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Top piston ring gap - whats really required?

General engine tech -- Drag Racing to Circle Track

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german4inline
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Top piston ring gap - whats really required?

Post by german4inline »

Guys,
I´m German and involved in serial engine development and some years in the past I was involved in racing engines like DTM a.s.o.
What I´m wondering about are the big ring end gaps what are recommended by the well known piston manufacturers. With respect, I don´t want to doubt about their knowledge, but sometimes I´ve a feeling that nobody is interested in producing engine failures caused by too tight end gaps, they would prefer 2 or 3 percent more pressure loss what maybe a small power loss to avoid any risk for the customers.
To be honest, in NA race engines with 2 Liters / 86 mm bore we used ring end gaps of 0,2 mm on engines with about 300 + HP, I don´t know about one single engine failure caused by this. As long as both cyl. and ring material is iron-based, the elongation of both is identical with the same temperature. We could assume that in TDC when sparking theres a short time more temp as deeper in the cyl., so the temp of the ring equals out over WOT ranges. A colleague of mine is responsible for piston development and he told me that he has tested end gaps of about 0,1 mm without issues on Turbo Diesel engines with 500 Nm torque out of 2 L. This is a matter of fact and I would be interested about your experiences and if theres any engine failure what the proved reason was an end gap too tight. Of course - we have to talk about real conditions, blower nitro engines have different conditions and may need much more end gaps due to temps in an different magnitude. I talk about NA engines with maybe 100 + bhp/per liter. I would suggest that 0,3 - 0,35 mm would be ok for an engine with 100 mm bore, according to bore and engine capacity the gap might be smaller. With my DTM engines, I saw pressure loss figures of 1-2 % with this kind of end gaps and had never a failure. Now I have to rebuild a Ford Pinto engine for a friend of mine and I have an end gap of 0,25 mm and the piston manufacturer considers it too small. Any thoughts / experiences are appreciated !
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Re: Top piston ring gap - whats really required?

Post by Nut124 »

Very interesting. Looking forward to hearing what others have to say.

Just to translate:

0.2mm = 0.008
0.3mm = 0.012
0.4mm = 0.016
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Re: Top piston ring gap - whats really required?

Post by SupStk »

How much ring gap? Just enough to keep them from butting.

The manufactures recommendations, I figure is based off years of testing and what has proven successful in the past for certain types of engines. For example Keith Black pistons have photos of top ring failures from tight ring gaps. Seen plenty of engines in the shop that look just like them. When checking out surviving cylinders, rings show signs of butting and piston land were stressed. Needless to say we follow KB instructions for gapping.

Another related point, Speed Pro had in their catalog, tests done on ring gaps. The same engine had incrementally larger gaps, power and blow-by CFM was recorded after each increase. It was amazing to me how large they got before seeing any negative affect on power.
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Re: Top piston ring gap - whats really required?

Post by miniv8 »

Ring end gap when hot as a passage is minuscule compared to even the slightest separation of the ring from the wall or a groove. Calculating a generous radial clearance versus even the tiniest clearance found in the circumference of a bore, will make all your worries about pressure loss through ring end gap wither away.

You can reduce ring flutter, the separation of contact from said components, by increasing the second ring end gap. Doing so might reduce the pressure between the two rings and therefore reduce the floating of the top ring from where it should seal against the pistons ring groove.

The ring end gap clearance is decided on how much heat to expect from the chamber, how much heat the bore is expected to draw from the ring, and how much heat the piston is expected to dissipate through the ring. the bore diameter is just a figure to calculate the expectations factors, and then we add the safety factor, as we don't live in a perfect world.

You can make all rings butt for a reason, just different reasons than following mfg.instructions. Butting ring ends is rarely a sign of skill in my opinion, luck has more to do with it.

Regarding those gaps, I tend to use the same principle as with bearing clearances; keep it wide and keep it to yourself. run it tight and people will find out.
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Re: Top piston ring gap - whats really required?

