Ring groove clearance

[Minto]

Hi folks
Just wanted to know if there's a better way to measure the ring groove/ring thickness clearance. I measured the top rings earlier, firstly by using a digital Vernier, then with feeler blades with the rings on the piston, very different readings. On inspection the internal measuring faces of the Vernier were slightly hooked so are basically trash. The feelers are telling me (after metric to real money conversion) the the right is.00591 (.15mm) and the left which is visibly looser is .00866 (.22mm)
The ring thickness of both are the same so the left groove is worn presumably.
The bike has since I bought it been a bit smokey on the left, but improved quite a bit after last year's strip and decoke.
Thanks for any help
Jase

[Swarfcut]

 Jase.  Standard set up was two compression rings, both 1/16" thick, and an oil control ring 1/8" thick.  Piston grooves when new would closely match these dimensions.

 Wear takes place in the piston  lands, but also to the top and bottom faces of the rings. If you examine  the rings  closely you may  find the profile is worn, changing from a  basic rectangular profile to being more "T" shaped with the top bar of the T against the bore. A quick test was a new ring into the groove and a measure of any  up and down play. 2 Thou max was an accepted rule of thumb. 

 This play in the ring/land allows oil to pass under the bottom face of the ring, as the piston descends, passing  around the inner face of the ring and from there to the top face  of the ring to enter the combustion chamber as the piston rises.

The result is the familiar exhaust smoke and high oil consumption.

 The Cords Piston Ring Company patented spring loaded piston rings to work with worn grooves, way back in the 1930's, and there is reference to these rings for motor car use somewhere in the Forum.

 Swarfy.

[RDfella]

Feeler gauges are the best way and, as Swarfy says, .002" is max. You either need thicker rings and machine the grooves to suit or new pistons.

[Minto]

 Thanks Swarfy for that superb explanation of what happens in there. You advised me on my last rebuild about checking on the clearance here, I obviously didn't look quite as closely as I shouldve. I'm not sure now whether new rings are in order or a new piston kit. I've looked at the IMD kits which, I see a few members have used with good results, but then I'll probably need new small end bushes and associated reaming. Does it ever end?
I remember reading some discussion between you and other members about Cord rings, very interesting. I'd never heard of them before, probably before my time. 
Gratitude as always
Jase

[Minto]

RD fella, you've just answered my next question thanks. That's what I was thinking, I've got a set in my watch list.
Reckon this might give me better compression and a bit more urgency???
Cheers
Jase

[Swarfcut]

 Jase. Sadly you  are in the all or nothing situation. Ultimate perfection requires rebore, pistons, small ends. Balance this  major cost against the use the bike will get. Unless the pistons are very badly worn, a set of NOS Hepolite rings from ebay will be the cheapest option and worth a try.  At least the metallurgy is correct and you have an accurate standard to measure the grooves. If it turns out the pistons are bad, then its time to think again and there is always demand for the rings to be sold on.

 Swarfy.

[Minto]

Thanks for the reply, I was banking on new pistons and rings anyway, new small ends aren't majorly expensive but not sure about reaming to fit the new gudgeon pins, think I'll need to get that done for me.
As for a rebore, I'm not sure that it's needed, both bores are within tolerance for the +4 pistons which are in it at present, measured with bore gauge yesterday both at .08mm (.00315") spec sheet says .003 to . 005 is ok. I'll recheck them later but if the same they'll get a bit of a hone to freshen em up ready for new +4 pistons and rings, unless I'm missing something that dictates a rebore??
It seems to me that the only thing amiss is the widening of the ring grooves in the pistons.
Cheers for your help and advice.
Jase

[berger]

make sure there isn't a wear lip top or bottom , they are ring breakers on rebuilds

[Minto]

make sure there isn't a wear lip top or bottom , they are ring breakers on rebuilds

Hello Mr Berger. Do you mean a lip on the cylinder bore? If so will honing take care of any lip?
Cheers
Jase

[berger]

yes the bore top and bottom, depending on how much of a lip there is you could try honing it out, just don't do what my mate did and think the slight raised bit at the bottom of his cylinders would be ok and the rings would compress! I told him not at the speed the pistons go up and down. he built it up and whoops a daisy the oil rings broke. he was well fed up

[mikeb]

Jase - if you are considering the IMD option, one thing i discovered recently was the new standard IMD pistons/pins/rings are substantially lighter than the old Jp +080s (which themselves were within a few grams of the old hepolite +060s, since you asked). we're talking 67g lighter per side (pistons, pins, rings). that translates into about a 9-10% increase in crank balance factor. so you may want to consider a re-balance  or just ride a lot faster to keep the revs in the smooth range .

mine was already at a higher balance factor so cases split to do the right thing. having said that, and still waiting for parts, I don't regret getting the IMDs - they're a modern affordable piston and should offer years of good service.

[Minto]

Berger, thanks for the heads up, I can't detect any lip at the bottom end but the top end has a ring of carbon that needs removing.

Mike, thanks for the info.
I hadn't even thought about that issue, crank balancing is definitely beyond my level of skill, and sounds expensive. Might have to reconsider my options if it's going to throw another major bill at the project.
Cheers
Jase

[groily]

To mikeb's point, there is a clear difference in where the vibes fall with IMD +40s in mine now, replacing original-spec flat-tops.
I didn't weigh them (should have!), but I'm not bothered enough to pull the thing down to re-balance it.
If the original factor was what, about 55%?, and if the difference is really as much as 9%+, then I'm still only about where I have some of my other motors. Anywhere not north of about 65% should be OK I'm guessing, for road use.
I'll know more when I've got more miles on and start using more throttle, but at about 500 miles only (due to nowhere much to go etc etc) I'm pretty happy with the pistons. The thing runs like a Swiss watch and I have yet to see even a whiff of smoke (even on first start up), nor used as much as a teaspoon's worth of oil.

[mikeb]

i agree with you groily. with a standard balance factor then i'd not be too bothered.
mine was already 63% and the +9-10% figure was from both my maths and the feedback from the balancing guy (an engineer and bsa owner).
as i said, just rev those imd's a bit faster!

[Minto]

Groily, thanks for your input. I was going to pm you to ask how yours is after reading some of your earlier posts re the installation of IMDs without crank work. That's good news for me then. How did you get on with the pins and small end bushes? Did you need to fit new bushes and ream to size?

Mike, it is only used on the road and I'm sure I can happily live with giving it more throttle to smooth it out. I'm not after perfection, just a reliable, reasonably oil tight, enjoyable ride. If it's too vibey I don't mind tearing it apart again, I'm getting quite quick at it now.
Cheers
Jase