I have two other threads regarding two ongoing projects I am working on. I started this topic in an effort to clarify some things for myself and possibly others. I have a number of questions regarding the interchange of top and bottom ends of 125’s and 175’s:

1. Piston port vs. Reed valve. 
     A. Are these the correct terms to describe the intakes for these bikes?
     B. Can I use a reed valve cylinder on a bottom end that originally had a piston port cylinder?
     C. I see pistons for sale with two rectangular holes in the skirt. My AT-1 piston port engine has a piston with no holes. Am I correct in assuming that piston port engines don’t have pistons with holes, and reed valve engines have pistons with holes?

2. I see pistons for sale with either one or two piston rings. 
     A. I assume one ring creates less drag, and therefore is used for racing, is that correct?
             A1. If one ring pistons are for racing, are they also higher compression?
  
If I get some feedback on these questions, I’m sure I’ll have more. Both my Yamaha enduros need top ends. I have bought some jugs and heads, all 175, and now need pistons for both. 

As always, thanks for reading.
Jamie Galvin
 

Answer to all of 1 and 2A is "yes".

Single ring pistons also tend to lose a little power at lower RPM.

The single ring pistons are generally from the "motocross" models and while there could be a very small difference in compression height, the motocross models had cylinder heads with quite a bit smaller volume to compensate for the loss of compression due to the higher exhaust port and to raise the compression ratio slightly.. 

One thing that is sometimes overlooked is the small end rod bearing is wider on the 175 so you need to use the wider 175 bearing.  Both part numbers are in the AT/ct parts lists but it's hard to tell them apart...  so I simply look up the upper rod bearing for a 1977-79 IT175 as there is only one choice in that list and it uses the same upper rod bearing as the CT and DT175's.

One more point...  the carbs on the piston port 125 and 175 (69-71) were identical.  Not even jetting changes between the engine sizes.

The reed valve engines had different carbs/jetting than the earlier models and differences in jetting between the 125 and 175. 

That makes the swap very easy in terms of carb on the early models if you stick with piston port.

(Motocross models had different/larger carbs and different jetting than the Enduros)

Thanks Mark, I’ll probably order pistons for both tonight. 

Are the reeds the same for 125’s and 175’s?
Do the MX versions have different reeds?

Thanks again
If Mark was my neighbor, I’d be his biggest pain in his ass.
 

You can look in the parts list for the reeds...  Partzilla and other sites will show you what parts lists the part shows up in.

Just guessing, I'd say the AT and CT reeds are the same, MX (motocross for our overseas friends) are likely different.

I might have some used parts if you need something. 

Thanks Mark, 
I get lazy sometimes and lean on the experts here rather than doing the research. I ordered a bunch or parts last night. I’ll report back on the “AT-1 running after 5 years” thread.