Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Ring Gap

Preparing to mount the cylinders. Checking the ring gap - it is well over 0.014" quoted in "General Data". This is a '72 T120RV engine with barrels bored 40 thousandths over. Pistons and rings purchased from MAP - EMGO pistons. The rings are Hastings with an Emgo label (+40), and a MAP label R11050/H040.
Setting the rings in the bore and squaring them with the piston, there is a gap of 0.024" plus or minus 1 thousandth. I have read that the ring gap should be 0.014" , but have also seen 0.017" as a maximimum gap - either way these are quite a bit over.
Is this a problem or will it be OK?
Thanks
Al

the piston to bore clearance is correct? I have done some sloppy rings that were ok....Some with experience say larger ring gap is ok...But .024 gap in a 2.90 bore seems like a lot ...Try and get another set .......

Whenever this question pops up I post this link. Read this and make up your own mind... http://institutiondiagnosticengineers.org/wp-content/uploads/2019/02/Ring-Gaps-vs-Knowledge-Gaps.pdf

SR

0.014” maximum gap is best described as wrong.

Late 1960s Service Bulletins specified up to 0.020”, on new bikes.

A top ring gap of 0.024” or more doesn’t cause my 650 any problems and I can’t see why yours would be any different.

There’s a modern(ish) recommendation that the gap on the second ring should be 25-30% bigger than the top ring gap.

What is the clearance between the piston skirt and the bore?

Thanks for the replies.
TT, I don't have an accurate way to measure piston to bore clearance. But an 0.005" feeler gauge when slid between piston and bore allows for no wobble of the piston. I can try and get a measure with a vernier caliper if that would be better.

First thing first, measure the bore and check with the piston supplier what the piston to bore clearance should be and what the different ring gaps should be. Hopefully this info is supplied with the pistons. Key to check with supplier as piston material has changed since factory workshop manuals or bulletins. If pistons or rings out of spec ask for replacements. Spent a few years working for motorcycle parts supplier and it was not uncommon for users to refer to the period manuals rather than the manufacturers data.

Good point! Some manuals have too-tight piston skirt clearance specs that were meant for Triumph’s own manufactured pistons.

Most modern cast pistons use same clearance as Hepolite. I wouldn’t go closer than 0.005” on a Triumph 650.

Very interesting read about the Ford kent engine experiment. Overall I can conclude the following - close fitting piston with loose rings will work well for a while. Or- Loose piston with close fitting rings will work well for a while. Logical conclusion is.........not hard to work out if you want a long lasting engine. On a separate note many years ago i had a Ford Escort which holed a piston on the M3. Despite this I managed to drive it on 3 cylinders into London and back to the South of England without losing much oil, around 100 miles all up. How i don't know but its true.

...many use 0.015 or so in the first ring then more in the second then the same as the first with the oil ring.

+1 with the reference that Stein Roger gives.
Just to add to the authenticity----I worked at the AE R&D Cemnter at the time these tests were carried out.
In fact I shared an office with the engineer in charge of the tests.
The reference quoted by Stein Roger is accurate except in one respect.
The results were so amazing to us all that the test was continued with a ring gap of 1/8"--- and no discernible increase in blow by was measured.
So to me the lesson is--- make sure you have at least the minimum ring gap-- but dont be concerned if the gap is greater than the maximum quoted in the manual.

your ring gap does not mean anything without knowing the exact bore dia .
because for every increase in bore of .001" ... the ring gap will grow .003" ( .00314" ) ...

your bore/hone ... may be on the larger size ? making for looser pistons ... and the larger ring gap
or the rings may be on the shorter size ?

If you're pistons are on the looser side , in the bore , better to know about it from the beginning .

For whatever it's worth, Phil Irving recommended increasing bore size by .001" (on twins / singles with bores of around 3") for racing. Triumph's recommended piston/bore clearance for 750 twins was .0045", therefore .0055" would be fine. That would make the rings have a larger than normal gap but as others have said, larger than normal gaps tend to work well (and are much better than smaller than normal gas)

Al, I'm not a machinist but a few years back I invested in a 3-inch micrometer and a 2.5 to 3.5 inch telescoping bore gauge, so that I could take my own measurements. I already had vernier calipers, but they're not accurate enough for this work.

