Cylinder differences - early, late, diecast

[markb]

I apologize if this has been covered somewhere but a search didn't turn up an answer.  I'm still debating whether to try to save the cylinder on #97 or replace it.  Are there any known differences between early and later cylinders?  Did the extra front hole start with diecast motors?  I have a lead on a cylinder off an early diecast motor that the owner claims does not have the extra front hole.  Possible?  That's one option.  Another is "borrow" the cylinder off the engine in 1553 (E1789) and repair 97's later.  Or repair 97 now which means repairing the fin and probably resleeving.  Anyway, the main question is, are there any differences?
Thanks,
Mark

[kp]

Mark, I have very very very unsure recollection that maybe the earlier cylinders had more of the small ivory rubber/plastic spacers than later engines without the extra bolt. I can take a set of comprehensive photographs of the cylinders I have but I cannot do this for a few weeks as I'm about to travel again. Maybe Mark and others could take some shots for you but be sure to check this area. I could be incorrect with this though KP

[markb]

After checking early (April, 1969) and later (CB750-K2) manuals, it looks like the number of rubber inserts was 22 up to diecast VIN 26143 and then it changed to 21.  The early cylinder number is 12100-300-020 and is listed as 12100-300-040 in my later parts manual.  Anyone know what the cutoff serial numbers are on these changes?  Or what the differences are?
Thanks

[hondasan]

According to my parts list, the 020 cylinder was fitted up to #1026143, with the 030 being fitted from #1026144 through to 2304500 (late K2 / K3?), followed by the 040 from 2304501 on (not going to bother with the later changes).
I guess the important thing is that it implies that ALL sandcast bikes used the same cylinder, unless someone observes differently.
The major change at 1026144 was the addition M6 x 90 bolt at the front of the cam chain tunnel. Also the mentioned change in number of rubber inserts.
The parts list is not actually correct from this point on as it suggests the 030 cylinder ran through the remainder of the K0, all the K1, and into the K2 models. The 030 barrel was in fact still "sandcast", but became "die-cast" mid way through the K1 model run, a change for which there is no parts list part number change. Another instance of the parts list being a useful, but not always accurate, guide.

Cheers - Chris R

[markb]

Thanks Chris.  That's exactly the info I needed.  I know there are opinions that one should use every original part on a restoration but with the broken fin and overbored cylinders I'm thinking replacement might be the way to go on this one.  If/when I pick up that other cylinder I plan on bringing the other one along to make sure it's an exact match.

[Wayne]

Mark, I have very very very unsure recollection that maybe the earlier cylinders had more of the small ivory rubber/plastic spacers than later engines without the extra bolt. I can take a set of comprehensive photographs of the cylinders I have but I cannot do this for a few weeks as I'm about to travel again. Maybe Mark and others could take some shots for you but be sure to check this area. I could be incorrect with this though KP

I just checked 576. Has 22 Ivory coloured spacers. Any replacements I have found are black. Any tricks to getting these things out so I can sandblast and paint my cylinders? Don't want to screw them up if they can be re-used and kept original.

[Steve Swan]

i use a thin piece of delrin and simply push them out.  i priced them a couple years ago from Honda and they were insanely expensive.  Like around, if i recall correctly, $70 for a set (sold individually.)  i imagine they would clean up in an ultrasonic.

[kmb69]

Steve is correct but I would use an egg crate or similar to keep record of which position they come out of so that you can put them back accordingly. I have found that over time, the engine heat tends to "mold" or "form" the inserts to the particular shape where they have been sitting for 40 plus years. Sometimes there can be enough difference in width between the fins or space they came out of that a thin insert can easily fall out of a wider fin spacing. New inserts are supple and are easily placed - the old brittle and heat formed ones require a little extra attention.