Saturday, 3 August 2013

HONDA CAM BEARING CONVERSION

HONDA CAM BEARING CONVERSION

Inspired by drturnip’s article on his low-cost alternative to the Honda single cylinder cam bearing problem, here http://www.thumperta...ad.php?t=819229, this is the story of my solution to the same problem, done before reading drt’s excellent article.

I acquired an early CB125 motor which would run ok at lower revs but was stuttering once the revs got up.  As opposed to the usual problem of the head bearings being burnt out, these were measured using manufactured go/no go plug gauges at 20.01 / 30.01.

The problem was traced to wear on the camshaft journals, 19.82 at the small end, 29.88 at the bigger end as opposed to 19.93/29.93 measured as new.  This was causing an end float of 0.3mm at the points cam, hence the running problems.

As NOS camshafts were coming in at about £75 / $120, I set about re-engineering the camshaft bearings.

Judging by the bearings that I saw on other camshafts, not knowing the loadings, and being a marine engineer where overkill is good, I didn't think that a thin bearing would be able to take the load.  Thus, I made an outrigger bearing holder with circlip for a 6004 bearing (20/42/12) to go between the head and points housing. 

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Once the alloy bearing holder was turned up (0.04mm undersize to allow for expansion) and fitted to check the axial clearance to the cam sprocket bolts, I turned up spigoted bushes for either side of the bearing to take the load onto the 15mm spigot and the 10mm camshaft points extension.  The outer of these 2 bushes was grooved on the internal bore for an O-ring to stop oil creeping along the shaft into the points housing (groove in yellow part). 

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The outer face of this bush acts as the running face for the points housing seal.  The bearing holder spigot into the head and points housing to bearing holder are sealed with the normal O-ring. The camshaft small end was left ‘as found’ as I didn’t want to machine a good head.

An extension was then made and turned concentric to the camshaft to take the advance retard unit, then drilled to take the ‘knock pin’ which locates it.

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A lot of work was involved measuring up to get the whole lot as narrow as possible (it's about 23mm wide), to seal the shaft where it comes through the bearing and to get the correct axial location of the camshaft once all tightened up. The whole lot is secured from moving by the circlip in one direction and the advance retard securing bolt in the other direction.

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