Some Models of Fuel Injection Pumps have bot O-rings and Copper Crush Washers to seal the Pressure Valve/Delivery Valves. Some only have the Copper Crush Washers.

While they can be changed the Copper Crush Washers seldom cause a problem.

Some other things might also have an effect on your shake at idle.

Is your idle speed within spec (there should be a tag just like on Gas Cars that has the Idle Speed Listed on it)?
Have your Valves been adjusted lately?
Have you tried re-adjusting your Rack Damper Screw?
I had a small air leak due to hard Fuel Inlet Hoses that allowed air in when the Hoses got warm. Tightening the clamps did not help. I replaced the Hoses and a little bit more shake dissapeared.

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84 300D, 82 Volvo 244Gl Diesel