Wednesday

Piston Ring Welding


full size image

Piston Ring Welding
by Don Terrill (c) 2005

The ring above is out of the engine that inspired the 740hp Engine Book. This was before we learned how to fix the problem.

What is it:
  • It's more commonly know as micro-welding.
  • It's where aluminum from the piston is "transferred" to the bottom of the compression ring. Check out the full size photo above.
  • The welding will most likely take place near the ends. The above ring shows no problems on the bottom or opposite side of the gap on the bottom.
  • It happens during break-in. If you can make it off the dyno you're probably home free.
If you haven't experienced this problem yet, you either haven't built enough engines or horsepower. It may have already happened to you and you don't even know it - start checking the bottom of the compression rings during teardown.

How to know if it's happened:
  • Low crankcase vacuum - If you run a vacuum in the crankcase you can easily spot a problem with micro-welding. When the compression ring sticks from welding, blowby will increase and thus offset some of the vacuum and lower the gauge reading. I've seen engines lose 50% of their normal vacuum.
  • Power off 2% plus - This is the first clue that something is wrong. When I built engines in Nascar it was easy to spot an engine that was off a fuzz. You build dozens of the same engine and you'll have a really good idea of how much each should make.
What causes it:
  • Softening of piston material - engine got hot or?
  • Poor ring groove machining - finish, flatness, ...
  • Wrong ring-to-groove clearance.
  • And probably some other factors no one knows about or understands yet.
How to avoid it:
  • Coating the bottom of the ring - phosphate dry lubricant. This is the fix we used and solved 95% of the problems.
  • Coating ring land - phosphate dry lubricant.
  • Tilting the ring land.
  • Anodizing the ring land.
  • Modified engine break-in method. If you run a vacuum in the crankcase, vent the motor during break-in so there is zero vacuum. This will allow more oil to reach the upper ring groove and may inhibit welding.
  • We even tried different end-gap locations during assembly - believing if you kept the top ring gaps away from the exhaust side of the piston it would help.
From Dana Corp:
New Techniques and materials are producing the advanced coating technology to meet the demand for longer life and better durability. Perfect Circle offers a patented PCF-152 polymer ring coating for micro-welding reduction. Prior to the introduction of this coating, engine manufacturers incurred substantial costs as they were forced to anodize the piston’s ring grooves. PCF-152 coated piston rings are currently used by our customers to considerably reduce the destructive effects of piston and ring micro-welding, thus reducing overall power cylinder cost. Dana piston ring coating image
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