Not sure on the pics you’re referring to but if 2.5 is the number yall think is best.  I will jot this down when mine all comes together.  The 3.4 came from the Yami Service Bulletin 235.. 

Cheers Guys.. 

drive.google.com/file/d/1hnZYm7FNdqyyZ3b...SZ/view?usp=drivesdk

Thanks for all the input. If the ports look right when I compare my original and the second set up I bought I will use them as much better condition

Still odd no capacity marked on the barrel/jug

Thanks all

Ian

Not sure what they were smoking in that service bulletin @3.4, but I want some to try lol. 250's are 3.2 with shorter stroke.

Hmm. Thought I had sent this reply once. Seems not.

So removed head and barrel last night. Barrel well and truly stuck on studs. Lots of penetrating spray down the holes and lots of wiggling with gentle persuasion saw it come off.

Heads are exactly the same. Good

Barrel ports exactly the same. Also good.

Differences are my existing barrel has machining of the fins for a decompressor but has a blanking plug and not sure it is fully “finished” for fitting one?
My replacement does not have this machining and is solid fins. So the replacement would seem to be an earlier barrel. Also good for my 1970 bike.

The existing barrel has the CC cast in it and the earlier replacement barrel is blank All considered not sure what that means, if anything.

Existing piston crown is stamped 96?

New pictures attached to show these points.

Replacement parts will be put back on after a good clean of all faces and the rusty studs.

Thanks for all input on this. Appreciate your time.

Ian

New photos attached link. share.icloud.com/photos/050TGtjs2C5pswO0_W_aQtdwg

I can see some sort of outline where the 351cc should be cast - any chance that it has been ground off for some reason? 

Hi Al,

That area does seem a slightly different shape to my original and slightly more proud if anything, so maybe built up? Though have no idea why anyone would do that unless someone fitted it to a 250 and tried to fool the mot tester! Not likely and probably wouldn’t fit a 250 anyway.

I guess it will remain a mystery

cheers, ian

I finally was able to check out the spare cylinders i have and they all have the decompression hole. I would say somebody ground off the 351cc on the replacement cylinder. I can somewhat see the 3.
Im thinking early 1970 360s had the mechanism on the flywheel and no decompression hole. 1971 im thinking they decided to change it to having decompression and no flywheel mechanism, Perhaps even halfway through the 1970 production year they decided to make that change. My dad had a 70 model and he said he never remembers it having a decompression release but that was many years ago for him.

It's not unheard of to remove the displacement or "doctor" it to show something different.   Somewhere on this site are photos of an AT1 (125) that someone swapped to a 175 and skillfully changed the "7" into a "2" so the 175 cylinder is now marked 125.  

Could be done for a legal (illegal?) purpose like some areas have different classes of motorcycle rider's license depending on displacement.  But probably was just done for "bragging rights".  A grinder and a new decal or two and you can have the strongest 250 in the neighborhood!  LOL

And great news!  The "96" generally means you have a standard bore!  

(P.S. the whole engine will fit a 250 frame, you are correct that you can't swap cylinders and make a 250 into a 360.  But with some genius creative work you can make an RT325 out of the two!)

Great info chaps.

Yes I suspect some skulduggery about engine displacement at some point Mark.

The standard bore cylinder with the 96 piston is in very poor condition and probably needs 2 sizes over rebore. The fins are poor too, but may be worth hanging on to for me to look at some point in the future.

I like the replacement item as the fins are complete and no decompressor as it was when the bikes were first made. The mechanical system to aid starting looks fine so that’s all sorted.

Will get this swapped over and then get back to rebuilding the clutch/ primary/tacho and kickstart gears.

So much still to do, but this will be one task ticked off the list.

Thanks again all.

In photo #4 from post #24 where there's a bung in the decomp--which i've done myself--be interesting to know if its fully blocked off or open constantly as comes straight out then straight down to the exhaust outlet so a short bung might not stop it. Then again without looking the thread might start way in so it might be a long bung & that possibility would be covered.