Pistons and rings

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[edit] Pistons

[edit] Parts of a piston

[edit] Diagram

Piston parts diagram1.jpg

Diagram Key
1 to 10 11 to 20 21 to 30
1. Compression Height 11. Pin Oiler 21. Skirt Thickness
2. Crown 12. Pin Span 22. SS
3. Deck 13. Pin Tower 23. Support Band
4. Dome Angle 14. Plug Relief 24. Tang
5. Dome Diameter 15. Ring Band 25. Valve Relief (VR)
6. Dome Profile 16. Ring Groove 26. VR Angle
7. Dome Radius 17. Ring Land 27. VR C/L Out
8. Lock Groove 18. Ring Root 28. VR Depth
9. Oil Return 19. Side Relief 29. VR Diameter
10. Pin Bore 20. Skirt 30. VR C/L

[edit] Piston design and material

Cast pistons are fine for engines that won't see nitrous or boost. Forged pistons are a better choice if the engine will see boost or nitrous due to the added durability. There are those who say using cast pistons and nitrous/boost is acceptable. And it is- providing the tune is spot on and the engine never sees detonation. If it does, the cast pistons will be the first thing to fail, and when they go they usually damage the bore and possibly the cylinder head as well. This is because cast and hypereutectic aluminum alloys are brittle and when they fail they tend to shatter into many pieces, where a forging will generally stay in one piece, with pinched ring lands or depressed crowns, etc.

The worst possible piston design if a dished piston is needed is a round dish with a 45 degree chamfer around the outer edge. That leaves a scant 1/2" band on the top of the piston to provide the squish/quench effect, and that's just not enough.

Round dish sbc piston.jpg

A much better choice if a dished piston is needed is a reverse dome or D-cup type piston (the one below, left is for a SBC 383 stroker).

The 10cc, 0.070" deep, round dished piston shown below right is a compromise between the D-cup and the undesirable type of round dish (shown above). As can be seen, the squish band is wider and the dish smaller in diameter,but deeper. This maximizes the quench/squish effect, but retains the "one size fits all" economy because one casting/forging will fit either side of the engine.

KB P-N 135 SBC 383 PISTON.jpg

[edit] Dish volume

Most dished pistons have published volumes. Those that don't or with pistons that are unknown as to brand can be estimated by using the following formula:

Warning Note: All measurements in inches.

  • 3.14 x [radius of the dish]² x depth of the dish x 16.4 (converts cubic inches to cc) = Volume in cc
    • Add 2 or 3cc for the valve reliefs if they extend beyond and/or below the dish.

[edit] Piston manufacturer catalogs

[edit] Rings

For descriptions of the various ring types of compression rings and their location (top or second), see Piston Ring Types from the Federal Mogul Piston Ring Handbook.

[edit] How to square the ring in the bore

Place a ring in the bore, then use a piston inserted upside down into the bore to square the ring.

[edit] Ring end gap clearances

Broken ring lands

Ring end gap clearance has to be checked and verified or severe engine damage can result. As a general rule, because ring end butting has to be avoided at all costs. If an error is to be made it is far safer to set the end gap too wide than to have too little ring end gap. Insufficient ring end gap will cause the ring ends to butt together, scoring the cylinder wall, severely wearing the ring(s), and a distinct possibility of piston failure due to the ring lands breaking.

The following are general recommendations. Consult the manufacturer's instructions and use that if they vary from these recommendations.

Unless otherwise instructed by the ring or piston manufacturer, a safe ring end gap clearance is:

[edit] Top ring

  • 0.0045" per inch of bore diameter for street/strip
  • 0.0050" for drag or road race
  • 0.0055" for supercharged or nitrous use
Warning Warning: Be aware that there are pistons like some made by Keith Black (KB info below) that use a wider top ring gap than what is normally recommended by ring manufacturers or other piston manufacturers. So be sure to check and follow the piston manufacturer's instructions, if they differ from the ring manufacturer's instructions.

