Had a good think on it, and I'm sorry for disputing said information. I never did time runs when I had the rt1mx head on the rt3 cylinder. Torque was definitely noticeable, but we all know how the seat dyno can be deceiving. I'm tempted to get that cylinder sized up and try that all again.

Hi ASCO, I saw yours on ebay, thanks.

Update. Picked up an parts bike 69/71 DT1 last weekend. Last spring, got a D2TMX bottomend(blown rod)/cylinder head very cheap. Then got a DT2/3 cylinder with a loose standard bore. Got me thinking so a bought $25 DT2MX piston. Comparing the DT1 vs DT2MX pistons, wrist pin location is different. Sharing the same wrist pin, the DT2MX crown is about 5mm higher(it has lower pin location) Put the DT2MX piston on the DT1 bottom end, same thing, the DT2MX piston extendeds about 5mm out of cylinder. It seems the DT2MX topend can be used with the DT1 bottom end if the cylinder has a 5mm spacer. Doing more precise measurements now to confirm the needed spacer thickness with two base gaskets. There are ebay vendors with 4 & 5mm 6061 Al plate. I'll take pics and post.

You can also swap rods, that's where the difference is.  

(250 piston port has a different length rod than 250 reed valve)

Mark, I believe you and that makes sense except...  The DT2 and DT1 rods share the same part#, per Partzilla and Rocky Mountain-431-11651-00-00.  They also list 510-11651-00-00 as a part#!?.   Only CSML has parts breakdown for the DT2MX and lists 510-11651-00 as specific to the reed intake DT2/3 and other 72-75 Yamaha 250's. Hadn't planned on a crank rebuild. 
 

It's silly what they did but the rod length DID change when the 250 went to the reed valve (1972).  Don't let whatever you're seeing in the parts lists fool you. 

They are "shared" parts lists and only the 250 changed rods with the reed valve.  The 360 stayed with the earlier 431 (or 214) "long" rod so that part number shows up in the 1972 list but it's for the RT2 not the DT2.

Spacer will work too but you will be reducing the primary (crankcase) compression ratio.  Not sure if it will make much difference as Yamaha had pretty high crankcase compression ratios compared to say an old BSA 125 that needed the crankcase "stuffed" for better performance.

 

Just having a laugh as I had a VERY STUFFED BSA Bantam125. Used to destroy it most weekends or after school poor thing, gearbox, frame, wheels, then the flywheel disc inserts let go so would spin on for a time after the motor stopped haha. My old 197 Villiers though had a Bob weight crank but had been fabricated into a full circle crank. Also the transfers had been cut completely out so totaly controlled by the piston with just one long slot each side. Worried me when I discovered it but it went like he'll compared to standard. Anyway-- i crashed into the ticket box at the local horse racing track & stuffed the frame so sold it as it lay. Well, I had to sell it to find the 25 pounds cost to repair the ticket box probzbly in about 1961 [way before we went decimal currency in NZ in 1967 lol].

But that's not what I wanted to say. long rods & short rods. Been through all that back in the day trying tricks & getting caught out. I know the 250 at some stage changed the crank pin to the bigger 360 size with smaller diameter rollers in the bigend cage. That change might not have been until yz250a. Happy to be corrected if I'm lost & giving 'fake news'.