Post by jed »

"The upper ring land of the piston will experience a considerable amount of growth due to thermal expansion. This can cause contact with the bore surface and/or close up the space between the upper ring land and bore. The ring end gap radial dimensional clearance is thus greatly reduced, in addition to ring radial expansion. The thermal expansion of the upper ring land also has the effect of choking off gas pressure to the back of the ring, reducing its pressure sealing capacity."
==
I found this quote in the foot notes by David Redszus. By reducing top ring gap to a minimum you could be limiting need gas pressure to actuate the top ring on the power stroke and adequately seal it. Maybe so maybe not. Only dyno testing would
answer the question.
I to have ran .007 end gap on 3" bore road racing 4 cylinders and was quite proud of it but now I may have lost 1/2 to 1 hp on
a 100 hp engine.
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Re: Top piston ring gap - whats really required?

Post by Steve.k »

Good friend of mine built 426 hemi super stockers and his brother chevy sb stockers. He told me many years ago on their hemi testing they had seen as high as .035 ring gap before the tell tale signs of butting disappeared. He said to me if not exactly sure what you want for gap looser is better in his view. That was many years ago and piston and ring expansion no doubt has changed. I’ve always stayed on the looser side since with no ill effects as far as I know.
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Re: Top piston ring gap - whats really required?

Post by engineguyBill »

.0045" to .0050" per inch of cylinder diameter is safe and will work very well. i.e. 4.000" bore will be fitted with a top ring with gap of .018" to .020". This would be considered the absolute minimum for top ring gap, unless piston manufacturer recommends otherwise (such as Keith Black hyperutectic).
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Re: Top piston ring gap - whats really required?

Post by bigmike »

SPEED-PRO PISTON RINGS - DESIGN, FEATURES, and
INSTALLATION GUIDELINES
Federal-Mogul Document #1205
RADIAL THICKNESS
All of the rings in the Speed-Pro line meet original equipment dimensions for proper fit and easy installation. Many
modern engines use shallow ring grooves, our sets for these applications contain appropriate reduced thickness
rings for optimal performance. Most other rings, with the exception of Pressure Back and Pro-Series rings, are
manufactured to the Society of Automotive Engineers (S.A.E.) D-Wall specification. The radial thickness (front to
back) of compression rings can easily be determined by the following formula:
Radial Thickness = Bore Diameter divided by 22
Example for a 4.00 bore: 4.00/22 = .182 radial thickness
SIDE (VERTICAL) CLEARANCE
Side clearance or Vertical clearance is the measurement of space between the sides of the piston groove and the ring.
Major piston and ring manufacturers have adopted the Society of Automotive Engineers specifications for ring and
groove widths. This combination of specifications results in a side clearance standard of .002”/.004". This clearance
is recommended for most Street and Moderate performance applications.
Racing engine builders that desire reduced side clearance may custom order their pistons with the top groove
machined to a specification less than the S.A.E. standard, but should maintain a side clearance of .001" minimum.
BACK CLEARANCE
Back clearance is the measurement of the space between the inside diameter of the ring and the base of the piston
groove, when the ring face is flush with the ring land of the piston. Optimum back clearance for a racing application is
.000". In other words, back clearance should be as small as possible without having the ring face protrude beyond
the lands when the ring is bottomed in the groove.
If the ring face is .018" or more below the land when the ring is bottomed in the groove, use .018" thick ring groove
shim stock to reduce back clearance to near zero. Ring groove shim stock for 1/16" wide grooves is available.
COMPRESSION RING END GAPS IN HIGH PERFORMANCE ENGINES
File fitting piston ring end gaps is not normally required for regular usage, but is a common procedure in racing
applications. Most SPEED-PRO rings are available in +.005" oversizes. Professional racers and engine builders know
that precisely setting the ring end gaps by "file fitting" is well worth the time and effort.
Comparative tests using a 350 cubic inch small block Chevrolet engine, reveal definite improvements in blowby
control and horsepower as top ring end gaps were decreased. Blowby was reduced by approximately 50 percent, and
horsepower increases ranged from 5 to 13 percent. The baseline test was run with top ring gaps set at .024". In the
second test, top ring gaps were reduced to .016". An additional test was made with the top ring gaps set to .010”. In
this final test, the results again showed a reduction in blowby; but a noticeable loss of horsepower was observed at
higher speeds. Examination of the rings indicated that the top rings were butting. Running with ring gaps butted will
result in scuffing of cylinder walls and/or flaking of moly from the ring face.

