[DeTomaso] Piston Ring Rotation.

Asa Jay Laughton asajay at asajay.com
Sat May 26 14:58:51 EDT 2012


Most of us are aware of the proper ring positioning when assembly an engine:
http://www.351c.info/gallery2/d/218-1/ringspacing.jpg
The two compression rings should be spaced about 30* (or about one inch) 
from the front centerline of the piston, one on either side.  The oil 
ring segments are the same on the opposite  side of the piston toward 
the rear.  The illustration above shows this quite well.

But do the rings rotate after assembly?

A few years ago at the POCA Fun Rally, Jack had an expert who was 
familiar with pistons.  He said indeed the rings do rotate, so in the 
end it doesn't really matter where you index them to start with... 
they're going to move.

Today I recorded the ring locations on my forensics engine.  I had been 
careful not to turn the rings on the pistons when I removed the 
pistons/rods from the engine previously.  Because I can't insure they 
didn't move at all, the relationships below are rounded.  Here are the 
results.  My "timing" is from the -front- of the engine, moving 
clockwise.  The piston position at 12 o'clock is the part facing front, 
the position at 6 o-clock is at the rear.

The first compression ring is at the top, the second on the bottom.

Piston    1st Comp    2nd Comp    Oil/Oil
#1        8 o-clock   4 o-clock     4/2
#2        10          11           12/3
#3        10          2            10/1  (probably the only one closest 
to installed)
#4        8           8             8/8  (All ring gaps were lined up)

#5        9           2             5/5
#6        12          9             5/5
#7        5           4             6/5
#8        9           3             3/7

As you can see, they are all over the place.  I'll work up a good 
illustration and post it later.

Does this mean I won't index them the next time I assemble and engine?  
Actually, I'll most likely still index them when assembling, only 
because I'm anal about those things and I do believe spacing them does 
assist with initial break in.  If they are spaced the same, I look at 
that as an easy path for compression gasses to get past.  In other 
words, gases might get past the lower tension oil ring segments and past 
the gap in the bottom compression ring but then they'll have to travel 
around the edge a while before they meet up with the gap in the upper 
ring.  In the end, it may not really matter.

Asa Jay
Pantera Research Institute, Pacific Northwest

-- 
Asa Jay Laughton, MSgt, USAFR, Retired
&  Shelley Marie
Spokane, WA
******************************
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