Topic: Cam question: Journals. 750 K1

[750deepsouth]

I have a cam to fit that has the two centre journals 7 thou undersize. {Dont ask why - I don't know how these came to be like this}

The two end journals are just inside tolerance.

What are the options. I am thinking of grinding the larger journals down to equal the smaller inner journals so all four journals have the same diameter, take several thou - perhaps 5, even 10 thou - off both sides of all four end caps, {that is, caps and the base}  then align drill to fit the cam's journal size [with the required 1 thou clearance]
This should result in a very nearly perfect fit.

Any comments welcome.

Andy 

[MRieck]

 I don't think it is worth the effort and I wouldn't worry about the extra clearance. The hot set up in bike racing is to align hone the heads for extra cam journal clearance- it frees up a little horse power. .001 is very tight. I have found quite a few heads the are not within factory specs either way (to tight or to loose). I'll take to loose any day.

[750deepsouth]

So, you would not be concerned about 7 thou clearance on those middle two journals ? I agree, too loose is better than too tight .. but don't want it actually rattling around in there..at the present clearance I can feel the play..

I guess we are allowed to confess stupidity in these forums. This is the cam for my race bike, I never measured clearances before getting it ground to megacycle 125-05 specs { Duh ! } and having spent $$$ on it I don't want to junk it. But I don't want to use it in its current specs if there is any risk of it making the top end fall apart...

[MRieck]

Well, now it makes sense. Megacycle and Web always polish the journals which removes some material. It doesn't remove .004 to .005 which means that's how the cam came from the factory. Web does grind the hell out of the base circles on hard welded cams which can be a real pain in the ass with shim/bucket setups as you have to go up 4 or 5 shim sizes- sometimes more. Anyway I personally think it will be fine and I know it won't rattle around- I'm sure it didn't when it was in the previous engine. The Web cams in my FJ are that loose with no problems. I can measure the clearance on a cam and towers I have at home tonight and let you know what it comes out at if you like. Sometimes to loose is bad with cams that are pressure fed as you will bled alot of oil and loose some pressure but the CB isn't like that.

[750deepsouth]

If it is no trouble to you, I'd be very grateful for your measurements. It does seem a bit odd that the two outer journals are much closer to the 'normal' size

Thanks,

Andy

Later: Just measured another cam I have. It is the same - the two inner journals are approx 5 thou smaller than the outer journals. I'm starting to wonder if they leave the factory with these measurements, or is this just the way they tend to wear.

[MRieck]

Andy...I couldn't get to it tonight but your PS(later part) speaks volumes.....don't worry!!

[750deepsouth]

It looks more and more likely that inner cam journals are slightly smaller when manufactured.

From Chris, who has expereience with these motors and sells performance valve springs and valve guides on ebay:

______________________________________________________________________________________________________________________
Hello, it seems that the undersize center journals is a normal thing. I don't have any factory cams anymore but I have two Megacycle billet cams that both have .006" smaller center journals and one of them is brand new! I guess you are fine so go ahead and throw that puppy in there. Perhaps Honda wanted heat expansion room as the center of an air cooled four runs hotter that the outer parts plus the fact the center journals are under the greatest stress as the cam is driven from the center. It's funny, even after over 35 years since production began these things are still full of little design suprises. Take care.
Chris

[750deepsouth]

From my mechanic - the final outcome of this discussion on my cam journals...for anyone intersted.


...just to give you an update on my findings with the CB750 head/cam. Bolted the holders onto the head
and measured with an internal micrometer 3 of the 4 weren't too bad.The 4th was fairly oval taking
measurements 45 degrees from vertical andhorizontal, which i feel may be due to some
misalignment between the cap and holder but as they dont have a dowl for more positive location
not much that can be done. Also bolted everything up with the cam in place and measured the working
clearance(vertical) using plastigauge and remembering that the service limit is .21mm, got
.15mm on the 2 outer journals and .23mm on the inners.(Due to design not possible to get the
internal mic into measure on the vertical plane) Going by these figures most of the wear in the cam
holders seems to be more on the horizontal than vertical and since there's really nothing that can
be done to "respec" on that plane I guess we should feel reasonably comfortable to carry on.