Revealing some ugly truths about my bikes past as I get into the strip down.
Long and short is my crank case (shifter side) is terminal - cracked !  I have found a replacement (I think) but on the inside face cast into the shell is ISO. My original has no lettering cast on the inside face.
I recall reading about this somewhere on here and know it is the thread type but my question is - what would my original be - and would the ISO marked replacement work?
Any help/advice appreciated.  

Hopefully one of the 68 experts will chime in...  I'm pretty sure 68-70 was the same cover going by the parts list.  Later covers might fit but could have had a different finish and Yamaha changed to recessed letters later on.

The ISO marking was something Yamaha seemed kind of proud of...  metric fasteners sometimes varied in size and thread pitch back then, especially the smaller screws...  kind of like sometimes buying something from China these days that has some oddball size fasteners holding it together.  The ISO marking meant the threads were "standardized". 

It doesn't necessarily mean any threads in the pre-ISO marked cover were different...  they probably aren't.

The only screws fixed into that sidecover would be the oil pump mounting screws ,
and the oil pump cover .
All the case screws only pass through to centre cases , so they will be unchanged .
Looking at my 68 parts list , it appears the oil pump mount screws were isometric ,
so your existing screws should be fine . The two oil pump cover screws were originally
not isometric , so these screws will need replacing with the updated isometric option .

I found an old Yamaha document... It says the change was to ISO (International Standards Organization) for all threads in mid 1968.  Yamaha sold a new screw assortment to dealers that contained small diameter screws (all under 6mm) and a few 12mm nuts to service the new ISO thread bikes.

My 1964 MG1T has a couple of goofy (and unobtainable) 5mm screws that have an obsolete thread pitch...  the ones I remember are the stator mounting screws that are 5mm x  0.7 pitch and everything these days is 5mm x 0.8 pitch.  I think some of the carb screws are oddball too and different from more modern Mikuni screw threads.

I think oil pump mounting screws are 5mm.  But Swoop says those are shown in the early 68 parts book as ISO so that's great!  It means they won't be the now unobtainable 5mm screws and will be the same as later models...  and you're switching to a later cover which would have the ISO thread pitch that's commonly available anyway.

Oil pump cover screws are 6mm and the pre-ISO 6mm case screws Yamaha used were all 6mm x 1.0 pitch (same as ISO) as far as I know so you shouldn't have any issue at all. 




 

In my experience, if a 1968 Yamaha part does NOT say that it is ISO, any threads in it will NOT be ISO.

My experience is that if a non-ISO case  has 5mm threads in it, they will be 0.90 thread pitch... NOT 0.80 thread pitch.



Just my experience....

So if the original cover has non-ISO oil pump screws as DEET says it will in his experience...  which makes the parts book Swoop has wrong (which doesn't surprise me about Yamaha literature)...  it really shouldn't be a problem...  in fact it's a benefit. 

If you were swapping to a non-ISO cover those pump screws might be impossible to find.  But since you are swapping to an ISO cover, the pump mount screws will be commonly found...  still available from Yamaha.  Just keep that in mind when swapping over to the ISO cover and don't try to thread your original oil pump screws into the ISO cover if the thread seems different....  buy new screws to fit the new cover.

I suppose the only drawback is if you ever would enter the bike in a concours event and a judge checked the thread pitch of the oil pump screws you might lose points for the threads not being original pitch. 

Thanks all for your valuable input. I think I will go ahead with the replacement and (eventually) post some progress.
FOR SALE: Anyone interested in an original cracked crankcase - excellent as a plant pot

That brings back memories of the iso change. I worked in the Yam & Honda then & was a pain for a while but soon got used to it. 12 & 13 hex nuts for 8mm bolts instead of 14 nuts if i'm thinking right. & fine threads on screws etc.

Here is a unique use of a discarded side case, a light fixture for the Kick Start cafe in PA. Credit to Donzilla’s many creative skills at Metro Racing.