Hi, probably going to start a debate here, but I would like some clarification on ring gap standards. My T140 manual says piston ring gaps should be .008/.013".. Now I have being doing a little research (reason why later!) and on the Steadfast Cycles website they are selling ring sets with the following quotes...
New industry standard for British motorcycles is to gap the rings at .020.
All rings from USA (Hastings) are using this standard, as are JCC and Hepolite.
New independant tests (in USA and England) show that any gap between .024 and .010 is efficient.
There is also an article on the Institute of diagnostic engineers website (Ring gaps vs knowledge gaps) which goes into a lot of detail to back this up... 'it's the tangential load of the ring that matters, not the gap...etc....'
My T140 is bored out to +40" and I have a new set of AE Hepolite rings which have a gap of .018 when inserted into the bores.Now if this 'new standard' is correct then I am guessing everythings fine. But talking to people is just giving me lots of different opinions, but no one with a Brit Bike I've spoken to has heard of this 'new standard'!!! Anyone spread any light on this?????
The first question that MUST come to mind when you put rings in a new bore and find excessive gap is the bore too big. This can be for three reasons: 1. The new piston dimensions falls within the recommended dimensions. For the Triumph T140 750 twin it falls between 2.9871" to 2.9882"+ oversize. 2. The person boring the cylinder, for what ever reason, gives you more piston clearance than recommended by the factory. 3. Or a combination of these.
If the piston diameter is .0005" over the high limit and the person finishing the cylinder gives you an extra .0005" clearance for good luck, you have your .018" ring gap.
The standard clearance for the top rings on the 750 twin is .010" to .014". This recommended gap falls in the range of the recommended piston clearance of .0036" to .0048". If at .0036" you have .010" ring gap, at .0048" clearance the same set of rings would give you an additional .0038" gap. So just within the factory recommended piston clearances your gap for the same set of rings would give you gaps between .010" and .014."
The people who make, and size the piston ring, can only do this to the factory dimensions. They have no control over the size of the piston or the diameter of the bore. For whatever reason we have the most comments about ring gap with the Triumph 650 rings. To address this we have made up, and machined to the correct diameter plus the oversize, checking rings which we can use to measure ring gap. This way when we get back a set of rings claiming they have too much clearance we can check the rings. More times than not the rings are within factory spec's.
So before you blame the rings, measure the bore and piston clearance and compare it with factory spec's.
0.018" gap is no problem. If everything else fits well and ring pressure is OK,you'd notice very litle difference if the ring gap was 0.060". I think I've read that article you spoke of. Here's something else to look at,which will tell you to use about 30% bigger ring gap on the 2nd ring compared to the top ring gap.If you have 0.018" top gap,that's about 0.024" gap on the 2nd.Current thinking (since the mid-'80s) is that a bigger 2nd ring gap improves power and oil control. Read what Barry R has to say,about 1/4 way down the page 2ND RING GAP I believe the man has some credentials.
Car guys have been using bigger second ring gaps...When you buy performance car rings you usually have the option of "file to fit" rings. So if the bore is slightly oversize it's no problem. I do some low buck rebuilds using a rack and pinion drive hone and bore gauge. Might have to go a thousand and a half oversize to get a straight round bore and a decent decent wall finish. So I wind up with .020-.025 rings gaps and a slightly sloppy piston fit. But the engine runs well, doesn't smoke, no blow by and the owner is happy. This is fine for a guy who rides around town 1000 miles a year.
650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
At first I thought you were saying 1000 mph,and thought I know people like that.
0.0015" cylinder wear is nothing.I remember a T140 I did with 0.007" cylinder wear.That alone would have increased ring gap at the top of the cylinders by 0.022". I honed out 1/2 the taper and it ran fine.It doesn't see a lot of regular use,and it's been fine for more than 30 years.
