So, I've just got to the part where I'm about to install my reed block. The gasket supplied with the kit is too big.
I asked my mate (on here) to compare to his 400. My inlet size is smaller.
Can anyone confirm if there were differences for different markets, or if not, point me in the right direction.
I've check everything, and cant find what is going on. See attached photos for the issue.

Mark
now seeing the pic on a pc, your frame looks different to mine as well as the barrel and head, look how chunky mine is to yours. Not to others mine is the last picture for comparison to marks, the 3rd picture

Look for a code cast into cylinder...  probably 5 numbers?  Usually at cylinder base or near intake.

My guess is someone installed a later (77-on) DT400 monoshock ("MX" in the UK) top end...  reed cage is smaller on those.

Hi Mark. The number cast into the base of the cylinder is 51000. See pic. 

I think you have a cylinder off of a 1975 MX400B
(510) is the part# prefix for a U.S. spec Yamaha 1975 MX400B
(500) is the part# prefix for a U.S. spec Yamaha 1975 DT400B

That number is for a MX400B cylinder and it bolts right on a 75-76 DT400.

I hopped up a DT400B using a cylinder like that plus a MX400B cylinder head, MX400B Piston and a 38mm carb from a IT400C.

The reed assembly is smaller on the MX400 than it is on the DT400.

Just to clarify for those in other countries, the MX400 mentioned is a dirt-only motocross race bike.  Should have substantially more power than a DT400...  if DT exhaust and carb were used that would likely restrict it somewhat compared to the motocross version.

Great info thanks guys. So, what next then?
Totally in the dark now.
I have just obtained a standard airbox for a dt400B. I’m hoping this is going to work ok with the MX cylinder. What are peoples thoughts on jet sizes for the carb regarding the above? What spark plug should I use - B9ES as standard?
I’m really not looking for anything lairy, I was just hoping for a fairly standard bike and power. Is the MX barrel going to be a bit too peaky for general use?

Couple of questions need to be answered first In regards to MX400B or DT400B before you can make a informed decision on how to proceed.
Which cylinder head do you have?
Which piston do you have?
Which carburetor do you have?
Which bottom end do you have?
 

The only one I know for sure is that the engine number falls within the range for a 1975 DT400B, and as I have had the crank rebuilt using standard 400B parts I would assume the bottom end is standard. Are there any markings on the head to identify its origin?
I can check the carb, again, any markings or model numbers I should be looking for….. excuse my ignorance, this is my first time.
The previous owner had fitted a new piston, but again I have to idea whether this would’ve been standard DT. I would assume that the cut outs in the piston skirt would need to be matched to the porting for the MX barrel.