Early Series 2 petrol had the pipe exiting the manifold upwards at forty-five degrees, changed to straight down around the change to 2a. This means a different front pipe.  
The shape of the pipes are also different for petrol/diesel, 88/109, RHD/LHD, 109 wagon, and Series 3 introduced a changed muffler location and hence different pipes. There is also a different pipe needed if there is a LH under seat fuel tank. 
As you can see, there are a lot of variations! As far as I am aware, all pipes are the same diameter for the 2.25 engine.
Hope this helps,
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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