...couple years ago or so I dismantle and rebuilt my T140 engine (still do not finished the bike so I do not fired it up!) so I was afraid of some dims like the pistons (STD) and the cyl barrels (brand NEW) from England; pistons are Harris and the cylinders are new made in UK, but I remember to see that the pistons had too much play inside the barrels, then I checked the manual and say that the factory had 3 types of barrels and put the measurements; however, this new barrels do not have any mark and the pistons do not too, so when I bought them I bought them STD and was thinking that the fit would be right...then I found that s not so incredible fit. The rings are AE Hepolite STD for Triumph T140 what was stamped on the box; after I assembled all the thing looked right but again still do not fired the motor...
Some interesting replies, thank you. All considered, I think I am happy to go with the rings, as they are, but I think I will measure the bore and the pistons today to see what I have (for peace of mind, if not just curiousity?)I will post what I find before progressing any further....
As long as you have a min .004" gap (slightly larger preferred) and a minimum ring gap of 010" (which 1-2" up the bore) and about .015" on the oil control rings you are not going to have a problem. One of the biggest things I see when some one passes me a barrel is usually a bore clearance of .002-.003" but worse is the rings have little to no gap. And not more of a gap after the barrel has been honed out.
Have just measured the bore. I have 3.0326" and the piston is 3.027".... It is +40" and the manual spec for +40 bore is 3.0321/3.0310
Also, just reading in the manual about cylinder blocks and pistons being graded to suit one another during manufacture!! L M or H (low, Medium, High) My block has M stamped at the back of the bores(on the fins)but the pistons are marked -040 L8. Does that mean they do not 'suit each other'.... God I'm baffled!!!!!
The "M" on the barrel no longer means anything.It's been bored twice since it was that size. Ideal piston clearance (brand new,by the manual) is 0.0037"-0.0047". You've got 0.0056" clearance,if you measured the pistons front to rear and 1/2" from the bottom.There not too tight,that's the main thing.
Pete,I'm asking Reverb's question too,... and ...in a nutshell, am I ok to go with what I've got? ie....018" ring gaps. Well I think you've already answered that.. But I guess what I'm asking is,does the spec's I've given lend any more weight to the gap sizes, or.. are the gaps down to the 'new standard' that started this thread? I hope that makes sense??? coz I don't think I know what I'm talking about anymore!!!!!
Again reading from Hastings Technical data: bores from 2.9525" to 3.5424 use .010" to .020". This places your .018" within Hasting's recommendations.
Remember these Hasting recommended figures are for an automotive engine which is liquid cooled and most likely has an engine management system. We have no such luxury. Air cooled engines can run hotter than an automotive engine. The hotter the engine runs the more ring gap that is required. A little bit of detonation, even when you cannot hear it, and that .018" starts to look pretty small. With the state of modern fuel, the inability for people to come to grips with being able to seat piston rings, and trends to run these engines at lower rpm's than they were designed to run at it is a pretty good chance the engine will suffer some detonate. While detonation was not typically a problem with fuel available 40 years ago, and riders didn't expect to putt around at 1500 rpm in high gear, problems with detonation was rare.
In a perfect world things would be different. These engines were neither designed, or manufactured in a perfect world. They were not designed to be run on modern fuel or oils mandated by EPA to give automotive engines better fuel mileage.
Ring end gap is there to keep the ends of the rings from butting up against each other as they heat up. If they touch they cause the ring to seize (micro-weld) to the cylinder wall. This starts all sorts of problems and given the right circumstances it can break the top of the piston off. Get some detonation and they heat up a lot. So in the imperfect world in which we use these engines having more gap is preferred to having less.
Some guys with Triumphs I have spoken to have told me they would only go with the settings in the manual (minimum if poss'), but if the manufacturers are making rings to this new standard, I don't see how they are going to!!??
I can only assume that rings are sent out with the intention of being filed to fit - its easier to file it back than it is to glue it back on. It also gives you the opportunity to keep within the factory specs when fitting the rings into a barrel honed out past the standard spec
Hi Allan, Yes, but the point is, if the rings are manufactured to give .020" when fitted, how are they going to filed to give .008 to .013" as per manual spec's? They can only be filed to give a bigger gap.....