A, b, or c piston

[18lstroup]

My 2017 300rr has 78 hours. Planning on top end at 100. How do I determine what piston size I need? Is it stamped on the bike or do I need to measure the cylinder?

[mtdirtbag]

Unless you're racing 100 hours seems premature for a piston change, but if that is your usual practice it's your call. Beta piston kits come in letter sizes of A,B,C & D with A being stock size, most replace with the next size larger but are usually going more hours. I would think you could just pull it down and re-ring it along with cleaning the power valve. I'm at 180 hours and it will get top end redone this winter, power still seems very good - I'm also going to have the head done while I have it down.

[Darkside]

pull your power valve cover. the piston size is stamped on the cylinder

[mtdirtbag]

pull your power valve cover. the piston size is stamped on the cylinder

Thanks for that, I'll need that this winter.

Tom

[18lstroup]

Should I just order the size piston that is stamped on my cover? or do I still need to measure it?

[aj_mariner]

I was told by a dealer that you just go by the stamp on the cylinder. The dealer’s thought was that if it requires a larger piston then you are close to wearing out the nikasil coating. A local machinist suggested that the nikasil is only 0.003-0.004” thick.

I am pretty sure the engine repair manual says to measure and choose accordingly. This seems to be standard practice and there are some on here who have increased piston sizes with no reported adverse affects.

For me, I am close to 200hours and plan to measure the bore, inspect the cross hatching, worry about and overthink it for a day, then most likely follow the manual.

When in doubt, email Beta USA directly.

[wwguy]

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.

[Motumatai]

My Beta RR300 Racing was gently broken in by doing 22 hours in the Gold class, Romaniacs back in 2014
I then purchased the bike and took it to 244 hours. Lots of single track, no racing, lots of tractoring in gnarly terrain. The bike just lapped it up.
At about 180 hours I started to feel a bit of a lack of power (or was that me getting more in tune with how to ride the bike!)
Several times (mainly when the pipe came off to pop out dings) I checked out the bore via the exhaust port. All looked good.
I took the compression immediately prior to tearing down. 165psi cold
Popped in a 'B' size piston and rings and way we go again. Back to its braaaping usual self . Cross hatching was still at about 75%. No scuff marks on the bore. Very pleased.
Take the left hand side power valve cover off, use a shop rag with break clean to wipe off the gunge and you will see an 'A' stamped there (80% of the time it'll be an A).
Getting an accurate measurement of bore wear with guages is extremely difficult unless you do it every day. The tiny differences will be difficult for an amateur to pick up, even with all the right tools.
Though it all depends on what work the bike has done, and how its been treated, you could either let her go to 2 hundy and go up one piston size, or follow the book and replace like for like at 100. Me, I'd rather spend that 300 bucks on travel gas and riding gas and enjoy my bike than replace stuff way before its due.
Racing your 2 stroke? just follow the manufacturers recommendations.

[18lstroup]

The main reason I am thinking of rebuilding it is because I work at a dealership and can get an new prox piston kit for 125 with my discount.

[LRM]

Question to all of the commenter above. Are you getting these kind of hours with or without OI? I wish to put a Lectron on my XT and it requires removing the OI. Just wondering what my experience my be.