Piston ring gap questions

[Pete.Silson]

Two piston ring gap questions for the price of one post:

Q1) What ring gap should I expect from new VB rings in a new (+060) bore?
The NOS rings that I have appear to give between 0.016" and 0.020" which seems a tad on the large side to me.

Q2) For my NH rebore I am using NOS +040 WNG rings with chamfered ends. Is there a technique for checking the ring gap of this type of ring?
I find that, because of the angled end, the feeler gauge twists the ring in the bore giving a false reading.

I am assuming that the gap should be the same as for square ended rings (0.011" to 0.015").

Pete

[Roger Gwynn]

Ring gaps are only a minimum to ensure that the rings do not close up when at running temps, according to a Hepworth and Grandage instruction. People tend to fret about larger gaps because the end gap is frequently used as a guide to bore wear. A little extra gap adds insignificantly to the very small space for the vapour above the piston to escape to below the rings. I did calculate it once and it turned out to be half of not very much at all. Other opinions welcome....
I cannot see any reason for the effective gap on the chamfered end rings to be any different.

[ian williams]

hi Pete;
i think some rings are pre gapped for a particular bore size .
if you have given the bore more running clearance than the rings were gapped for then the gap wil lbe wider .

i have an AJS that i bought years ago which had a partial seizure on new +.040"pistons and rings , i found it was mistakenly given only .001" running clearance using original information from factory for reboring using original wire wound pistons , but the new pistons are quite different in specs and require different running clearance so they dont seize , so i honed it out to give me .006" running clearance because the new pistons expand somewhat more than originals .
of course this in turn then made the ring gaps become wider at .025" , and although the gap was still within the .030" upper limit it was too much in my eye .
what i did was to get a set of +060" rings and then could file and gap them to the .010 - .012" " that i wanted for the +.040" pistons .

to measure the ring gap all i do is stick the ring 1" down the bore using end of the vernier to push it down and then drop feeler gauges in to see what the gap is , doesnt seem to matter what the ends of the rings are like i can still get a good measurement .

not sure what you can do with the vb if you cant find rings that are longer but .016" to .020" gap isnt too bad .

when i did my running clearance i set the bore gauge zero to the nominal bore size using slips in a gauge master and then honed the bore out bit by bit to give me the required running clearance shown on the bore gauge .

cheers
ian

[tony.barnett]

So how big is too big a gap? I have rings in my 52 VH with a 40 thou ring gap. Too big?

[volker.halbach]

Hi Pete,

I have the NOS piston rings from the club with the oblique gap in my NH. I don't know how to measure the gap accurately, but I remember that it looked to be rather big, about 0.025" with the gauge 45 degrees to the (used) bore (I think the chamfer is 45 degrees). Since we are interested in the horizontal gap, the effective gap is increased by the factor √2. I guess that's also the reason for the oblique gap; It gives an extra safety margin for fitting rings without checking the gap without increasing the blow by too much. Just a guess.

[nevhunter]

Just another "Bothersome " feature that does little but make things more difficult. You have to get the ring dead correct flat in the bore. Push it there with a piston or such. Getting a much larger ring and filing it a lot is a bad idea as the ends are bearing firmer on the bore. Rings generally run hotter than the Barrel does so a bit of excess gap is safer. Sealing all round the ring is desired. NO WARPING.. On a 500 I wouldn't set less than.010" .25 mm. A rougher hone pattern will make this figure greater fairly quickly so if you're careful you can "Crib" a few thou, there. Taper wear limit is about 5 thou which will give a 15 +thou variation of the ring gap in itself. IF you are no where near the wear limit just hone and re ring it. It will often run better than a risky tight new bore and piston. You can't replace metal once it's bored out . Sleeves can cause issues and cost a lot. Nev