So going back to the expander rings the piston ported 70-71 years did have the expander rings and the reed valve engines 72-73 didnt?

RT325 i meant 80 thousands over. so yes the 80mm+1.75. Im putting that bike on hold for the moment because im dealing with this other 360 and restoring a DT250. I am curious about this kit you mentioned?

0.080" over is a fraction more than 2mm over.  If that's what you really need a 1.75mm over piston will be too small.

2X5 is the model code for a 79 YZ400...  RT325 has discussed modifying and using that piston here a few times...  should be able to find with a search.

Im just going by what my local shop told me i havent really dove too deep into that particular one yet. It currently has a 1.5mm in it but i need to go back and measure to be sure. Im currently fighting another 360 as well as restoring a DT250 so its been kinda hard to get to that one.

Not sure what your shop told you...  but 80 thousandths over is 2mm not 1.75.  That's all RT325 was trying to point out.

Yamaha went in relatively small steps (0.25mm or about 0.010") up to 1mm over.  Yamaha considered 1mm over "maximum overbore".  Aftermarket often goes up to 2mm over.

Aftermarket industry in general uses bigger steps of 0.020" or 0.5mm.  So once you get over 1mm you usually only have a choice of 1.5mm and 2.0mm. 

(Suzuki also used 0.5mm steps and sometimes only offered one oversize...  after that it was aftermarket.)




 

I would like to jump into this with some recent trouble I had. One of my RT3 regular riders with the 4th oversize piston and a lot of miles on it developed an air leak at the base between the cylinder liner and cylinder. Had to install a NOS cylinder and piston. I measured the thickness of the cylinder liner on the new one at 4mm. A NOS DT3 250 cylinder liner is 5mm.

I was told by the machinist not to go past the 4th oversize piston on the 360. The liner will be too thin.
 

I see, thats good info Mark. I will definitely be checking back here before i do anything but its gonna be some time before i go back to that project. I could use what i have however it may rattle a bit more then it should. I was told it needs to go to the next size up. It would be nice to re-sleeve it but i was estimated at $350 to do that and thats what i paid for the bike! Im probably going to end up getting a different jug in good time and sell that one as is, will see im still thinking about my options and that bike is currently gonna have to wait her turn.

As to your question about swapping pistons back and forth between reed valve and non reed valve engines, you cannot run a reed valve type piston in a piston port engine.  It would seriously compromise the primary compression (bottom end compression).  I'm not sure what would happen if you put a piston port type piston skirt on a reed valve engine but would think it would also have a very negative effect.  I would say that if you use a piston port type piston in a reed valve engine you should trace and duplicate the windows of a reed valve engine.  

As far as catastrophic failure in ring end position, although I have both PP and reed 250's and 360's I've never compared exactly where the transfer ports are or the slot above the intake port but would GUESS there is no reason for concern BUT it's easy enough to check.  Just be sure that the ring gaps are not in a port anywhere.  A sharpie mark on the top of the piston and then sticking it in the cylinder with no rings installed will make it obvious.
 

I probably said this before but cutting holes in a cast piston skirt is asking for disaster.

Some really good reed valve engines did not have windowed pistons...  or extreme intake port timing...  and ran strong...  so not sure running a solid skirt with a reed valve would be that bad.  I raced a reed valve Honda 350 (no windows in piston skirt) in a very competitive class and some engine builders windowed the piston.  It didn't seem to help much if at all.

But yes, running a windowed piston in a piston port would be bad.  Probably would have trouble getting it to even start. 

I decided to go with the piston its supposed to have which would be the reed type. It just came in today and looks great! i plan to put it on tomorrow. I dont really like swapping stuff out like that, its one thing for cosmetic stuff like fenders and gauges but when it comes to engine parts i like to keep it stock just in case something doesnt work quite right and grenades the engine. I was just curious if it would work or not. I appreciate everyone's help on this, this community never fails to amaze me.

SOLID LIFTERS #17. Here's mine drawing the inlet onto the skirt. Not recommending & hang my head in shame haha. Worked good though--just got scared of how long it'd hang together but was in there for a while. Only option at the time But if i did it again i'd try it without any holes. Gotta work, just be interesting how much the performance would change without the boost port having full affect.SOLID LIFTERS #17. Here's mine drawing the inlet onto the skirt. Not recommending & hang my head in shame haha. Worked good though--just got scared of how long it'd hang together but was in there for a while. Only option at the time But if i did it again i'd try it without any holes. Gotta work, just be interesting how much the performance would change without the boost port having full affect.