letting it soak for a few days might’ve loosened them up again.

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Hi Andy, That's great news! Looks like you have some oil crust there. If motor runs well & oil consumption not to bad as you say, you could leave it alone & run the rest of the life from rings/bores. Looks like bores have some wear. They'd have to be measured for roundness & taper. Hone & ring often doesn't work out on worn bores. You'd probably be better to leave bores alone.

You didn't say if you were going to reseal cyl base. If you take cyl off, DO NOT remove rings from pistons unless you will replace them. You can simply lube well, put cyl on with the old rings that you didn't touch. I've seen this low compression on both cars & many Triumph motorcycles. Motors just do this. I've seen this hundreds of times at work. With electric starter the motor may turn over a few more times to start after sitting. Kick start you feel every revolution so you feel things that would be ignored with electric start. Took the apprentices a while to grasp this when testing compression.

A perfect valve will not leak solvent/gas at all. Zero! You have a tiny seep. I expect a trace of carbon on seat. I agree 100% it is good plan to lap valves. The valve & seat wear out of round on these motors. So if a valve is removed & not replaced in the very exact position you now have a non matching sealing surface like it was before. The out of round is not much, but enough to make a difference. So yes, lap them.
The seat wears wider & the 45deg angle wears in a curve. To restore the flat you must grind both valve (or replace valve) & grind seat. There is little extra metal on both valve & seat. The deal is the curve of the seat doesn't matter in real life. Just lap it. Wait until you need guides & then seats must be ground after guide replacement. The curve of seat may reduce efficiency, but on a street motor you'd never know it. On paper the curve might reduce sealing, but after lapping it seals well. Again I'd lap in your case unless you find a really worn guides.

Rocker adjuster tip wear is a problem area on Triumph motors. I don't know why, but rocker geometry & lack of lube on tip of stem come to mind.

It can be a good plan to have the tip of valve stem reground. The shop will want to grind 45 as well. DO NOT ALLOW THAT!!! Just clean up tip removing least metal as possible. If you let them grind the 45 you'll have to lap the crap out of the seal or grind it. Trust me on this.

Regarding rocker adjuster screws, I don't know which is best quality or which last longer. I've refinished the end of several & they lasted as good as new ones. Or should I say as bad as new ones.

There are mushroom tip available. I don't know if this is better or not. The way I see it, it's still line contact.

The swivel foot ones... I don't know either. Porsche 911 motors have short rockers like Triumph in a sense. They have swivel foot & prove very durable & don't wear stem tip. However.... Porsche 911 motor parts are of the highest quality & they have huge volume of oil to head/rockers. Does weight of adjusters really matter? I don't know but at 7000 rpm things like this tend to matter a lot.

On a side note, I've been tracking adjuster, stem tip wear on my bike. I lapped valves & ground stem tips, refinished adjuster screw tip. Compared to old Castrol GTX 20-50, Mobil1 vtwin 20-50 has cut wear in about half... At work we learned different oil can double motor life.

You can put adjuster in electric drill or drill press. Use emery cloth wrapped around a file. Use oil or solvent for a cutting fluid. Or buy new ones. Some new ones the face is very rough. I'd polish those to remove the grooves.
Don
 

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Hi Chris, subject to measuring the bores etc, looks like a good clean up and a valve lap might be all you need for now.

Those pistons are scary though - my 750 pistons are almost flat across the top! I do have the same as yours in my Daytona though.

I'd strip the head and have it and the rocker boxes vapour blasted - it'll look like new.

Andy
 

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Discussion Starter · #14 ·
Some more pictures;

Valve clearance adjusters…soon to be consigned to the bin. The pushrods are all straight and I think the cup ends are ok but the other ends have a circular wear pattern which doesn’t look ok, should I replace them as well? As an aside I’m amazed that these little alloy rods even stay in one piece at high revs, they’re tiny.

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Hi Chris - yup, those adjusters are shot. Get new nuts while you are at it - think they changed the threads before 72 so take care to get the right ones. Not sure about the rods - one cup looks dubious and one of the bottom ends, but not sure about these. Can't remember what mine looked like.
 

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Years ago, many people had serious wear with these aftermarket hex adjusters. They looked exactly like yours with bits of metal breaking off. The mushroom head ones also suffered wear problems. This is one of those dont change parts that are serviceable moments. I have had the adjusters on my engine for near 50,000 miles so far.
It is very rare i buy new pattern parts as i prefer to find original used parts except for bearings.
Rusty, it is quite normal to have a ridge at the top of the bore and it is often a carbon build up. Do not scrape it off or you may find your engine will burn oil until another ridge builds. I would be careful who sells you the new tappet adjusters. They will probably be Harris ones which should be good. Buy from the big dealers of parts. I use Montys in recent years but previously Len Craig and Sons. Both excellent service and good price.
 

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I work for a company that sells all manner of valve adjusters for coventry and meridan makes and have seen a few failures. Mushroom heads have a problem with rotation changing the clearance. The Allen head ones maintain their clearance better. Andy is right about the TPI change at 72, be aware of that, use a thread pitch gauge (BSCy) if you have one to check the old ones. A lot of unburned fluids on those pistons but nothing a decoke kit isn't made for.
 

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Hi Rusty1 & Andy, Sorry I got the name wrong.

I've removed carbon from piston top & around top of cyl many times & never had a problem. However shop manual speaks about leaving it.

In any case clean tops of pistons. If cyl are not removed. Vacuum off any carbon you can from piston tops after scraping. Then stroke piston down slightly. You'll see loose carbon on the walls. Wipe with rag. Move piston more, wipe more. Since rings were oiled they will deposit most of the loose carbon on cyl wall. Move piston up & go again. You can add some oil around piston & it will release more loose carbon. Do this until it shows clean.

In any case, at work we changed hundreds of head gaskets. Scraped piston tops & combustion chambers. Some techs didn't clean off the carbon as well as others. That always bothered my, but I didn't see one scored cyl by their sloppy work. So don't stress over it, just do the best you can.
Don
 

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Hi, Speaking of the deposits on the pistons, at work we found the "detergent" gasolines, specifically Chevron premium & Shell premium, the additives indeed tended to keep combustion chambers much cleaner as well as intake valve stems.

The difference between Costco & Safeway gas to the above was staggering regarding deposits. The additives in the Chevron & Shell are the real deal.

California demands a minimum standard for additive package. Seller of fuel can make as much better as they want. The minimum standard is certainly not enough. This is California. I don't know about other states.
Don
 
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