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ring to groove clearance, minimum?

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ring to groove clearance, minimum?

Postby A_VAS » Thu Oct 09, 2014 11:07 am

I have a used set of pistons, 1.5 1.5 3.0 ring setup. No gas ports..just a 4032 forged flat top

New set of Total Seal rings. The instructions with the ring set say .0015 to .003 clearance required for the ring to groove check.

I can get a .0010 shim stock in there, but not a .0015. So it appears to be a little tighter than what they asking for.
Should I be concerned and or try to make adjustment to the ring thickness via 'lapping' or similar?
both top and 2nd check the same way. The rings are all very consistent in thicknes when mic'd to within a tenth.

Thanks
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Re: ring to groove clearance, minimum?

Postby Baprace » Thu Oct 09, 2014 12:19 pm

If the rings are free and spin easy in the groove I would go ahead and just install them, I do like lapping the sides of the rings. Don't forget to look at the radial depth, make sure the rings are at least flush or inside the piston face, they can not hang out. JMO
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Re: ring to groove clearance, minimum?

Postby A_VAS » Thu Oct 09, 2014 3:09 pm

thanks..yes back clearance is good. The rings do seem to spin free in the groove even dry. I just wasn't sure if some more clearance would be good, to allow them to flex / roll or let pressure get in behind.
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Re: ring to groove clearance, minimum?

Postby #84Dave » Thu Oct 09, 2014 3:50 pm

If the rings are too tight in the grooves, it will be more difficult for the combustion gas to get between the ring & the groove to the backside of the ring, in order to pressurize the ring outward & seal against the cylinder wall. Even if the ring spins freely in the groove @ .001" clearance. Also remembering, you're checking those vertical clearances @ room temperature? I have a pair of nice thick gloves I've used for years. I put the piston, with the rings installed, in my 300-degree pre-heated kitchen oven for 15-20 minutes. Then remove with my thick glove & immediately check the vertical clearance with my feeler-gauges. Yes, the clearance changes with the heating process. As it would inside a running engine. How much? Depends on the manufacturer & alloy of the piston. Oh...... and I slide the 'floating' pin into the piston for the same heating process. -Dave-
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Re: ring to groove clearance, minimum?

Postby MadBill » Thu Oct 09, 2014 8:33 pm

The thermal expansion of typical aluminum alloys is ~ 12 PPM/° F., so if a piston is heated up from 70° to 300°F., a 1.5 mm (0.059") ring groove will grow by: (12 x 0.059 x 230)/1,000,000 = 0.000163" , or just over one and a half tenths of a thous.
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Re: ring to groove clearance, minimum?

Postby littleben » Thu Oct 09, 2014 8:54 pm

The groove is empty space, with a bunch of aluminum in its root, but it has ring lands above and below it so we have to consider the expansion of those, that's what the ring see's.
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Re: ring to groove clearance, minimum?

Postby MadBill » Thu Oct 09, 2014 9:55 pm

It doesn't work that way. The groove width grows exactly in proportion to the piston. E.g., if you had a thin wall aluminum ring and heated it up, the ID would grow per the formula, as would the OD.

BTW, if the ring heated up by the same number of degrees, it would expand by about half what the groove did, so the net clearance at 300° would increase by only ~ 0.00008", less than one tenth of a thous.
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Re: ring to groove clearance, minimum?

Postby engineguyBill » Fri Oct 10, 2014 9:34 am

.0010" is pretty tight for ring to groove clearance. Basically it boils down to the quality of the pistons and the rings. Up until the past few years the ring grooves in the typical high performance forged piston were just not consistent enough to run rings as tight as .0010" clearance. Current production of quality name brand pistons are probably able to use rings with tight clearance, but it is still an issue as the rings may not be consistent enough in axial width to be installed as tight as .0010". I would recommend lapping the rings using a flat piece of glass, very fine (150 grit or finer) sandpaper and light oil such as 3 in 1 or Marvel Mystery Oil. .0015" side clearance would be much better.
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Re: ring to groove clearance, minimum?

