Topic: Currently rebuilding a CB750K8, need a sanity check regarding piston rings

[myersn024]

So I'm currently rebuilding a CB750K8 engine. A few of the pistons that came out of the bike have scored skirts, and the cylinders those pistons came out of were too damaged to be revived with a dingleberry hone. I've bought a set of jugs and pistons from eBay that were supposedly out of a 77 K model. However, I found out after making the purchase that there are two different pistons stamped 392. The pistons that came out of my engine look like this.


The ones I bought look like this


Based off my research, these are more likely out of an early 76 F model and will work with the K8 head since they're the same between the earlier F models and the later K's. I have a set of rings that I ordered, but from what I'm reading the K8 rings won't work with the other "392" pistons.

Assuming my research is accurate, the rings I need should be these, right? https://www.ebay.com/itm/CB750-Piston-Rings-STD-4-Sets-Honda-69-76-CB750K-75-76-CB750F-Do-4-pistons/201521336507?hash=item2eeb9b90bb:g:CFAAAOSwgvFbOQ4N

[PeWe]

Or new pistons with rings?
Need a fresh tight bore.
Total clearance max 0.025mm
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

[Jerry Rxman Griffin aka MuthaF'er]

The earlier rings are 1.5mm, 1.5mm, 2.5mm. I BELIEVE the later rings are 1.25mm, 1.25mm, 2.5mm. There is a difference however the 392 pistons and to my knowledge it is basically just the dome. You should be able to tell from your engine serial number and a parts book.

The 77/78 K used and is based on basically the 75/76 F  xx-392 engine. You just found one difference. Another is the 78 K used the different pistons I BELIEVE as well as different valves and keepers with stronger springs. Cam timing is a little different too. The heads are identical between the F's and the K's of 75/76F and 77/78K. Your new pistons will bump compression slightly and will work just fine. Measure the ring lands to be sure. 1.25mm vs 1.5mm. The xx-300 rings are what you want with the 1.5mm lands

[myersn024]

Or new pistons with rings?
Need a fresh tight bore.
Total clearance max 0.025mm
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

Yeah, I’ve been trying to do this without having to take anything to be machined. Was it the right choice, probably not but I’m already eyeball deep and I just want it back together. Will it bite me? Time will tell. If I had it all to do over again, I would have bought a running bike instead of a basket case.

The earlier rings are 1.5mm, 1.5mm, 2.5mm. I BELIEVE the later rings are 1.25mm, 1.25mm, 2.5mm. There is a difference however the 392 pistons and to my knowledge it is basically just the dome. You should be able to tell from your engine serial number and a parts book.

The 77/78 K used and is based on basically the 75/76 F  xx-392 engine. You just found one difference. Another is the 78 K used the different pistons I BELIEVE as well as different valves and keepers with stronger springs. Cam timing is a little different too. The heads are identical between the F's and the K's of 75/76F and 77/78K. Your new pistons will bump compression slightly and will work just fine. Measure the ring lands to be sure. 1.25mm vs 1.5mm. The xx-300 rings are what you want with the 1.5mm lands

Thanks for the info. I already have the xx-392 set, so I’ll pick up a set of xx-300 rings. One of them will work, the other I can return or resell.

[Jerry Rxman Griffin aka MuthaF'er]

Double check the ring land width of your pistons first

[myersn024]

Will do. Hopefully I'll be starting reassembly later this week.