Hello, 

I bought a 1978 DT 400, that is in pretty good shape. After tearing down the Engine, the cylinder & Piston have some slight scoring. It looks like the IT/YZ's of the time had a supstatial power increase over the DT's. (Im only 30 so these bikes are way before my time)

With that said it seems like you can use the Piston, Connecting Rod, cylinder & head from an IT and it will bolt right up to A DT Case/Crank. 

Am I missing anything, and can someone confirm the part numbers I need to Make this happen? Also any reputable engine guys in case I have to bore the cylinder, seems like most are not in great shape that are for sale. 

Thanks!

Welcome from NZ. Man you've got some big plans for the upgrade.
What i usualy do to see if ideas like yours are in with a chance is compare base gaskets on Partzilla or similar.
I had no luck looking above year 1976 under YZ's as list is incomplete & showed no gasket, but i've not looked at IT's.
Base gaskets are the same number for a "76 YZ400C compared to you DT400 so that's a start.
Here's two links but on the YZ parts list the numbers are hard to pin down but gasket is listed on the first part of the list with no number down the side, just 0 but says 1M2-11351-01-00 same as DT400.
www.partzilla.com/catalog/yamaha/motorcy...-cylinder-yz400c-d-e
www.partzilla.com/catalog/yamaha/motorcy...linder-cylinder-head
I'm no help really but there's a hell of a lot more to consider before ya go swapping bits.
Then also there's the exhaust to get the advantage from it.
Best of luck with what you decide but let's see what others say before ya jump in.
Now that reminds me of a lifetime ago--well more than your lifetime for sure--[im nearer 80 than 70 haha] & i had a DT250 about "77-"78 monoshock model that i put an IT400 complete motor in it i bought from the wreckers.
It was a weapon & had a good exhaust off a YZ i 'made fit', but motor had been overly ported & so i had to tame it down to spread the power lower down so was good overall & not a mad light switch power hit.
Never got any pics of that as was before digital easy camera use.

This has been done before. You might be able to find threads on here with the parts needed. If I remember correctly, you need the earlier model parts?

I found a thread on here that sounds like it discussing what parts are needed using google, but i think that it is too old because it throws an error when I try to open it. 

Yes, site is going through some growing pains. I think it was the 75 MX400 and 76 YZ/IT400 parts that were the closest "bolt on" Here are a couple threads I found before the search broke on me.

yamaha-enduros.com/index.php/forums/74-7...chamber-volume#95938

yamaha-enduros.com/index.php/forums/74-7...lock-question#146223

WRT a good source to get cylinder boring and similar work done, I think a lot of members here use Bill Bune in Anoka MN.

billbune.com
 

I second the Bill Bune doing your machine work. They did the cylinder work on my IT400 and did an excellent job. Getting ready to send a crank to them to rebuild.