Cowboy 3200, 4500, Juki 441 and similar

[chrisash]

When you look at these and many others compatible makes, side by side they all look the same, so what makes the difference between a lighter thread limit and a very heavy thread limit and can you modify say a 3200 to a 4500 with a few parts on the pressure foot to increase the thickness that can be used, or are they totally different inside

I do realise that some clones may be better made than others, but that is not what i am enquiring about, Just really how the difference is applied to the same basic design

[SolarLeatherMachines]

Each machine may "Look" the same to an untrained eye, but there are subtle but important differences in each machine.  Some machines, such as the 441 clones can swap certain parts, but not ALL parts.  

With regards to "making the 3200 into a 4500", it's really not possible.  While the interior parts are very similar in appearance, there are many small differences that prevent cross part swapping (believe me, my life would be easier if this were the case!).   Not to mention that the casting is greatly different between the two, ultimately limiting foot lift to a certain distance.   NOW, that said, you CAN adjust the 3200 to sew thicker by sliding the outside presser foot bar upwards in the upper block.  But, this comes at a cost:  the presser foot will no longer touch the needle plate by that same height you raised the foot.   
I guess conceivably, a sharp machinist could rework a 3200 into having the same sewing thickness as a 4500, but by the time the guy is done making all the new parts, and grinding the castings, he could have worked enough overtime to have paid for a 4500 in the first place.