After inserting the bore gauge in the cylinder, you lock it and then measure it with the micrometer. Not quite as good as an inside micrometer, but on my last three engine jobs, the machinist's measurements matched mine.

If you do go shopping, I found it's more cost-effective to buy good quality used tools (from a reputable source of course) than poor quality new ones. For example, I snagged the used 3-inch Starrett mic for $40. BTW, I also invested in 1 and 2-inch micrometers so I can measure bearing journals.

^^^i dont understand the "tangential force" decreasing with ring wear. i have always thought ring pressure against the cylinder walls was a function of combustion pressure behind and on top of the ring.

how does this force decrease with reduction in the radial dimension? is it just less surface area for the gas to press on?

what am i missing here?

kevin, what you need to know is if the OEM's and best performance engine builders fuss to keep ring gaps to a close tolerance....And if they do, why?

The ring is a spring. If you grind the outside of say a valve or clutch spring down, it will progressively lose its spring force.
To take the ring situation to absurdity, grind half of it away, as in ridiculously worn by the bore.
There is so little spring remaining to exert force against the bore.

Also the ring spring is only compressed a little to enter the bore, to provide the spring pressure required for sealing (but not too much to produce excessive friction breaking through the meagre oil supply from the cross-hatch).
I think this is the critical bit, and once the rings have worn by say 10 thou all round, they will have lost significant springiness, and the gaps will be ~60 thou over spec.

As the article alludes, its not the increased gaps, but the reduced springiness that leads to issues.

I also wonder if by this time the original spring constant of the ring has been reduced by the temperature of operation.

I think you are right that the pressure of combustion behind the ring contributes to sealing, but it has to get there in the first place (perhaps by wear in the ring lands, which maybe helps extend usability in a worn situation?). Just guessing.

None of us have to “think” it’s right or wrong.

It’s a fact, long-established by experts. That’s why rings seal.

When there's combustion pressure in the cylinder this is what keeps the rings pressed against the wall. On the compression stroke the increasing pressure helps, but of course not nearly as much. The expansion force from the rings themselves is basically all they have.That's why an engine can feel low on compression when kicked over but still go reasonably well. When coasting, on a closed or partially closed throttle, the rings are left to their own to maintain a seal, which is why when worn, the engine sucks oil by the rings and into the combustion chamber. White puffs of smoke is the visible result. The radial width of the ring is what governs its spring force. As with any steel spring a change in thickness alters the spring constant (k).
There's more than one thing that the ring package has to contend with, and we haven't even talked about the oil control rings yet.

SR

^^^that makes a great deal of sense.

theyre looking for that last 1 percent.

IMUO (where the "U" is for "uneducated"), I would think the spring action of the rings would have to be a factor in keeping the rings firmly against the cylinder walls. That spring action would be reduced as the outer diameter of the ring is reduced. I'm hard put to see how combustion pressure pushes the rings outward, as this force is perpendicular to the ring. (I know, this blows a hole in the "ride it to break in the engine" theory.) Perhaps the answer is that both forces are required to provide good compression.

Combustion pressure gets behind the ring and pushes out.
That's why it's important to fit rings the correct way up as most are
shaped on their ID to form a pocket.

"Whoever heard of rings with 1/16” gaps – a ridiculous figure – but the interesting thing was that the
increase in blow-by and oil consumption at 0.0625” was only marginally above the figures obtained
with 0.015” gaps.
Practical tests established that the gap was not the villain of the peace. To all practical purposes the
size of the gap didn’t matter. It is important to stress at this point that we were dealing with
compression rings that were brand new when fitted to the test engine."

One is left to wonder what the increase in oil consumption and blow-by will be after 5000 miles.
I can't find compression ratio figures for the Kent engine, but it certainly used a different combustion chamber.