Hey guys i have 2 1973 RT3 360 cylinders that need work. The first one is currently bore 60 over (1.5mm) and it needs to go to 80 (1.75mm) but cant seem to locate a piston that size for it. The 2nd cylinder has a small crack down at the bottom and is standard bore. Does anyone know if these can be re-sleeved and if so where to get a sleeve?

I run a 2X5 piston kit from a "one year only" "79 model YZ400F. 82mm. Need about 5mm of the skirt base to match the original piston except i forgot to do that on one of mine & is ok. Just gets close to the crank at bdc. Top of the cutaway is about the same as top of the windows on original. Head needs a slight tweak in the lathe to take the edge out a couple of mm to clear. Not sure how far past 82mm you'd get away with but i have an 82.25 so i'll find out one day maybe. Here's a link but only a piston. Should be complete kits out there. Good luck which ever way you decide but buying a good cylinder might be cheaper.
www.nospartsnow.com/nos-yamaha-1979-yz40...ton-2x5-11631-00-96/

Tempting however machining isnt exactly my strong suit. I wonder if i could use a standard DT400 piston which would be 85mm but that may be too far. Or better yet are there over size rings for 1.5mm over?

Have you checked with the original Enduronut on a piston and/or rings?  He would be a likely source to have whatever piston/rings you need.

A member in the UK (if I remember correctly) had a nice resleeve done.  You might also check with...  drawing a blank on his member name...  Rich's Taylor'd Porting in Utah.  richstayloredporting.com/  Rich might be able to help or might know who can resleeve.

I can’t help with a machine shop but yes, they can be sleeved successfully, I have a few hundred miles on my 400 now with no issues.


  

Fins just about fall off at 85mm. Tried it on an RT2MX cylinder with a YZ400D piston & was into the aluminium in places & still not quite there. Was kicking myself when the machine shop rang me with that news.

All good advice i will reach out to rich for sure and go from there but its probably better to just find one that needs a hone job. I know it will be cheaper that way especially since i have a standard piston all ready to go.