Hello All,
  I am in the process of renovating an old Brown Junior, that is in very poor shape. The compression is passable rotating in the normal direction, but is superb when rotating in the reverse direction. I remember reading that this is not unusual with ringed engines. It got me to thinking WHY? The piston doesn't know which way the crank is turning so why the difference in compression?
  The only thing I can think off is that the piston is rocking around the wrist pin, because of wear on the rings. It is inclined one way for forward rotation and the other way for reverse rotation. Even if this is correct, I still can't figure out why the compression is different.
  The only other thing to state is that the big end of the con rod is very worn (Note to self, do I have another one or must I make one!). Anyone got an explanation for this difference in compression?

Regards,

Andrew.

Thanks Jim
  Ah, so it depends where the gap is! Didn't think of that one. Mind you I still don't see why there is a difference. Have you any explanation for it? I really can't work out the physical reason for the difference in compression. Any mechanical engineers out there?

Regards,

Andrew.

     The difference in compression depending on direction of rotation is due to the ring gap, per Jim Kraft.  The change is due to the side thrust of the rod driving the piston against the cylinder wall.  If the gap is on that side, the ring is seated deeper in the groove, making the ring groove mask more of the ring gap.  If the ring gap is on the other side, more of the gap is exposed when the side thrust pushes the piston to the other side of the cylinder.  IMO, anyhow.  One of these days we'll figure out how to send a diver into one of these things while it's running, and end all the speculation.

LOL Tom; But I think you are exactly right. It is the side load on the piston that makes the difference. There is always some clearance in a ringed engines piston to cyl., and the rod is pushing it one way or the other. And the fact that compression starts when the rod is at a high angle. In other words, my O is the same as your O. LOL.  Maybe we can get Tim and Al from Tool Time to go into the engine. They did that on a V8 on one of their shows. Only they would have to be made a whole lot smaller.