Topic: Rebuild using two engines (K3 and K8)

[crabbyjay]

We're at a point in our rebuild to start putting things back together, using the best parts from two engines.

Is it possible to use a K3 head with a K8 crankcase?

Thanks

Jill

[crabbyjay]

These are both CB750's that we took apart.  Have engines and frames but want to use the best bits from each -- both were seized when we took them apart.

[axehole54]

Someone else please jump in but I believe the oil passages (or something to that effect) don't line up between k3 and k8 but I think there's a way to remedy that... oddly enough I have both of those bikes too

[scondon]

Someone else please jump in but I believe the oil passages (or something to that effect) don't line up between k3 and k8 but I think there's a way to remedy that... oddly enough I have both of those bikes too

It's the 77-78 F model engine that has the oil passage concerns.

K3 head should bolt right on, no problems. Be aware that the the K8 valves, springs, and retainers are different than those on the K3 head. Not sure if you will run into any fitment problems if you try and mix and match these parts. Cam towers, rockers, and cam should be ok to swap between heads.

[crabbyjay]

Thank you so much, that's great information, especially on the rockers.  We managed to keep the valves altogether without any mixing of parts, but we've mixed up the rockers.

[scondon]

Thank you so much, that's great information, especially on the rockers.  We managed to keep the valves altogether without any mixing of parts, but we've mixed up the rockers.

When disassembling it is suggested to keep the rockers, and shafts together as they came apart. If everything is now in a jumble, then using the parts with the least amount of wear is the next best option I suppose.

At some point in their development Honda changed the cam towers and rocker shafts to use two small bolts to keep the shafts from rotating. Do all your rocker shafts have two small threaded holes in them? Through some close inspection of parts you may at least be able to determine which rockers and shafts came from which head.

Also, the cam bearing caps that bolt on the cam towers should have letters stamped on them which correspond to letters stamped on the towers themselves. These caps cannot be mixed and matched between towers.

[madmtnmotors]

Be careful with the clutch case covers. If you are using the later (78') clutch with the earlier (73') case cover you may have an issue. I used an earlier clutch case cover on my 78' 750. The earlier clutch case cover (the aluminum clutch case cover, not the tin outer linkage cover) has a 1/4" more material at one of the outer cover screw bosses that will "Pin" the clutch basket in place to the point that you would swear the engine is siezed.

[Old Scrambler]

According to others on this site........the k8 tranny and clutch are improvements over the k6 and earlier models. The K8 pistons have a slight dome to add about 0.2 compression.  I have no idea why they have a gear indicator on the cover, but if you are using the K3 frame I would use the older cover.

[GammaFlat]

According to others on this site........the k8 tranny and clutch are improvements over the k6 and earlier models. The K8 pistons have a slight dome to add about 0.2 compression.  I have no idea why they have a gear indicator on the cover, but if you are using the K3 frame I would use the older cover.

The K8 output shaft is set up for a 630 sprocket.  You can mount a 530 sprocket to it (same spline) but the attache method is different.  Also, the K8 sprocket is 3/8" more outboard.