The connecting rods and some other stuff is a little better in the newer engine. From what I have seen on here the Speed water pump impeller is a good mod. I'm going to run the OEM water pump with a Speedwerx auto bleed kit. It is supposed to fix the water pump issues and costs much less. The Speedwerx PTO shaft is another item I'm gonna upgrade to. I also plan on going with Evens coolant. Ive been reading a lot on here since before I got my XX and the issues people seem to have with the engine have been the water pump impellers , the cause seems to be air getting into the system some how. And clogged oil filters, clogged with assembly lube , or a clogged oil pick up. If you think you might want to do a turbo , the rods and pistons are an upgrade you would want to do. I have not torn my bad engine down yet. Depending on whats bad in it I was thinking of rebuilding it and building it for adding a turbo. There are people using the same engine in some gyrocopters and it is supposed to be a very reliable engine. When mine first started to knock I read every thread I could find on here and emailed anybody who I saw online that had a XX engine issue. I too want to put mine back together as strong as possible. Another reason I got the newer engine with the better internals. I tried checking part numbers to see what had changed but they " Updated" the internal part numbers to the parts in the new engine on the website I was looking at. And all of the stock motors that failed were from a lack of oil. either the filter clogged or the tube that sucks it into the oil pump and an oil pump failed on one other car. It seems like if the engine is stock, the only issues are the assembly lube that is in them to start, and then if you get air in the cooling system the water pump impellor can fail. I read some YXZ forum stuff too since they are close . And I saw some people have issues with the cam chain tensioner. I was looking for mostly issues with newer lower mileage XX's though . I found more people that had an issue around the same mileage as I did ,than had issues with higher amounts of time on their cars. I'll keep an eye on this post and possibly add some of the items people suggest. Either to the new engine or when I rebuild the old one, or both. Another thing that I saw a lot of was the issues with the XX having not been put together correctly. Like a loose connection in the wiring, or bolts loose, etc. Again it was mostly on the 18 since that was what I have and what I was looking for. I knew all of this when I got mine. At the time there were not any new ones around. After I bought the engine I'll be around the price of what a new one would have been. I had hoped by getting the second hand XX I could avoid some of the issues I saw people have in the first 500 miles. But it looks like the guy that sold it to me either saw them coming, or got lucky and he avoided them and they happened just in time for me to take it on a long ride. But nothing I can do now but fix my XX . And I have gotten to know my XX better than I might have if it had not broken. I was blown away at all the sand and dirt I have cleaned out. I'm sure my trans axle will go at some point too and I'll get the newer one with the wider gears. And after that I hope its solid for years to come, and I can rip it around my small c track I'm also making on my property. More of a trail system , its only a mile right now and it needs a lot of work still. Just not enough time in the day to do everything.