[edit] Second ring

  • The second ring should be gapped 0.004" to 0.008" wider than the top ring

[edit] Oil ring

  • Minimum of 0.015" for the oil ring rails, do not file oil ring expander or rails

Warning Note: See the info at SBC oil ring depth, below.

[edit] KB ring gap and piston to wall clearance instructions

Warning Note: The following info on the KB Icon/Icon FHR (Forged Head Relief) series and various KB hypereutectic pistons used to be on the KB site but has been replaced w/the above link. Refer to manufacturer's instructions to verify these specs as revisions/redesigns may cause them to vary, and for other brands and types of pistons.

KB pistons can be installed tighter than other performance pistons. A close fitting piston rocks less, supports the rings better and seals the engine for maximum power. When a loose fit engine is desired the rigid skirt design of the KB piston allows the builder a choice without fear of piston damage. See the clearance chart below for minimum and realistic maximum loose fit clearance for KB pistons.

[edit] Special notice on Keith Black piston top ring end gap

The Keith Black piston's unique thermal conductivity, ring location and varied end use requires special attention be paid to top ring end gap. KB pistons make more HP by reflecting heat energy back into the combustion process and, as a result, the top ring runs hotter and requires additional end clearance. Increasing ring end gap does not affect performance or oil control because normal end gaps are realized at operating temperatures. Failure to provide sufficient top ring end gap will cause a portion of the top ring land to break as the ring ends butt and lock tight in the cylinder. The broken piece may cause further piston or engine damage. Safe top ring end gaps can be found by multiplying the bore diameter by the appropriate ring end gap factor from the clearance chart below.

Example: 4" bore "Street Normally Aspirated" = 4" bore x 0.0065 = 0.026 top ring end gap. NOTE: Second ring end gaps do not need extra clearance. Dyno and track testing has shown that 34 degrees or less total ignition timing makes the best HP and time. Excessive spark advance, lean fuel mixture or too much compression for the fuel and cam used will make heat sufficient to butt piston rings with as much as 0.060" ring end gap. The entire top land can expand enough to contact the cylinder walls, when close to melt down temperatures are reached.

Ring End Gap Factor Suggested Piston To Wall Clearance Ring End Gap Factor Suggested Piston To Wall Clearance
Bore to 4.100" 4.100"-up Bore to 4.100" 4.100"-up
Street - Normally Aspirated 0.0065" 0.0015" - 0.0020" 0.0020" - 0.0025" 0.0040" 0.0035" - 0.0045" 0.0045" - 0.0055"
Street - Towing 0.0080" 0.0015" - 0.0020" 0.0020" - 0.0025" 0.0045" 0.0040" - 00.0050" 0.0050" - 0.0060"
Street - Nitrous or Supercharged 0.0080" 0.0020" - 0.0025" 0.0025" - 0.0035" 0.0050" 0.0045" - 0.0055" 0.0055" - 0.0065"
Circle Track -
2 BBL or Restrictor Plate Gasoline
0.0070" 0.0015" - 0.0045" 0.0020" - 0.0050" 0.0040" 0.0040" - 0.0050" 0.0055" - 0.0065"
Circle Track - Unrestricted 0.0080" 0.0025" - 0.0045" 0.0030" - 0.0045" 0.0040" 0.0045" - 0.0065" 0.0055" - 0.0075"
Circle Track - Alcohol Injection 0.0060" 0.0025" - 0.0045" 0.0025" - 0.0050" 0.0040" 0.0045" - 0.0065" 0.0055" - 0.0075"
Circle Track -
Carb Alcohol
0.0080" 0.0030" - 0.0045" 0.0030" - 0.0050" 0.0045" 0.0050" - 0.0070" 0.0060" - 0.0080"
Drag -
0.0075" 0.0015" - 0.0045" 0.0020" - 0.0045" 0.0040" 0.0050" - 0.0070" 0.0060" - 0.0080"
Drag -
0.0065" 0.0015" - 0.0045" 0.0020" - 0.0045" 0.0040" 0.0040" - 0.0070" 0.0050" - 0.0080"
Drag - Supercharged or Nitrous Gasoline 0.0095" 0.0020" - 0.0045" 0.0025" - 0.0050" 0.0050" 0.0060" - 0.0090" 0.0070" - 0.0100"
Drag - Supercharged Alcohol 0.0085" 0.0015" - 0.0045" 0.0025" - 0.0045" 0.0050" 0.0050" - 0.0070" 0.0060" - 0.0080"
Drag - Supercharged Fuel 0.0115" 0.0030" - 0.0050" 0.0035" - 0.0055" 0.0060" 0.0070" - 0.0100" 0.0080" - 0.0110"
Marine - Normally Aspirated 0.0080" 0.0030" - 0.0045" 0.0035" - 0.0050" 0.0040" 0.0045" - 0.0060" 0.0055" - 0.0070"
Marine - Supercharged 0.0090" 0.0030" - 0.0045" 0.0035" - 0.0050" 0.0045" 0.0055" - 0.0070" 0.0065" - 0.0080"
Air Cooled Baja 0.0075" 0.0030" - 0.0045" 0.0035" - 0.0050" 0.0040" 0.0050" - 0.0070" 0.0060" - 0.0080"
Propane 0.0065" 0.0015" - 0.0045" 0.0020" - 0.0045" 0.0040" 0.0035" - 0.0070" 0.0045" - 0.0080"
Monster Truck or Mudbog - Supercharged 0.0090" 0.0025" - 0.0045" 0.0030" - 0.0050" 0.0060" 0.0080" - 0.0100" 0.0090" - 0.0110"

