Hi, I'm a new member living in the UK. I have several bikes, and I'm now looking to buy an early 70s Yamaha trail bike. I'm looking at DT 400s, DT 250s and an RT360. Should these bikes have matching frame and engine numbers? If the numbers don't match, is the value affected?
I look forward to hearing from you guys,
Atb, Phil.

 from near Salisbury.

Yes, frame and engine numbers should match. In my opinion, non matching numbers won't matter that much on these models (unlike on the RDs!).

I would go for a 250 (DT1, DT2, DT3 or DT250) as they are easier to live with (ie no kickback when starting).

Thank you, all help gratefully received 
 

Technically, Yamaha published that engine and frame numbers might not match exactly...  they could be off a few numbers from the factory...  such as XXX-00333 on frame and XXX-00336 on engine. 

In practice, this is rarely the case.  Numbers almost always match perfectly.  

One thing to watch out for is major mismatches as that often does affect value quite a bit...  A 1970 250 frame with a different serial number 1970 250 engine is one thing and more "acceptable" than a 1970 250 frame with a 1970 360 engine.   It's also not uncommon to see a "bitsa" bike with say a 1970 frame and a 1973 engine.  The more the mismatch, the more it can potentially affect value.

Worst case is the 1968 DT1...  even if it has a matching engine/frame, but if it has (completely functional!) "revised" parts not original to 68 that came on later models, the value will be affected significantly.

If you are looking for a "rider" then the above and "matching numbers" is not as important...  if you want to buy or build a "show bike" and/or one that you hope to sell for top dollar someday, not having matching numbers will hurt in my experience.  

One of the bikes I'm looking at has the following numbers
Frame RT1-128241
Engine RT1-149078

I'm told it's an RT360.
Does this sound right?
Atb, Phil.

There are excellent US model VIN tables in the Tech Library here. 

At the risk of sounding pedantic, technically the "RT360" model name may have been used by Yamaha in some countries...  from what I've seen that was most likely in the 73-74 era. 

The US models were the RT1, RT1b, RT2, and RT3.  All had the "RT1" prefix in the VIN.  The VIN numbers you posted may be a 1972 RT2 frame with a 1973 RT3 engine.  Engine should have a reed valve intake and not the earlier piston port top end.

It's also possible it's not a US model bike...  but the year models I gave should be the same...  mismatched but the 72 RT2 and 73 RT3 were virtually identical except for cosmetics and a couple of minor mechanical changes so functionally that later model engine is the the same as what the frame came with. 

 

Thank you all for your help, every day is a learning day