SPEED-PRO PISTON RINGS - INSTALLATION GUIDELINES -cont'd
Running with "ideal" end gaps is certainly the goal, but the results of this test show that is better to have a slight
amount of additional clearance than to have too little and risk scuffing. When fitting rings to cylinder bores, every
.001" change in bore diameter changes the end gap by approximately .003". (Diameter changes affect the gap by
the factor of pi…..3.1416").
Example: An increase in bore diameter of .002" increases the ring gap by .002" x 3.1416" = .00628".
RING END GAP RECOMMENDATION GUIDE (FOR 4.000 INCH BORE)

Speed Pro Top Rings
(ductile iron, 4" bore) 4.000" Example Minimum Gap Factor
Moderate Performance .016 - .018 (.004 per inch of bore diameter)
Drag Racing, Oval Track .018 - .020 (.0045 per inch of bore diameter)
Nitrous Oxide - Street .020 - .022 (.005 per inch of bore diameter)
Nitrous Oxide - Drag .028 - .030 (.007 per inch of bore diameter)
Supercharged .024 - .026 (.006 per inch of bore diameter)

Speed Pro 2nd Rings
(cast Iron, 4" bore) 4.000" Example Minimum Gap Factor
Moderate Performance .020 - .022 (.005 per inch of bore diameter)
Drag Racing, Oval Track .022 - .024 (.0055 per inch of bore diameter)
Nitrous Oxide - Street .024 - .026 (.006 per inch of bore diameter)
Nitrous Oxide - Drag .028 - .030 (.007 per inch of bore diameter)
Supercharged .024 - .026 (.006 per inch of bore diameter)

Notice: Most of the second ring gap recommendations are larger than the top rings. Recent testing has proven that a
larger second gap increases the top ring's ability to seal combustion. This larger "escape" path prevents inter-ring
pressure from building up and lifting the top ring off the piston allowing combustion to get by. Many engine builders
have reported lower blow-by and horsepower gains at the upper RPM ranges with wider second ring gaps. Also,
almost every new car made is using this inter-ring pressure reduction method to lower blow-by and emissions and to
increase engine output.
TO AID IN FILING RING GAPS, WE OFFER THE FOLLOWING HINTS:
A) Place the butt end of a small sharp file in a vise. If several sets of rings are going to be filed, you may want to
consider the gap filer Part No. MT-135 or MT-141.
B) File from outside face toward inside diameter to avoid chipping the face coating or leaving burrs on O.D. edges.
C) Filing only one end of the ring allows you to verify that you are keeping the gap straight and parallel.
D) Remove any burrs created by the gapped process with a fine stone.
Use the above chart as a guide to normal ring end gaps. The 'ideal" end gap will be somewhat different for each
engine. It can be as varied as the contributing factors that influence it. Piston rings should be file fitted to the desired
end gap with the torquing plate attached. In worn cylinders, make sure that ring gaps are checked at bottom of ring
travel. In seeking for the optimum end gap for your particular engine, choose the proper application in the chart and
gap the rings to the high limit. If the ring's end surfaces show shiny spots after usage, it is evidence of ring butting.
This means that your rings are operating at a higher than average temperature and require additional gap. If there is
no indication of butting, then the end gap can be narrowed until you reach the "ideal" condition. Remember, stay on
the safe side!
There is some controversy as to the effect of water temperature on ring end gaps. Some racers feel that if their water
temperature is low, compared to another engine of identical size, they could narrow up on ring gaps. This is not true!
Basically, piston and ring temperatures remain the same whether the water temperature is high or low. Theoretically, if
you consider thermal growth or expansion, be it ever so slight, the engine with hotter temperature would have bigger
bores. The engine with the lower temperature would have smaller bores. The above chart was developed for "normal"
engine temperatures. If your engine water temperature tends to be low, you should run a larger end gap than recommended to compensate for the smaller bores.

https://static.summitracing.com/global/ ... 902-30.pdf
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Re: Top piston ring gap - whats really required?

