For example, I bought a not too cheap top from the US company called Topsonline. They offer a 72 month warranty, OEM fabric, comparable quality rear window etc.
What you will not know is that the OEM top has some plastic parts stitched in which are specifically made to fit in the recess around the curve of the main window towards the back and across the top of the main window area all the way to the front. It also has a small plastic fitting which is stitched in which fits left and right at the front adjacent to the locking mechanism and which screws down providing a secure water proofing channel there. The previously mentioned plastic strip also screws down in a channel around the main window.
These items are not a part of the non OEM top I bought, and I would hazard a guess that they are unique to BMW.
Further, on OEM tops all the requisite holes which allow screws to be fitted through the fabric in various places are pre-punched correctly. The non-OEM tops are not pre punched, and although they are sort of marked in some instances, they cannot be counted on to be accurate. Consequently on has to make these holes as you go along which is scary from point of view that the entire top has to eventually be corretly positioned to give a smooth and professional finish. If you're out in one place, one can't be sure of the consequences down the line.
To sum up, the non-OEM tops are good in many respects, but undoubtedly not the real deal. They involve more work as a consequence of not having the appropriate stitched in parts as mentioned above, and this means having to cut them out of the old roof and re-use them as best you can. Some of the plastic bits mention weren't in good condition on my old roof, and I had to fashion new components from 2mm thick softish plastic. All time and hassle when you want to actually do a job.
The final fit in general is extremely good on the top I bought, but again, careful is a watchword when making holes etc.
Now to the actual job:-
I downloaded an article from Bimmerforums titled " How to replace your Z3 Roadster Soft Top" It is excellent in all respects regarding the dismantling and removal of the old top, and is accompanied by photographs which a re clear. The person who put the article together replaced with an OEM top. Consequently only some parts of the re-assembly were useful. I think if you are replacing with an OEM top it will be almost as good as you can get interms of a write-up.
Some small but critical aspects which were not mentioned and which are so important as to make the task a breeze.
These are:-
There are 2 options on removing the top from the roof well behind the seats. The structure there comprises a SEALING STRIP and a TACK FRAME / STRIP.
The sealing Strip is held by 4 self threading bolts to the car body inside the well. Between the Sealing Strip and the car body is a layer of Butyl and a strip of stick on foam rubber (The type of stuff you buy on a roll with one side sticky with a peel off paper) below the Butyl. The Sealing Strip is made of plastic, and it is easily damaged/cracked/split. So removing it without the intent of replacing it with a new item isn't an option. I didn't remove mine in my job.
The Tack Frame is made of steel and is a pressed item with a fattened "U" channel roughly central to the width, and running the length of the frame. The frame starts just behind the body panel to which the door datch is located - but somewhat higher up -and ends on the opposite side of the car in the same location there. It is held to the car by 18 large self threading bolts (M10). When it is fitted, it overlays the "Sealing Strip" and has a layer of Butyl between it and the Sealing Strip, and also a strip of foam rubber as per previous mentioned on the Sealing Strip.
The "flttened "U" channel in the Tack frame has a thick (1/2") slab of softish plastic in it and is held there by the channel which is slightly enclosed at the open end. The exposed surface of this plastic is about 5/8" wide and runs the entire length of the fram on the side which will fit to the Sealing Strip.
To remove the Tack Frame the 18 bolts must be removed, at which point it is held by the Butyl. This is seriously sticky stuff, and becomes very hard when cold. I used a fairly powerful hairdryer and started at one end and after about 20 minutes I had the frame out by working my way around to the other end. It seems that once the Butyl heats up, it stays soft for some time.
So, first point that you don't get in the instructions is the hairdryer as a tool.
Next, removing the Butyl is a pain, and the best is to gather a ball of it and use the ball to lift of more and more together with a small pallette knife.
Tip No.2 The Butyl will warm in your hand as you work. This diminishes it's effectiveness as it begins to stick to your fingers and the butyl still on the car. Let the ball cool whenever you find it becoming too soft. There is Butyl on the car, and on the back of the Tack frame..it all needs to come off.
Tip No. 3. Have at least 2 x 750ml bottles of Xylene. The final cleaning will need Xylene.
Tip No 4. The bow frames where the clip on plastic tubes are and by which the fabric top is held to the bows are removed during the dismantling process. The Latex type glue which was used by BMW to hold the fabric is worse than the Butyl. Xylene will soften it to a jelly consistency, but it os still extremely sticky and has prefers to stick to anything it touches. In removing it I used the pallette knife and an old piece of bath towel with Xylene. The pallette knife was useful in scraping most of it into larger lumps which refused to depart the bows. The piece of old towel and Xylene softened the glue to a jelly, but it doesn't wipe off. Wiping in one direction only will move the stuff along the bow frame until it gathers into a large enough mass that can be plucked off. When my old piece of towel was eventually gummy, I had a facecloth size piece of micro-fibre cloth lying close by, and continued with that. IMPORTANT...the micro-fibre actually managed to wipe the stuff off !!! So, don't waste your time with an old towel...USE MICRO-FIBRE CLOTH.
Tip No. 5 The new top must be stapled to the plastic strip in the flattened "U" The center of the fabric will be marked, and the center hole on the Tack frame is the only round hole on the frame. Line up the fabric at center and punch 3 or 4 staples 1/2" apart either side of center. Now go to one end of the Tack frame and screw the previously removed small self tapper through the end of the fabric top and into the plastic strip in the "U" so that it lines up flush. D the same at the opposite extremity of the Tack frame. Now pull the fabric over the Tack Frame. The fabric top will have a strip of hard-ish plastic stitched ito the seam, and which will "clip" into and on top of the plastic strip lodged in the Tack Frame "U" channel.
Once you have this done. Apply staples to the corners of the set-up first. Then go back to the center and work out from there in both directions.
Tip 6. Before attempting the process described above, leave the fabric top in a heated room overnight, and leave the Tack Frame outside in the cold... It'll help for the fabric top to stretch and the Tack Frame will be as contracted as you can have it.
Tip No.7
Remove the seats, Storage box,Roll Hoops,Lateral trims....You will be able to move about in the car with greater ease, and as you know, with all the fittings behind the seats, just cleaning the rear screen is a pain, so get all that stuff out. The seats get in the way, and instead of kneeling on a seat and working over it, get it out and kneel on a cushion. It's a whole lot easier!!
The rest of the installation is straight forward. At various points the Bimmerforums instructions mentions using a 3M double sided tape, and indeed BMW does so too. I did buy and use this where applicable, but franky it's a waste of time, and I can't see the point of it. Use it or not... I only did because it was in the instructions.
When you finally complete the job...it's a relief. Also you realise that the saving in product cost of the non OEM top is a false horizon. It took me 3 days in all from removing stuff to complete replacement of everything.
BMW suggest wetting the roof for a few days, and sunshine / heat to allow the fabric to stretch / shrink / stretch shrink etc etc.
Finally, closing the new roof requires at least two people, but preferably 3. One on each side and one inside to apply the locks. Once you have it down, leave it down and do the water / heat thing. If you open it, you'll be needing those extra hands again... I know..
I hope that the above will be useful insomuch as it's the little things that aren't in the instructions which will make the job more do-able for the average person. Some muscle / pain tabs are good if you're not a youngster....
