Topic: CB750 F2 engine questions

[KJ790]

Hey guys, let me start by saying this site rocks! I've learned a ton about these bikes from here so far. I just picked up a 1978 CB750 F2 with 27,000 miles and I've done some searching, but I still have some questions.

I've heard about the F2's valve guide issues. how big of an issue is this really? I like the bigger valves of the F2 head, but if the bike isn't going to be reliable then I will just buy a K model head and cylinders and go from there. The bike runs good, but looking in the exhaust port I see I have one valve seal leaking so I'm trying to decide what to do. I'm into head porting so I was gonna work the head a little while it was off, but don't want to invest the time if this head is just going to be problems down the road. Is the F2 head worth refurbishing or should I just put the money towards putting a K model top end on my F2 bottom end?

By the time I pull the motor out and pull the head, I might as well refresh the pistons and rings. I was thinking an 836 kit, but again the F2 head causes problems. I know the larger combustion chamber of the F2 head means lower compression if I use the normal 836 kits. How much would I have to deck the head to get the compression back up if I choose to go this route. Is this all worth it for the gains?

I really don't know what to do with this engine. My options now are

1. Refurbish this head with new sales, guides, springs, and valves if needed, port the head, and leave the rest of the engine as is.

2. Same as option 1 but refresh the pistons and rings with stock sized parts.

3. 836 kit with the refurbished, ported, and decked F2 head.

4. 836 kit with K model cylinders and head on F2 bottom end.

Which of these options would be best bang for my buck with a decently reliable engine? Thanks.

[brandEn]

Welcome to the site! Just as an FYI the 1977 CB750 SS is an F2 and the 1978 like you and I have are an F3. They are the same bike however. Do not confuse those with the 1975 F0 and 1976 F1 as they are quite a bit different in many ways. When I rebuilt my motor I had the very same concerns as you. I learned that the F2-F3 heads have head valve guide wear due to the valve retainers being made of cheap stamp steel. If you rebuild your head with new guides, springs, valves and new retainers you will be good to go. I went with all Cycle X parts and the cheaper hard anodized alloy retainers rather than the more expensive titanium ones. I also threw in a new set of Wiseco 836 pistons for good measure. Finding new piston rings for stock pistons on our bike can be a challenge but not impossible. A few have actually had new ones made. If you want contact member Wohali, she had a group buy for these not long ago. I also think Hondaman has said that you can use the K pistons without much loss of compression but don't quote me on that. Hope this helps!

[jtb]

Someone's making an 836 kit for the 77-78 F's now, I think.  Search the forum.

[KJ790]

Welcome to the site! Just as an FYI the 1977 CB750 SS is an F2 and the 1978 like you and I have are an F3. They are the same bike however. Do not confuse those with the 1975 F0 and 1976 F1 as they are quite a bit different in many ways. When I rebuilt my motor I had the very same concerns as you. I learned that the F2-F3 heads have head valve guide wear due to the valve retainers being made of cheap stamp steel. If you rebuild your head with new guides, springs, valves and new retainers you will be good to go. I went with all Cycle X parts and the cheaper hard anodized alloy retainers rather than the more expensive titanium ones. I also threw in a new set of Wiseco 836 pistons for good measure. Finding new piston rings for stock pistons on our bike can be a challenge but not impossible. A few have actually had new ones made. If you want contact member Wohali, she had a group buy for these not long ago. I also think Hondaman has said that you can use the K pistons without much loss of compression but don't quote me on that. Hope this helps!

Great, thanks for the information! What did you use for a head gasket when you went to the 836 pistons? I've heard that the F2/3 engines use a different head gasket than the others.

[brandEn]

They do. I used a Cometic head gasket. If you get the head decked Cycle X has a MLS gasket that would be nice. For headwork forum member Mreick is the go to guy for high end stuff, Hondaman, and Cycle X also do this service.