[edit] SBC oil ring depth

GM has used both a shallow oil ring and a deep oil ring on SBC engines. Up to 1985 the oil ring/piston groove was 'deep' (0.195" or thereabouts). From about 1986 to the end of the L31 Vortec production, the 'shallow' rings (around 0.165") were used- but there could still be deep groove pistons and rings being used from about '95-back. A page on what to look for is Oil Ring Groove Depths.

The difference is enough between them that if a deep oil ring were to be installed on a shallow oil groove piston, that the ring would bottom out in the groove. This leaves a portion of the ring extending out past the piston and will prevent the piston from being installed. The piston can be damaged if it's forced, and if somehow the piston were to be installed in the bore with the wrong oil ring, the bore and piston could be damaged.

Using a shallow ring in a deep groove will function normally, but installing the ring on the piston and the piston into the bore will be a chore due to the oil ring trying to pop out of the groove.

[edit] Metric rings

Metric rings (2mm x 1.5mm x 4mm) were used from about 1992 to 2002 when the Vortec bowed out. If you have access to the vehicle VIN and the original engine is still there, the metric ring engine is VIN P. There may be other VIN's that use metric rings so this has to be checked, regardless.

[edit] BBC oil ring depth

The BBC engine from about 1991 to 2000 used a shallow oil groove piston and ring.

Warning Note: Info on this is incomplete at this time.

[edit] Ring end gap location

The idea behind staggering the ring end gaps in a certain way is to have the engine start up without any gaps being aligned. The rings tend to rotate on their own during the life of the engine, so starting them out as shown will prevent them from aligning, at least not right away.

Hastings has instructions if there's a problem with installing their rings on some pistons, located here.

While there are different thoughts on ring end gap placement, unless the manufacturer's instructions say otherwise, the following diagram will work fine:


[edit] Metric rings

And if that wasn't enough, there were metric rings (2mm x 1.5mm x 4mm) used from about 1992 to 2002 when the Vortec bowed out. If you have access to the vehicle VIN and the original engine is still there, the metric ring engine is VIN P. There may be other VIN/applications that use metric rings so this has to be checked, regardless.

[edit] Changes to popular rings, including Sealed Power E251K rings

Ring dimensions.jpg

As of about 2011, the Sealed Power 4" nominal bore (including oversizes) x 5/64" width top compression rings have changed. The ring is now thinner, radially. Instead of a ~0.180" radial thickness to fit a 0.200" deep ring groove, they now have a ~0.140" radial thickness. So when installed in a "normal" piston, the ring backspace is now 0.060"-0.070". Sealed power claims this was an engineering change to improve bore conformability.

