Topic: CB750k 1978 Cam Lobe Measurement Issues when compared to manual an online number

[Knightro]

Hey all, I have a 1978 "R1" cam that doesn't look like it's been molested in any way. I've just done a top end rebuild. Everything looked really good, cylinders, lots of oil on top of cam, head straight and stock thickness give or take.  The Cam chain had some excessive flex so I put on new Cam Chain with rivet link. And APE studs. New rings, o-rings, gaskets and button this puppy up.

All this being said, as an after thought i decided to check the Cam Lobes before installing as that is the stage i am at.  Because everything else looked so good and being my first rebuild, like an amateur, I left this till the end. I figured with everything looking so good, how could the cam be worn out. That being said, .... I also had to replace my intake valves and exhaust guides. ... This is where i'm at. I was hoping to keep it pretty close to stock though now i'm thinking the CX-1 Cam from Cycle-ex might fix my problems.

In looking online and doing many searches and much reading, i'm having difficulty. The Clymer manual suggests (when taking engine apart and checking specs) the 'all models exept Cb750A' 'the camshaft should be replaced if the inlet cam height is less than 1.411 inch or the exhaust cam height is less that 1.392 inch.'

My numbers (give or take, still figuring out this micrometer) are on average about 1.405 to 1.4091 (depending on if i'm measuring the inside of the lobe (cam gear side) or the outside. One is a larger measurement presumably due to wear.

My lift numbers are around .3013 on average. I get this by subtracting my base circle from overall lobe height.

Now I'm having a hard time determining when/where these online and manual measurements are being taken. Many people talking about measuring 'at the spring or at the rocker'.

I've also read that the valve lobe measurement will never be what my clymer manual suggests it should be, which is also, super odd and confusing. WTF clymer?

So, After i've bought and learned how to use a micrometer, i'm still going to get my machinist buddy to mic the cam... as these numbers are too far off for me to think i'm doing it right.

a. Do these numbers sound like it's time for a new cam (for the price, considering the rest of the engine doesn't have to be upgraded, that cam sounds like i wont mess around too much before I decide to just buy it).
b. Am I doing this measurement right and is the clymer number right if the cam is out of the engine and you are stright up measuring the lobe? Am I supposed to measure the cam while it's loaded up in the rocker. Like I said, cam is out, clymer says my measurements are too low.
c. If i'm getting an 'aftermarket' cam, do I have to now get an adjustable cam sprocket or is this a basic standard move when trying to get these old beauties running 100%? Will I be able to set timing on new cab without an adjustable sprocket?



Spoke with Cycle Ex they said they can't even remember what it's like to talk stock specs on these and what works and what doesn't... but their cam is great! lol... either way, I'm thinking i'm going to end up buying one of their cams cause i don't want to mess around, and if this OEM cam is shot, ... well... then it's a no brainer.

Thoughts, suggestions?

Thanks all,

This forum is pretty much my favorite thing on the web... You guys are great!

[scottly]

I've got a K7 cam in decent condition I can measure.

[PeWe]

I bought a good low mile CB750 K7 -78 cam, it had slightly less lift than my old CB750 K6-76 cam. That K7 cam sit in my K2 project, just bolted in.
Measured direct on cam. Not that exact as measure in engine when timing and open/close can be measured too.
 [mm]
              base circle mm    IN lobe+base     IN lobe   Lift mm   inch     EX lobe+base         EX lobe     Lift mm      inch
K6 original           14,05         35,95                 21,9      7,85     0,309"        35,3                   21,25   7,2          0,283"

1978 original      14,05           35,7                  21,65       7,6     0,299"        35,25            21,2      7,15          0,281"

I cannot resist to add something to compare with
RC295 marked:  13,2                  35,8                  22,6       9,4     0,370"        35,25            22,05   8,85          0,348"

"RC295" when timed @ 0 lash  1mm (.004") lift:    Lift  9,930 (0.391")/9,180 (0.361")    Duration: 259/257   IN 25/54   EX 56/21
 With running lash .004"/.006") and 1mm (.004") lift      9,700 (.382")/9,000 (.354")                  254,5/251      22,5/52       53/18
Running lash is what counts.

