Hi there, l started putting my motor back together pressed new crank bearings into cases , fitted rebuilt crank into left case , new seals etc fitted gear shafts with all the parts that came out - shims ,washers etc. Following rebuild from Haynes manual tapped the right case on everything seemed fine until I tightened the case bolts (hand tight)the gear shafts are very tight almost impossible to turn ,if I slacken bolts everything spins freely and I can move selector drum and can here gears  clunking in , is there something when rebuilding that's not mentioned in the haynes manual I should be doing split it apart twice to check everything is there that should be ,any help/ advice would be welcome 
thanks

Apparently you didn't check crank clearance before installing.
That would have alerted you to a potential issue.
With the empty cases together, measure the distance between bearing faces.
Measure the width of the crank at the bearing face contact area.
Subtract crank width from bearing opening. That tells you the clearance.
Divide the clearance by 2 and that gives you the shim dimension.
Shim goes on the primary side.
The Yamaha service manual tells you the crankshaft maximum width. It can't be any wider than that.
Pressing bearings into cases is not the recommended method of installation.
A crankshaft installation tool should be used for crank installation. Install primary side first.

Take some measurements... you should soon see the problem.



Just my opinion

You are absolutely right,I didn't measure the crank or the gaps in between bearings , I didn't even consider  that would make the gearbox shafts tight , I'll split cases again measure up and try again
thanks

I'll take a guess though that the problem is with the gearbox side of things but i'd leave all the gears out & slide the case on to see if its crank width which would be real bad luck if thats the case as there's a slight tolerance for width. So if that's ok, then could individually try each shaft & close the case & selector drum etc just to try to eliminate things. Not sure about Haynes Manuals. Also not sure what year you have but here's a "74 dt250a on partzilla & probably from "74 on are much the same. Make sure 28 is still on 27 except that would make a big difference if it dropped over the edge of 27 not a small difference. Partzilla is pretty good most of the time to view stuff & looks good in this case from what i remember. Hope it all works out as pretty frustrating.

www.partzilla.com/catalog/yamaha/motorcy.../dt250a/transmission

Thanks for the ideas , I'll give it a go tonight 

cheers

Update on my comment...
I assumed your crank was binding thus causing the gears to bind. 
The crank could be OK but your transmission shaft widths could be wrong (shims), or the bearings installed wrong. They are measured the same way. 
You could try removing the crank and assembling the transmission in the cases to find the issue.
Maybe do one shaft at a time. 

When you first noticed the transmission binding, was the crank binding also? If so, then look towards the crank. If only the transmission was binding (the crank still turned freely) then look towards the transmission. 

The Yamaha service manual should be available in the tech section of the forum. It should have the dimensions listed.