As MR340 mentioned the factory 340 caps do have a little more beef to them; the rear cap I have never measured but looks identical to the 318 cap.Bearing in mind that any 340 caps are likely to be 40 years old any only offer a tiny margin of additional support I would go straight to the aftermarket and obtain a superior product in every way.( I wish I had ditched my 340 caps when I set my 340 up;live and learn)