[scottly]

I took the measurements at the centers of the lobes:
Intake height 1.4065", base circle 1.1055", lobe lift (not to be confused with valve lift) .301
Exhaust height 1.3905", base circle 1.1065", lobe lift .284"

[Knightro]

Thanks for the #'s guys to compare. It's helped, and i'm ... still unsure, but that is mainly cause i have to watch the $$ on this build and i also wanted to button it up this week, but i want it right.

After going to the machinist he gave me:
in            EX
1.  1.406   1.390
2.  1.405   1.389
3.  1.406   1.390
4.  1.406   1.390

With an average base of between 1.105-1.107 (which is what i was getting as well give or take.)

Lift #'s
     in      ex
1.  .300   .284
2.  .299   .283
3.  .300   .284
4.  .300   .284

Clymer manual says cam should be replaced if the lift is less than .312. (1.411 in and 1.099 out min)

So far i have seen no one with stock numbers anywhere near these and the machine runs well according to the reports. So what is clymer smoking? I understand these after market manuals take 'learned Technique' into the writing of them and don't just follow the original honda manuals. Could this be clymer just trying to get across that you will like the cb750 more if you get a hotter cam? lol

The other thing is:
My #2 has the worst numbers both when my machinist measured and when I measured.  What would cause one valve/lob/spring/guide wear our on one particular piston more than others? is that all tappet? or is there one of the variable that is out or not working well in that one piston/valve train that causes wear on the whole unit more than the rest? 

Finally, are these #'s ok? is this cam at the end of it's functional or enjoyable life?

[seanbarney41]

Maybe post a few pics of your cam and rockers...if your bearing journals are good, lobes not pitted or galled, and hopefully not worn through the factory hardening...run it.

[Jerry Rxman Griffin aka MuthaF'er]

Your numbers are in line with Scott's. My Clymers says the same thing.

My original FO cam with ~ 6000 miles measures 35.7mm/1.41" and 35.4mm/1.38"

Perhaps Mr Clymer and Mr Honda service manual was measuring a new K0 hot cam?!

Use it       

[HondaMan]

First of all, don't believe the Clymer/Chilton numbers, and you will save yourself a lot of grief and angst. 

RXman has it right: the only production cams that ever came close to the published 1.415" overall height were found in the sandcast K0 engines. Beginning in the diecast K0 they dropped to less than 1.410 brand-new, and over the years until the F0 engine they became even less. The duration was made to open the intake valves later in the F0 engines (0 degrees instead of 5 degrees BTC), to improve scavenge for emissions reduction reasons, and this pushed the powerband RPM higher.

If the cam has at least 1.350" overall height, it is fine: they start to lose top end if this drops below 1.30" (like mine, at 143,000 miles, 1.29n" intake lobes, 108 MPH top speed instead of 121 like it used to be). They were Parkerized when new, so they immediately wore off 0.002"-0.004" of whatever height they started with while they mated to their rockers, entirely normal.

[Knightro]

Here are some pictures and a quick video of the lobes on the cam. They all look like this. They are pretty well polished. No pitting really. the rockers are smooth as well. No issues that I can see there either.

At this point I feel like I should have just ordered the other cam and have it 1/2 paid for already. It would be in the mail. Argg... when will I learn. lol

Thanks for the advice guys. feeling like i might regret not heating up the cam on this bike.

[Jerry Rxman Griffin aka MuthaF'er]

To me it's kinda difficult to open up my 750 engine and not make it a little better before reassembly. A cam is THE easiest way to do so and probably the cheapest too. IF you do so you should consider stronger valve springs depending on how much cam you put in. I missed a hard shift and floated a valve with my stock F cam and springs and destroyed my engine back in '76.

[HondaMan]

At the risk of calling on the popcorn vendors...
...I usually try to envision (or have the rider do it) the purpose of the bike before indulging in cams, more than any other part. If you're planning on touring a lot with the bike, skip the cam: if you feel you might need a little more torque for some reason, consider a step-bore (or worst-case, big-bore) instead. The cam always requires more engine RPM to develop significantly more power in this engine, because the cylinders are small. In most respects, it appears to have been designed as a 760cc-ish engine (62mm bore) as regards the structure and carbs, cam timing and a host of other things, and per Honda's 1960s mindset (i.e., earlier engines) was built that way to ensure a long future for it (Sochiro's most famous comment was, "Honda will ALWAYS (in Japanese, 'forever') make parts for their motorcycles"), so the .25mm step-bores, unheard of but for Honda, and the ability to reach well past that if required, was built in from the start.