Topic: CB750 Cam Tower Journal sizing...

[garyS-NJ]

I had a top end failure  in my '78 cb750F3 (not sure but there was a loose cam journal nut that got under the cam for a bit and then stripped cam tower bolts and loose studs & broke a tower.  wiped the cam and rockers...).  So I bought a pair of cam towers with clamps but cleaned them so good the cap match markings washed off.  BIG question here, we know Honda made the inner journals 0.006" smaller and I assumed the journal caps and bases were machined/matched to the small and large radius' but now I'm thinking no because that would command part numbering as a right and left tower... but they are the same exact part.  so wtf, Honda designs a nominal tolerance around 0.005" (my cam journals are nominal and caps look factory fresh and I'm seeing .005-.007" clearances) and then makes the inner cam journals 0.006 smaller than the outers, and then makes the journal clearance service limit 0.008".   comments??

[Jerry Rxman Griffin aka MuthaF'er]

Gary,

Not sure where you got your numbers (??) but I'm looking at my Honda brand Manual. here are some numbers that may help.

Camshaft:

Camshaft bearing diameter standard value 0.8669" - 0.8678"   serviceable limit 0.8701"
                                                                     22.020 mm - 22.041mm                    22.00mm

There is no distinction between center or outer camshaft BEARING diameter but there is a difference for the CamSHAFT

Camshaft:

Shaft CENTER diameter                               0.8578" - 0.8587"    serviceable limit 0.8559"
                                                                              21.789mm - 21.8190mm                    21.74mm

Shaft END diameter                                     0.8637" - 0.8646"                              0.8628"   
                                                                              21.939mm - 21.960mm                      21.89mm

From these specs one might ?? ascertain that the holders are line bored equivalent and the cam dimensions creating the differences in clearances
 

[PeWe]

Maybe you have other marks scrathes indicating where the caps have been mounted before? They can only be oriented in one way to fit.
My set from a 1978 are marked 34 on both bearings on one cam holder, the others are marked 20 on both. ink marks faced on the more narrow side of the towers, both rocker arms closer to each other. BUT!!  both caps have same numbers but are marked L vs R with thin mechanical stamp. These R, L should be visible on the caps that can only be oriented one way.

This might help you to place yours right if your towers and caps are like mine 1978 set I recently purchased.

[754]

I told how to fix them in your other thread.

[garyS-NJ]

thanks Jerry, I guess my point was with nominal high side sizing on the journal bore of 0.868" and nominal low side center cam bearing at 0.858" we're already at 0.010" clearance (over the 0.008" max clearance spec and add another 0.002 if the bearing bore is at 0.870" service limit.) for the inner journals.  I swapped the L caps and then swapped towers and then swapped L caps and ended up with 0.005" clearance on the outers and 0.009+ (almost .010" with the plasti-guage?) on the inners.  My cam measured 0.864" on the outers and 0.858 on the inners.  thought I was home free but when I cleaned it up and oiled to make sure it was spinning OK I found it binding and could only fix by swapping the L caps again.  I got 1-4 at clearance .007", .0095, .0085, .0055  (I'm marking these puppies now and calling it done.

And PeWe, I couldn't see any scratches going across Base to Cap to link them and my ink markings were washed off.  And 754, that is a nice idea but I got close and don't want to take a chance on making things worse... 
thanks all!