Back for some details!
So, as I said, I think I can add some details that might help anyone with parts and TZ/RD interchangability. I cannot go into everything as there are myriad details. Also, if you want to build a real TZ, you will have to do a lot of research yourself to be happy with what you do because, not all the history is available to mortals.
A great info source is Greg Bennett's great book "Yamaha two stroke production roadracing motorcycles" vol 1.
So, I will start with the crankcases.
Generally, the TD3/TR3 and TZ250/350 up to the late 70's had cases with the same dimensions as the DS7, YR5 and the RD's. The style and detail changes sort-of follow the road bike development but, there are loads of anomalies (like when the different side cases were changed to later styles-there are loads of variations even in Official Yamaha portraits) and, you will have to satisfy yourselves about that! There was a big change around '76 model year when the cases changed and the clutch cable became attached to the left side case, rather than through that small square cut-out on the top of the main crankcase on earlier engines.
The genuine TZ250/350 cases are rare because they got raced, crashed, repaired, rebuilt and swopped about. So, there is no doubt that RD cases can be modified and substituted, even on gen TZ's. The engine numbers usually did originally match the frame in the same way as the RD but, they are often different today or, remarked.
The engine id code can help. Overall, in this period the engines on the racebikes stuck with "DS7" for the TD3/TZ250's and "R5" for the TR3/TZ350. The next 6 numbers start with 99 followed by 0 for '72 and 1 for '73 etc...probably!
The usual Yamaha system of reserving the first 100 numbers for development bikes seems to apply. So, the first production TD3 engine (for '72) would have been DS7-990101. The first TZ250 engine for '73, DS7-991101. Now, these numbers are not cast in stone but, any DS7-99---- or R5-99---- cases are worth checking out. Of course, cases can have been remarked or replaced so, the numbers on their own are not full provenance. Buyer-beware.
Yamaha had different dies for the TZ castings, you can tell because there is no small gbox oil transfer hole below the gearshifter cam mounting on the dry-clutch crankcase. Also, the TZ crankcases continued to have the steel crank bearing mount inserts fitted, though I don't know if or when they reverted to the all-alloy construction like the RD? I would not be surprised if the bare castings were machined and processed on part of the RD production line as the fit and finish is very much identical to the roadbikes. However, the racebikes do have the extra crank "C" ring grooves, the front mountings bored out for the rubber mounts and the gbox oil drain point is drilled and tapped in the centre of the case, unlike the RD which is tapped in the boss further to the right.
Now most of that is only important to a TZ owner/builder. However, it does show that the RD cases are good for TZ conversions and, shows you just how close they are!
5port
