18lstroup wrote:Should I just order the size piston that is stamped on my cover? or do I still need to measure it?
As mentioned above, Beta pistons are available in 3 diameters. Not sure about your specific RR model, but on my 2016 300 Xtrainer they are :
"A" is between 71.935 and 71.945 mm
"B" is between 71.945 and 71.955 mm
"C" is between 71.955 and 71.965 mm
Beta's shop manual call for 0.1 mm of play between cylinder and piston. On new engines the cylinder and piston are matched with like sizes (usually "A") which provide this clearance out of the box.
On your rebuild your goal is the same as when building a new engine: To match piston and cylinder with appropriate free play between them. To do this you have several strategy options:
1) If you know the piston and cylinder have minimal wear (i.e. preventative maintenance) you can make an educated guess whether a new "A" piston will snugly fit an "A" cylinder with 100 hours, or
2) If you suspect cylinder wear due to long hours, hard riding, OI system failure, poor air filter maintenance etc. (i.e. corrective maintenance) you should probably have the cylinder professionally inspected and measured before determining which piston to use, or
3) Some riders that know the piston and cylinder are worn just opt to go up one size on the piston and call it good.
Bottom line is that the piston and cylinder need to be inspected and/or measured before determining a definitive action. None of us can tell you what you need for sure because we don't know anything about the condition of your top end.
For what it's worth here's my most recent experience:
I changed my Xtrainer piston at 140 hours as a preventative maintenance activity. The old piston still had good compression but I wanted to swap it out before riding another season of 60 to 80 hours. After removing and inspecting it I could see that it was in very good shape. So I chose to replace it with another "A" piston and it's been working great for 70 hours or so. In hindsight I'm confident that I could have waited until 200 hours to change it.