Compression/Rebound side on Shock Piston

How is a fool proof way to determine which side of a piston is the Compression side and which side is the Rebound?

We know that when the shock is right side up like it is on the bike,that the Compression side of piston is at the bottom and rebound is on top.

 

But if one was to go.....to say....like a shop and a shock piston was just laying on a bench......which side could you tell is the Compression side right off the bat?

On 99% of bikes, the compression face shims are a larger diameter than the rebound face shims.  There will be a matching difference in the piston port sizes.

One exception to this rule that comes to mind is the early CR500's.

Big shims go on the comp side.  Small shims on the rebound side.  Haven't seen one yet that didn't conform to that but there may be some weird European crap out there that uses the same shim diameter on both sides.

On Showa 47s the comp side of the piston has a slight recess bored in the center to accept the 8mm id floating shim’s post. The rebound side has a flat surface.

 

Big shims go on the comp side.  Small shims on the rebound side.  Haven't seen one yet that didn't conform to that but there may be some weird European crap out there that uses the same shim diameter on both sides.

20161024_202315.jpg
Say like looking at the piston from this angle. Which side could you tell is the side of the piston that would be facing toward the swing arm (Ground)?
Is the raised ports the rebound side? And the lower flatter ports the Compression side that face the Ground or clevis?

Both ports are raised, but the diameter on the top appears to be larger and the corresponding ports on the bottom appear to have greater area than those on the top.

Compression speeds can be much higher than any desirable rebound speeds, so the rebound shims should always be smaller and will likely have less port area.  Smaller shims on the rebound side allows more flow area through to the compression side. 

As far as any other dimension on the piston it really makes no difference.  If both sides are flat and there is no recess you could certainly flip the piston and run them upside down.  Would not work well at all but it would certainly go together.

With the shock shaft was assembled, if the piston was installed facing the wrong way,

as a visible clue the rebound side shims (36mm) would no fully cover up the compression side ports (40mm).

*typical diameter shims used in KYB shocks

Edited by mlatour

The rebound ports area is usually smaller than the comp one.

16 hours ago, CaptainKnobby said:


20161024_202315.jpg
 

 

4 hours ago, texasthierry said:

The rebound ports area is usually smaller than the comp one.

Wow!  Does it really even matter how big the ports are if the area which feeds them is so restricted due to the short raised area?  I do realize a shock damping rod moves lots less than a fork damping rod/mid valve thusly not flowing nearly as much oil but still, lots of restriction there.

Both ports are raised, but the diameter on the top appears to be larger and the corresponding ports on the bottom appear to have greater area than those on the top.
Compression speeds can be much higher than any desirable rebound speeds, so the rebound shims should always be smaller and will likely have less port area.  Smaller shims on the rebound side allows more flow area through to the compression side. 
As far as any other dimension on the piston it really makes no difference.  If both sides are flat and there is no recess you could certainly flip the piston and run them upside down.  Would not work well at all but it would certainly go together.

I see what ya mean now. When I was looking at the piston, I noticed both sides were raised. However .....like you said......the rebound ports were raised higher. The compression ports were more flatter or flush but the area was greater.....(the holes where the fluid go through).

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