John at Aircooled advises be real careful when measuring for new rings.My question is, how do you actually measure ring size?
For example my 90.5 bore would possibly need 1.5 X 2 X 4mm. What exactly do these measurements represent?
I was told by a friend that he believed the measurements must be taken inside the cylinder bore itself with a special micrometer which I don't have, but why couldn't the measurements be taken off the old rings especially when they still had their sharp edges? If so - how would it be measured?
Measuring for new rings.
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Marc
- Moderator
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Re: Measuring for new rings.
Those dimensions are referring to the widths of the rings - top compression ring, second ring, oil ring.
Early 90.5s (and the AA Chinese clones of them) use 2.0x2.0x4.8 (commonly referred to as 2/2/5) while the later ones are 1.5x2.0x4.0.
Just measure the widths of the old rings if you have them, or the widths of the piston grooves - it needn't be particularly precise to tell the two types apart.
Most replacement ring sets are sold with ample end-gap and require no special filing to fit. To see what the gap is, put the ring into the cylinder (near the bottom, below where there's any wear, works best) then push in a little with a piston top to set it square; measure the gap with feeler gauges, if it's over ~.014", there's nothing for you to do...rule of thumb for conventional rings is ~.004" minimum per inch of bore. Slightly more end gap is usually preferred for racey applications, and special rings like Total Seals have their own recommendations (scroll to middle of page): http://www.totalseal.com/TechPage.aspx
With the new rings fitted to the pistons, press them inwards and see how thick a feeler guage you can slip in alongside. On a new piston it should be ~.003" top, .002" second. More than ~.002" over that says the piston's pretty much at the limit for being worth reringing.
You can use the ring end-gap measurement as a shadetree substitute for a bore micrometer to judge how much wear is on a cylinder. Compare the gap at the bottom of the cylinder (or at the very top after the carbon's removed) to the gap when the ring is just below the "ridge" formed by wear, at the end of the top ring's travel. Divide the difference by pi (3 is close enough) to approximate the taper wear on the cylinder. For instance, if the gap is .018" at an unworn spot and .023" just under the ridge, the cylinder wear is approximately .0015" - still plenty good to reuse, ~.010" being the absolute upper limit.
Early 90.5s (and the AA Chinese clones of them) use 2.0x2.0x4.8 (commonly referred to as 2/2/5) while the later ones are 1.5x2.0x4.0.
Just measure the widths of the old rings if you have them, or the widths of the piston grooves - it needn't be particularly precise to tell the two types apart.
Most replacement ring sets are sold with ample end-gap and require no special filing to fit. To see what the gap is, put the ring into the cylinder (near the bottom, below where there's any wear, works best) then push in a little with a piston top to set it square; measure the gap with feeler gauges, if it's over ~.014", there's nothing for you to do...rule of thumb for conventional rings is ~.004" minimum per inch of bore. Slightly more end gap is usually preferred for racey applications, and special rings like Total Seals have their own recommendations (scroll to middle of page): http://www.totalseal.com/TechPage.aspx
With the new rings fitted to the pistons, press them inwards and see how thick a feeler guage you can slip in alongside. On a new piston it should be ~.003" top, .002" second. More than ~.002" over that says the piston's pretty much at the limit for being worth reringing.
You can use the ring end-gap measurement as a shadetree substitute for a bore micrometer to judge how much wear is on a cylinder. Compare the gap at the bottom of the cylinder (or at the very top after the carbon's removed) to the gap when the ring is just below the "ridge" formed by wear, at the end of the top ring's travel. Divide the difference by pi (3 is close enough) to approximate the taper wear on the cylinder. For instance, if the gap is .018" at an unworn spot and .023" just under the ridge, the cylinder wear is approximately .0015" - still plenty good to reuse, ~.010" being the absolute upper limit.
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D7
- Posts: 218
- Joined: Sat Dec 29, 2007 1:32 pm
Re: Measuring for new rings.
Great reply and just what I needed to hear.-Thanks Marc.