Postby piston guy » Fri Oct 10, 2014 11:02 am

A_VAS,
You should be fine at .0010 axial clearence on a used piston and new rings. "Some" of the phosphate coating on the ring will wear off and the clearance will increase . Plus as has already been mentioned the piston expands upward as well as radially so the clearamce will increase as the piston warms up. Nascar guys do run them closer but they typically have coated ring grooves to avoid micro welding. .0010 clearance will cut down on possible ring flutter at higher rpms too.
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Re: ring to groove clearance, minimum?

Postby A_VAS » Fri Oct 10, 2014 3:20 pm

thanks for the replys
I think I will give them all a light lapping on some 600-800 grit paper with light oil, just to feel better about it. Might give a tenth or 2 more and take the matte coating off the ring

these are Autotec pistons and the grooves seem to be very consistent. Grooves look great even though used pistons... and rings all mic within a tenth on all 8 (better than I thought)
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Re: ring to groove clearance, minimum?

Postby Dan Timberlake » Sat Oct 11, 2014 3:07 pm

what is the measured ring width dimension?

I'd spin the oiled ring around a dozen times, remove and clean, and measure again.

Machining ring grooves is not a easy task. The sealing face (side of the groove) must be perpendicular flat and smooth. Grinding is preferred to "turning" with a cutting tool.
Perfect circle says 0.0008 inch minimum. Nearly everybody else sys 0.0015 minimum.

The first problem encountered as ring groove clearance is reduced in top rings is delayed pressure buildup BEHIND the ring necessary to seal on the power stroke. As I Recall in some piston or ring tech article Kevin Cameron mentioned the late new England cam grinder Domenic Ravesi was developing a radical for the time bike engine, and among other things tested very "tight" ring side clearance. I believe the result at some tighter than stock value was power essentially shutting off at high rpm.

For over 30 years Drag racers played with reduced side clearance in a quest for better ring stability at crazy rpm. To make the rings seal in "too tight" grooves "gas ports" are added (and needed) to pressurize the volume behind the ring. Wiseco FAQ used to have a discussion about gas ports but it is gone now
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Re: ring to groove clearance, minimum?

Postby Walter R. Malik » Sat Oct 11, 2014 4:52 pm

A_VAS wrote:I have a used set of pistons, 1.5 1.5 3.0 ring setup. No gas ports..just a 4032 forged flat top

New set of Total Seal rings. The instructions with the ring set say .0015 to .003 clearance required for the ring to groove check.

I can get a .0010 shim stock in there, but not a .0015. So it appears to be a little tighter than what they asking for.
Should I be concerned and or try to make adjustment to the ring thickness via 'lapping' or similar?
both top and 2nd check the same way. The rings are all very consistent in thicknes when mic'd to within a tenth.

Thanks


It depends upon whether the piston is "gas ported" or not. The required minimum side clearances ARE different.
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Re: ring to groove clearance, minimum?

Postby piston guy » Mon Oct 13, 2014 12:07 pm

The Autotec piston has a gas distribution groove ( tiny relief just above the top ring groove) for imporved ring sealing. Still no need to lap the ring for more clearance. .0010 is fine.
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Re: ring to groove clearance, minimum?

Postby A_VAS » Mon Oct 13, 2014 12:20 pm

i did see that small step cut above the ring...can't recall if it was on the top or 2nd groove.
FWIW, I did lap the rings on both sides on a piece of 800grit, but still couldn't get a .0015 shim stock in there so I don't think I affected the clearance by much. Rings spin free and are now in the engine...we'll see
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Re: ring to groove clearance, minimum?

Postby piston guy » Tue Oct 14, 2014 10:01 am

Dan T,
Respectfully, the primary function of a gas port is to put pressure behind the ring to improve the ring's seal to the cylinder wall. This is especially true for reduced axial ( thickness) and radial (depth) rings. Some compression rings (Nascar and NHRA Pro Stock among others) are almost the thickness of an oil scraper ring. Without a gas port to "load" the ring , it "could' easily be blown off of the cylinder wall. I recommend gas ports for rings 1.2mm and thinner in axial height. The ring groove is cut about .0004 tighter to allow for the increased heat the gas ports add to the top ring land. Most drag racers prefer top or vertical gas ports where circle track racers prefer side or lateral gas ports as they don't "carbon up" like verticals can do when run for long periods. Gas ports do shorten ring life a little but the hp gain is worth it.
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