Topic: Advice needed on worn cam and tower bearings

[Kickstart]

I'm trying to build a dedicated race bike (as I've currently been racing my street bike), so I've started a rebuild on a K2 engine I purchased a couple years back with the idea using it for racing.  My plan is to still race in AHRMA's production class for another year or two, so I wasn't planning on doing anything major... just new valve guides, springs, and bore out the cylinders to 2nd over.

Unfortunately, the engine has a lot of miles on it (50k) and it looked a little worse inside than I had hoped (everything has black sticky oil residue on it).  The cam looks a little rough... my first thought was I need to look for a replacement, but I noticed most of the cams on eBay didn't really look much different so I decided to mic my cam and see what I was dealing with. 

I took some measurements of the journal and corresponding bearings (on the towers) and found that the cam's outer most journal-to-bearing clearances were within spec (between 0.005" and 0.006") but the inner two were over the recommended wear level of 0.008" (between 0.010" and 0.011"). 

Pics below

Some odd things I noticed (at least to me):

• Even though the cam journals look rough (the one in the picture below is the worst looking of the four) there wasn't much tapper in them, if any (at least not enough that I could reliably measure).  Which makes we wonder how they can look so rough but still not have a measurable tapper.
• The IDs of the four came bearings (in the towers) were all similar.  The larger journal-to-bearing clearance on inner cam journals was due to the journals themselves having a measured OD slightly less than the outside journals.  Which really seems odd to me as I'd expected the softer aluminum to wear much faster than the OD of the cam journals... which leads me to think the cam's inner journals weren't within spec when it was built in the first place.

The rocker shafts, rocker shaft bearings (tower holes), and rocker ID's were all almost like new spec wise (I guess this makes sense being a K2 with the free rotating rocker shafts).

So I'm looking for advice on what to do (keeping in mind I plan on road racing this engine).
What are my options here? 

• Just throw it back together and see how she holds up for the race season?
• Just find a better cam and drop it in, or do I need new towers as well?
• I assume I can re-use the rockers and rocker shafts since they look, and measured, ok (no pictures attached) - or will I be asking for trouble using them on a new cam?

Thanks!
Chris

[Old Scrambler]

Those bearing surfaces look good from here..............I would add an F2 or early K0 cam to your parts list.  Probably don't need springs.

[Kickstart]

Thanks for the feedback... definitely makes me feel better about the tower bearings.

EdwardMorris (here on the forums) saw this thread and graciously offered to sell me a hardly used K2 cam... so I think that's the route I'm going to take.

I've heard from others about dropping in an F cam (but I wasn't sure if that meant an F0 or F2)... Can you drop and F2 in there? I know the valves on the F2 and K7+ were at a different angle so I wasn't sure if the cams were interchangeable.
I'll have to re-read that section in Hondaman's book, I know K0 and earlier were probably the best cam lift wise... I was hoping the K2 was still pretty good.

Thanks,
Chris

[MRieck]

How about a real cam in there?

[Old Scrambler]

All CB750 motors (including F2-3) have the same valve angles.  The cams are interchangeable. The F2-3 have shorter valve-stems.  The K0 cams continued into the early K1s............beyond that Mark's book is the best source. I built my salt-flats bike with an F2 head..........the original cam looks very similar, if not slightly more aggressive, than the K0 cam but I have not measured the difference.

I don't know your racing-class rules, but the Web-Cam 41A is mildly aggressive and is still considered a 'drop-in' application..........meaning it will run with standard springs and retainers.  BTW, the F2-3 springs and retainers are slightly stronger and were also used in the K-8. However, the collet-groves and keepers are different from all other models. 

Check your rules................you may want a more aggressive cam. Its called 'racing' for a reason

[Kickstart]

So... I don't know.  As I recall, the rules aren't that specific about the cam in my class (novice production historic)... and I was thinking about dropping in a Webcam 41 or 41a. I'll have to re-read it.   Now that I have a "core" I don't need, I might just go ahead and get one.

I also know the other guy in my class with a cb750 is running an F cam (which I now assume is an F2), so I was thinking no one would care about having a mild webcam. 

I figured I would play it safe rules wise and stick to what would officially be considered stock for now.  I might change my mind though
The original plan for this engine is something reliable that I can build upon. So I don't mind racing this season with the K2 cam and then maybe upgrading later on.  I'm still at the stage where investing in track days and classes will have a greater return than squeezing out few additional hp's.

Either way, is it OK to change the cam and re-use the rockers and rocker shafts?  I assumed so, but I also remember reading in Hondaman's book that if you replace the cam you should replace everything above it.  Of course, I'm not sure what I would replace it with even if I was willing to spend the money to do so.  I know my rocker shafts and rocker ID's are good, so it seems safe to re-use them.

I guess dropping in a new(er) cam will cause additional wear on the rocker feet and tower bearings... which then brings me back to maybe just going with the webcam, so I don't have to put the rockers and bearings through one more break-in cycle if I upgrade later.  Decisions, decisions... leading to decision paralysis

[70CB750]

Following

Good luck, Chris!

[edwardmorris]

Chris, I've got a webcam-41a as well, I can ship that one instead, better it get some use than collect dust here. I'll wait to hear back  before shipping the K2 cam.