Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board 72 TR6 piston clearance-ring end gap?

What are the correct clearances for this engine? We are having a fit trying to make this engine live...two new sets of .020 over pistons and rings, both seized. The 1st time the teardown revealed Drive side piston 3 thou larger than the other post mortem. We assumed bad manufacturing tolerance. Now we are assuming heat expansion.

This time it was a mild seizure but still seized. (Cooled and restarted)

Drive side piston skirt showed scuffing on the left/intake side. These are Emgo pistons and rings.

What am I missing?

Not to be snotty, my friend, but measuring the pistons and matching them to the bores is part of building a top end. Manufacturers ship bad parts all the time. It's up to the mechanic to verify the parts to be installed are correct. So you may want to own up to some of the responsibility for yourself.

The piston clearance is .004" min and the ring end gap is .010" min. These are in every Triumph manual printed, even the really bad ones. Does this mean you're building engines with no workshop manual?

What oil are you using and what break-in procedure are you following? Everything has to be spot-on for one of these engines to hang together.

Is it possible that with 1 piston .003" bigger than the other that the holes were bored correctly and you got the pistons switched?
Were clearances confirmed on the second go round?
Timing off on that side?
Intake air leak?

Just fishing........
Mike

I'm no expert. mater of fact I a bit of a hack. I screwed up my first top end build so I paid for and experienced, respected Triumph mechanic to do it the second time. Guessing the right answer is very expensive, so I didn't mind paying for the lesson.
Did you know that after a cylinder has been honed, that it can take on the shape of whiskey barrel? Smaller at the top, wider in the middle and tight at the bottom. I didn't.
Not just anybody can bore and hone these things. He mentioned at least a half a dozen ways my jugs were wrong. I thought he was being a fanatic and was wondering when he was going to mention that I should only torque the engine with the bike pointed north and under a full moon and quench the bolts with stump water and eye of newt.
But I did what he told me to do, to the letter and the pistons ran in in the first ten miles.
These Triumphs are their own beast and the advise that the measurements have to be spot on must be followed.
Or you can always do it a forth time.

Yes, the builder IS responsible. I have been working from the ancient Clymer manual I have used for the 27 years I have had the bike. We couldn't get the bore done by the machine shop that I have used for the 66 TR6, 66 A65, 68 T120 (as yet un tested) the two MG engines and the GM 400 small block that I have done in the past 30 years because they have finally gone out of business. Ask me if I miss them...

Thanks for confirming that the bore clearance and ring end gap we have been using are correct.

We shall look elsewhere.

Thanks to all.

Oil pump is piped backward?

That would totally depend upon the brand of hone you use, and the honing technique employed.



Now that is total BS!

Everyone knows that before torquing, the front wheel must point toward Coventry, and the mechanic must first say a prayer at their personal Edward Turner shrine.

Ignition timing.

:bigt

Yes....(sigh)...static timed only, not strobed....

I BET IT WILL THE NEXT TIME!!!!!

Thanks Professor.

To be pedantic, static timing actually means setting the timing to the idle setting. Did you do this? It is best to set the timing to the fully advanced figure with the aau locked against the springs as this is where the engine requires accuracy. This should be ok if carefully done on each cylinder. I set the engine to 36 degrees rather than 38 to cope with any bad fuel and do strobe it but the timing would have to be a long way out to damage the engine. Mine will run ok after the battery and bulb technique (or ammeter flicking to zero with ignition on).

You can't set a Boyer without strobing as it could be miles out

Yes, set with no centrifugal advance, idle setting. It is the original points/condenser ignition. I agree, it seems as if I would have noticed pinging (pinking I think you gents call it) from an engine so far out of time to cause it to heat seize, but I'm grasping at straws here.

The face of the rings, color of the wrist pin, condition and color of the underside of the piston, condition of the carbon on the top of the piston, and the actual type and location of the seizure will tell you a lot.

It no doubt would tell YOU a lot and I'm willing to learn but as of now it would unfortunately tell me very little.
If pictures would help, I'll get some.

Aren't we here to learn. I do every day!

I hope we are a little bit farther along than Bob Seger's Night Moves. But as it is we ARE "working on mysteries without any clues". Description of colors, location, etc. would be nice... Pictures would be better.

Close up of the face of the rings (important), piston dome inner and outer, pin to show how much heat it has been exposed to, and the location and type of seizure on the piston.

It might tell us nothing, but then again it might give you some clues as to the reason of the seizure.

The only thing we know at this point is "the piston got bigger than the bore."

Not really associated with your problem, but this did remind me of an article I have at home concerning piston ring "end gap". I dont normally read such technical articles, but the more I read, the better it got! They took an internal combustion engine, and added new rings at a wide variance of end gap. The first, I think, was just about flush, or close to it, and they ran the engine and took readings and so forth. They again, removed the piston and rings, and using something like .080 gap on the last run, and all along positioning the gaps in line with each other, opposed, and just about any way you can put them, and there was no significant difference in any of the results....I may be wrong on some of these statements (disclaimer), but I'll bring the article with me to the BIBR...(If I remember)....

DPO: There is ring gap as measured when the rings are new, and there is ring gap after many thousands of miles of use.

When new, rings have a lot of spring pressure they exert to seal the face of the ring against the cylinder wall.

When old and worn the rings have lost most of their tension because of heat and wear and can no longer can seal as they once did. The result is loss in compression and high oil consumption.

When we take the engine apart, what we can see is the rings now have a wide gap and we attribute our problems to that. What we can't see is the face of the ring, and the cylinder bore has worn, and for each .001" of wear the ring gap increases by approx. .003". So it is easy to assume that the gap is the problem, while ring tension is overlooked.

While one would not assemble an engine with an .080" gap, what some would assume to be excessive will have little, if any effect upon performance. But go less than recommended and the ends of the rings can come together and cause all sorts of expensive problems. So in ring gap more isn't bad, while less can be very, very bad.