Hey everyone! new to the site and loving it! anyways i have a 1974 Yamaha DT250 and a spare RT360 motor. I just got the DT running today but unfortunately its making some serious knocking when i kick it, i suspect crank bearings or possibly top end. My thought was can i take the crank from the 360 and stick it in a 250? Also will a 360 top end work on a 250 lower end? from what i can tell those 2 motors are very similar, if you ask me a 360 is just a punched out 250 but i dont know lol
Thank you!

Visually similar, but the 73/74 years Yamaha made a lot of changes. If you can record a video, upload it to youtube and post here and people can give you some feedback on what the sound is probably from. Top end is no big deal to fix compared to a crank issue. To my knowledge on these bikes, the only one that was truly just "punched out" on the engine side of things is the AT125/CT175 series.

from the UK Enduro nut.

You're username is very similar to one of our Moderators!

When you refer to "RT360", do you mean the pre 1974 RT1, 2 and 3, or the 1974 DT360A? If you could post the first half dozen characters of the engine number, then that will confirm. Also, the DT360A was fitted with CDI as opposed to points.

Yes as Gr8uncleal says above RT1--2--& 3 were the same except for non reed on RT1 but same otherwise. & DT1--3-- & 3 were the same, without getting into rod length on 250 non reed DT1 then reed on DT2 & DT3. So in 1974 it all changed, total engine on 250/360 plus frame. about the only thing interchangeable through the years is the points magneto or the CDI mag, So-- now the out of the way-- I've swapped cranks both ways on the models before "74. I ran a 360 with a 250 crank just to see how it'd run. Then had [still have] a 250 with a 360 crank. Things to consider are what to do when the piston comes out the top by 6mm when using a 360 [70mm stroke] in a 250 that has a 64mm stroke. Think i put a base plate under that one. But the 360 with a 250 crank i think i used a 322 rod i had which put the piston at the bottom of the ports at bdc then shortened the top of the cylinder by 6mm which then gave it about 180 degrees exhaust duration. It went well but needed lots more compression as had the std 360 head & less stroke to compress it [my theory without really knowing what i'm talking about]. I didn't persevere in the end as just wanted to prove it worked. The 250 long stroker went like hell but just parked it & never bothered anymore but you never know--might get back to it. Somewhere i have a video of the 360 running & can see its low compression as ticks over like a trials bike.

Found a couple of videos. First is with 250 crank & i'm sure the second is back to 360 & can hear the difference in pulses at idle. In saying that the cylinder & piston were stuffed but had new rings. Confusing myself now lol. 250 crank would have worked good if i'd rebored it & modified the head or got an RT1MX head, & could probably have even squeezed the MX head a bit harder too considering its only a short stroke. Close ratio gearbox would be good instead of the overdrive top gear. Needs lower gearing than what it has. Just listened with the headhones on & blimey she rattles--hard to listen to lol.

Maybe a "senior moment" on my part, but I've just re-read your thread title..................!!

Oh bugger, my senior moment too. Was thinking ya dealing with all the earlier models but i now see you want to mix early & later. I'll have another cuppa & a think but think i'll sleep on it.

Thank you everyone for the replies! it is a 1973 RT3 360 motor i also have a somewhat running RT3 360. Sounds to me like im better off keeping 250 parts with 250 parts lol. Ill have to tear it down today this aint my first rodeo with these enduros, i have restored an LT2 100, been into an AT3 125, DT400, ill have to get pictures on here one of these days lol