To address concerns regarding the increases backspacing, the following bulletin was released by Mahle (Mahle also makes Sealed Power, Federal-Mogul, and Perfect Circle rings):

TB-7001 Issued July 27, 2011 (also released March 20, 2012 as AERA Technical Bulletin TB 2588):

Steel Compression Rings with Reduced Radial Wall Thickness

MAHLE Clevite Inc. is transitioning its popular MAHLE Original ring sets from a grey cast iron top ring to a carbon steel top ring. The advantages include; 35% more strength, 30% less weight and double the resistance to side wear which is a common problem on late-model engines.

Since the steel is 35% stronger than cast iron, we can achieve the same ring tension with 35% less weight. That weight savings is made by reducing the radial wall thickness of the steel ring.

The net result is a ring that's lighter, so it has a higher effective RPM potential, stronger - so that it weathers the abuse of today's high output engines better, and actually seals better because the reduced radial wall allows it to conform to the cylinder wall better.

Having said all that, there's some additional explanation needed when it comes to back clearance - that area behind the ring which is calculated by subtracting the ring radial wall width from the root depth of the ring groove in the piston. For example; if I have a ring radial wall of .170” and a groove depth of .195” then my back clearance is .025”. Reducing the radial wall of the replacement carbon steel rings does increase the back clearance because they are going into piston grooves designed for the thicker radial wall of the cast iron rings.

It's been a well-accepted piston/ piston ring engineering design criteria, that for optimum performance, ring back clearance should be minimized. This comes from the fact that the top compression ring needs the pressure from the combustion gases to get in behind the ring and push out on the ring to maintain proper seal on the high pressure, or combustion stroke. The logic was that the smaller the area created by the back clearance, the quicker that pressure would build to push out on the ring, and the quicker the ring would react to its sealing requirements job. That logic is all good but what about the reality of the concept?

Since MAHLE makes both components in this equation - pistons and the rings, our MAHLE piston ring R&D lab did some testing in conjunction with one of our OE customers to see if engine testing could tell us what the right amount of back clearance should be. What the lab folks found surprised most all of us! The engine wasn't nearly as sensitive to ring back clearance as it was to ring side clearance.

The reason was the gases have to get past the top side of the ring in order to get around to the back of the ring to push out on it. When we tightened up the side clearance to less than .001", the ring went unstable even at normal operating RPMs. Blowby, which is gas leaking past the face of the ring, increased dramatically. Changing the back clearance didn't show us anything, negatively or positively either direction. We realized it was all about having enough side clearance to let the gases flow back to the back of the ring not how much back clearance we had!

We came to the realization that in any cylinder, you have hundreds of CC's of gases available to fill that relatively small volume behind the ring (0.4 cc's), Those gases are at a very high pressure, several hundred, or even thousand PSI, so they will fill the small space behind the ring very quickly if you give them enough room, a.k.a. side clearance, to get there.

Most OE and aftermarket side clearance specs are in the .0015" - .0025" range. For our regular, non-racing, customers; changing from a cast iron top ring to a steel top ring won’t cause any adverse effect at all from the increased back clearance because the side clearance is adequate to allow the gas movement. Remember, we have hundreds of cc’s of gas to fill the very small amount of space change.

The performance engine builders are a different story, but they take care of the issue with custom groove specs. The MAHLE Original performance ring catalog supplies radial wall specifications for every ring for just that reason. Many of the true high performance engine builders specify ring grooves with less than .001" side clearance, but they add gas ports, small channels drilled in the piston, either from the top or side of the piston, to give a direct path for those combustion gasses to get directly to the back side of the ring. The engine doesn't care how the gases get back behind the ring, it only cares that it does, and in enough volume and pressure to do the job.

In summary, you can replace cast iron top rings with carbon steel, get all the benefits we’ve talked about in this bulletin, and rest assured we’ve done our homework on the engineering side!
End of TB

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