Post by modok »

The idea that the ring and cylinder are the same temp must be wrong.
Thew ring is cooled by the cylinder.
In order for heat to transfer from the ring to the cylinder, the ring MUST be hotter than the cylinder.
the MORE heat is being transferred, the higher this temp differential will be.

so, you have to guess how hot the ring will run. many factors involved
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Re: Top piston ring gap - whats really required?

Post by jed »

The Federal Mogul Document 1205 has a lot of good information but is I am guessing it is at least 20 years old and specifications
for ring side clearance and back clearance have changed.
Also rings are cooled by the cylinder but I'm not sure that rings are a big player in cooling the piston. The surface area ratio of
the piston to ring is small and I can't see how the ring can absorb enough heat from the piston to cool it. I think the piston skirt
in contact with the cylinder and oil have a bigger impact on cooling the piston than the ring package.
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Re: Top piston ring gap - whats really required?

Post by Nut124 »

Excessive rich running can affect ring temperature.

Long time ago, I built an engine with big new carbs. They were supposedly jetted for my application but the idle mix screws were wide open.

The engine started right up. I could tell it was running rich but did not think it was urgent to adjust. After a few minutes of idling, the engine stopped, seized. Ring end gaps closed due to extra rich running.

After cooling down, it started right up again. The rings had 0.004 per inch of bore gap. Years later when taken apart, the rings had worn to 5-6 thou per inch. Some of this could have happened at the first running.
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Re: Top piston ring gap - whats really required?

Post by rebelrouser »

I follow the .004 per inch of bore, for N/A engines, and the ring manufactures recommendations across the board for nitrous, blown, etc. I have a vacuum tester used for checking valve seal, it has a disc big enough to vacuum check a cylinder on say like a 350 chevy, with the head off. In just playing around with that tester, ring end gap does not have a big factor in the cylinders ability to seal, if gapped to the spec, especially if the ring end gaps are staggered properly. Just some redneck scientific testing I guess. I would also go along with the comments that a round true bore is much more important in keeping a cylinder sealed than pinching a couple thousands of ring end gap. Many may say I am not correct, but I use a cylinder leak tester on my engines on the stand before they are fired, and anything over 20% leak, I take it back apart and find out why. When I give the customer their engine, the cylinder leak is noted on the build sheet, I strongly recommend to my customers the leak tester as a way to gauge the health of the engine. I tell them keep track of the leak, it should get better as the rings seal, then when it starts to rise, save your pennies, it's time to freshen. I have had engines leak as much a 40% with no real impact on performance, at least on the drag strip, but I know it is getting worn.
I always leak an engine first before diagnosing any performance problem, just to rule out engine seal. I had a customer one time say " I wish you wouldn't bring out that tool, it always costs me money!" But when a guy says it has a slight pop on the big end, it must need more jet, and it has a hole leaking 75% they need to spend some money before it scatters all over the track.
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Re: Top piston ring gap - whats really required?

Post by Walter R. Malik »

A bit to loose is much better than being anything to tight.
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Re: Top piston ring gap - whats really required?

Post by jed »

I also have played around with a vacuum pump trying to evaluate ring seal. I would hone cylinders to different surface finishes
then install the piston with rings push the piston to the bottom of the cylinder and see how much vacuum it would take
to suck the piston to the top.
The first time I tried it I didn't thing I could build enough vacuum on a 4.030 bore with a .022 ring gap to push (lol) the piston
to the top of the cylinder. The piston was jerked to the top of the cylinder and hit the vacuum plate with a thud.
That was a good old DAY and a fun one. Every one should try it as a learning experience.
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Re: Top piston ring gap - whats really required?

Post by ptuomov »

modok wrote: Mon Oct 05, 2020 3:28 am The idea that the ring and cylinder are the same temp must be wrong.
Thew ring is cooled by the cylinder.
In order for heat to transfer from the ring to the cylinder, the ring MUST be hotter than the cylinder.
the MORE heat is being transferred, the higher this temp differential will be.
so, you have to guess how hot the ring will run. many factors involved
I agree. In particular, I'd highlight the role of piston oil cooling jets. Without them, the rings must run hot to cool the piston. With them, they don't need to. Logically, piston oil cooling jets should have a first order impact on the minimum safe ring gaps.
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