I have been building for a loooong time just don't understand the difference in bottom ends
There are three basic geometries we're seeing, they aren't new either, many of us were thinking the "half Breed" engine was a new cobbled up thing being shipped over lately, but I found out that it isn't when I tore into one of my engines that was bought on ebay from LEB (Lucky Early Bird) 4 years ago and found that it has a 38mm stroke crank marker ZAE50 just like the current ones we're seeing now and then, and it has the 110mm long con rod and uses the low wrist pin piston.
Most of what we see are the "PK80" type which are 40mm stroke cranks, 110mm rod and use the low pin piston.
Also the GT5 type is more and more popular it seems now, they are 38mm stroke cranked engines, use a con rod that is longer at 114mm and they require a Type B high pin pston.
The actual engine case between all of these geometries will interchange, the difference is the stroke of the crank which is determined by the position of the lower rod journal pin and the length of the Connecting Rod.
These are the three basic 47mm bore 66cc -68.5cc engines that one is most likely to get, with some vendors its a crap shoot which you'll end up with no doubt.
If the con rod is marked Z-L and is scalloped up top its a 40mm stroke engine that likely doesn't have a caged bearing on the rod journal but rather loose pin bearings, If the rod is copper washed looking and has no marking on it and the piston is a low pin type A piston, its gonna be a 40mm stroke engine.
If the rod is marker ZAE50 it a "half Breed type engine and will use the low pin piston but uses a 38mm stroke crank and 110mm length rod, usually these engine can be identified by just removing the head, rotating crank until piston is at TDC and you'll notice that the piston sits way down in the hole/cylinder, normally about 1/16" or about .062"
The GT5 type engines that are popular will have a rod marker FM80 or ZAF80, these engines use what we call a type B or high pin piston and have a longer con rod than all the others @ 114mm
The very knowledgeable engine builder say that the 40mm stroke engines have the least amout of port timing issues, which I have also found to be true.
This is what I know about the most common differences, there are others also, but these are the three geomtries we are gonna see.
Here is a link where Fred explains some of it also, I have three GT5 engines myself and two of them have the rods marked ZAF80 and one is the FM80 marked rod, the one I have running is from rose326a and is a fairly good running engine, the others are waiting to get built up soon.
Hope this helps a little.