piston size help; PLEASE

[bildough]

does anyone know if I am running a 54.43 piston in my 125 how many over it is. thank you for any help you can give

[firstturn]

Bill,
  A little under 1st over.  First over is 54.5.  Which means that you may not have to bore when you get ready to clean up the cyclinder.  You can just go to a regular 1st over.

Ron Carbaugh

[bildough]

that is great news Ron. The cylinder is a little sloppy but not bad at all. A little scratching is all. I think most of the problem was the torque on the head. I hope that all these things led to the inability to start the bike easily. Any other recommendations I should explore.

[bildough]

Ron
could you clarify a little more. What would a standard bore be and what would a first over and second over be. If the piston I remove is a 54.43 and is a little sloppy but not bad. It could use a bore as a set of rings wont close it enough. What would you recommend doing and how many mm should the proper ring gap be.  Is the 54.43 piston the original piston possibly?

[firstturn]

Bill,
  I guess I will do last things first in this case.  I have purchased Original pistons of all sizes i.e. they actually measured the piston instead of just assigning a number such as 54.50mm.  Noval idea!
  Next, I don't believe the piston is the original piston or bore.  I have had pistons from 53.97 to 54.04 on original new bikes.  On a bore job I think I would consider honing out the cylinder and possibly fitting a 54.50 piston.  This all depends on what the condition of the cylinder and who you trust to do a new bore and not blotch the the job itself.
  Last the question ring gap.  I will let someone else address this since I approach this step a little differently.  I will say that I fit a piston with 2.5 to 3 thousands clearance.  I like to put the bike on a trailer or in the back of the truck and start it up and idle it around 2000 RPM and drive at highway speeds(for air circulation -it is a aircooled engine) for about a hour.  All sounds crazy, but I have been doing this for years.  The reason I don't address ring gap is that I have a step that I don't recomend for other people.  I cut my own rings to fit the cylinder.  IF ANYONE TRYS THIS AND MESSES UP A CYLINDER OR BREAKS THE RING/S PLEASE DON'T CALL ME.
  Bill, as always if we need to talk, email me.  Also thanks for the Monark pictures and say hi to your Brother.
Ron Carbaugh

[Kip Kern]

Actually, It could be 1st over or 2nd over.  All depends on the piston manufacturer and cylinder.  The 1976 KTM 125 starts at 54mm standard, 54.25mm 1st, 54.5mm 2nd, 54.75 3rd.  Wiseco sells pistons in .010 oversizes, Tarabusi also goes 1/4 mm sizes so actually it really depends on the manufacturer, i.e., Elko, Mahle, Tarabusi, Wiseco and many others.  The key thing is to measure the cylinder correctly, and purchase the correct size piston for the overbore.  Remember also that the cylinder could be out-of-round and may need more than the next size piston.

[firstturn]

Kip is correct, but to lessen confussion I tried to stay on track with what I thought (possibly incorrectly) that we were dealing with a OEM piston which are the only ones I have ever seen with such accurate measurements.  On to the difference in bore  as Kip brought up, I was sticking to the shop manual/parts books definition of 1st over, 2nd over..........and so on.
  To the next subject on out of round cylinder.  It should include out of round pistons....YES OEM pistons can be out of round.  If either of these two situations exists (providing everything else was up to specs) you would have tremendous acceleration up to about 60% of you power curve then nothing.
  With all this said to quote Kip "The key thing is to measure the cylinder correctly, and purchase the correct size piston for the overbore."  And with that said always buy the piston first.  You won't believe how many people call for a piston and have the bore job done without piston in hand.  If that happens, I wouldn't do any further machine work business with that shop.
  Well I hope this helps.

Ron Carbaugh

[DKWRACER]

Bill, another consideration in regards to Sachs motors, is that the wrist pin is located low on the piston which leads to a barrel-shaped bore in time....so as Kip recommends, accurate measurement of the bore before deciding on what piston to go to, can play a part in the final outcome. If a new piston is in the plans, you can add oil squirt holes to the piston on the exhaust side at approx. 10 and 2 o-clock below the bottom ring, vertically between the ports, and or have the piston coated with some of the newer technologies; Swaintech...Good luck with your project!!!