Oil Ring Tension

Oil Ring Tension
By Don Terrill (c) 2005

A major portion of the frictional losses inside an engine comes from the pistons, more specifically the piston rings, and even more specifically, the oil rings.

Ring Sets usually come in two flavors, standard and low tension. Usually the only difference being the oil ring expander (photo above).

Measuring oil ring tension:
  • Fish Scale - This is the method most people know. You put only the oil ring on a piston, install the piston upside down in the bore with either the pin or rod installed and then use the fish scale to measure the force required to move the piston through the bore. Focusing on the reading while the piston is moving, not what it takes to get it moving. Should the bores be oiled? As long as you measure the same every time, it probably doesn't matter.
  • Piston Drop - This could be best described as the feel method. Install only the oil ring on a piston, install the rod on that piston and then install the piston, right side up, into the bore. Now you just tap the piston down the bore and note how it acts. If you tap it once and the piston falls out the bottom of the cylinder, you may be a fuzz light on the tension :) Do be careful to not let the rings past the bottom of the bore though - it will make for a tough time getting the piston out.
What dictates tension? The oil ring expander end-gap.

Adjusting oil ring tension:
  • Try a different size expander - Want less tension; use an expander for a smaller bore. Want more tension; use an expander for a larger bore. You can also try expanders from low and standard tension ring sets.
  • Bend tabs - Can't find the exact expander you want? Well, maybe you can make your own by just bending the ends of the expanders. Needle nose pliers work perfect for this. It's better to find an expander that will work un-modified, but if you can't find one, bend away.
How much...?
  • Drag - I'm not going to throw out any fish scale numbers, but what I would recommend is testing every combination you can get your hands on. You will soon get a picture of how loose you can go.
  • Expander gap - Again, I hate to throw out numbers, but one thing is clear, you don't want the end-gaps touching. I've routinely used a .150 gap, but don't go off my numbers, learn what works for you. This number applies only to the Speed Pro SS-50U style oil rings - which the majority of ring sets use.
Other Tips:
  • Save expanders - I have a cabinet full of oil expanders - some are new, some are used. I just tag them with their bore size, new or used, and if they're standard or low tension.
  • Re-use expanders - Once I feel an engine has got the right expanders, I don't want to mess with them again, so I just reuse them during future rebuilds. Keep in mind that expanders will relax some over time - which may be a good thing.
By the way, don't give me any crap about the above photo including the little plastic pieces that keep you from overlapping the ends. I don't use them; I just thought it made for a better photo.

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