Topic: cb750 K2 & K3 head swapability?

[CBPete]

Hi all,
I've lurked around this forum to try and find an answer, but couldn't, so my apologies in advance if this topic has been covered before.
I'm putting together a cb750 motor from various donor parts from different years and models. I've got most of it sorted out, but I bumped into a bonehead mistake that I made a little while ago that I wasn't aware of until last night. Basically I have K3 or newer cylinders and a K2 head.  In my haste, I didn't bother to check their compatibility and now I'm in the hole for machining work that I had done on the two parts that don't seem like they will fit together properly. The cylinders (K3) have the enlarged cylinder stud holes for the dowels and the rubber collars that should mate to the enlarged holes in the head. But since I didn't check that part out before I went to the machinist, the cylinder stud holes in the head are the smaller K2 variety. My first thought to fix my mess was since I have more money invested into the head, I should keep that part and replace the cylinders with the correct K2 ones and then get those re-bored. That seems like a safe, but costly solution. My second thought is I could take the K2 head back to the machinist and have the cylinder stud holes enlarged on the head and fixed to make a solid fitment to my existing K3 cylinders. Either way, It is going to cost me some more dough, but I'm thinking it may be less to modify the head and keep the cylinders that I have. Has anybody ever done or heard of doing something like that? Any help would be greatly appreciated.

Thanks,

Pete-

[seanbarney41]

...pretty sure you can just leave the dowels and elongated rubber seals out altogether

[Old Scrambler]

You can leave the dowels out but risk an eventual oil leak......especially if the motor is not properly retorqued after a heat-cycle.

I would cut the dowels to fit just proud of the cylinder deck to hold the o-ring in place but short of contacting the head. Be sure to use the correct head gasket.

You will like the K2 head with the floating rocker pins and extra fin supports.

[CBPete]

Thanks for the replies. Where are the extra fin supports located on the K2 head? I've never heard of that and now I'm curious

[Old Scrambler]

There are more 'posts' between the fins through early K3 than later models. When you look directly at the end of the heads you will see the difference.  Also note the shape of the fins on the exhaust side......yours will have a curvey step near the #1 and #4 ports......later units just have a slanted fin design....no step or notch.

[CBPete]

Thanks for the info. You learn something everyday!