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Honda cam bearing conversion

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Inspired by drturnip’s article on his low-cost alternative to the Honda single cylinder cam bearing problem, here http://www.thumpertalk.com/forum/showthread.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.


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).



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.



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.



I've done about 500 miles on it so far but all good.

Drturnip’s article has got me thinking again that it may be possible to make a new points holder housing with a deeper bearing / seal socket for the same effect but a lot narrower. The limitation is on the 15mm spigot on the camshaft which is only 10mm long.

Total cost about £30 / $50 and of course, about a week later a NOS camshaft turned up on ebay for £45 /$75!!

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You over killed that,its a lot simpler to put the bearing in stock backing plate.I beleve I was 1 of the first to do in the early 70s.Some were out side bearings and some in and outsde.

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Cool pictures. That setup should last a long time. Was it a pain to assemble everything? Does the bearing seem to get enough oil? I also find it strange that the camshaft would wear before the aluminum head. Maybe Honda knew what they were doing after-all. Imagine that. :thumbsup:

Edited by drturnip

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I made up plug go/no go gauges to measure the head, so they are accurate sizes. The engine's got 38000 miles on it.

Assembly was easy, it was the measuring-up that was a pain. You can see from the imprints how close the points cover thrust face is to the bearing bush here


It's not shown on the drawing but on the inner bush there's a hole to allow the oil coming through the camshaft to spray on the bearing, seen below. To be honest there's probably enough oil thrashing around in there anyway, roller bearings don't normally need pressure fed unless they're highly loaded like thrust bearings.


I subsequently bored a drain hole for the oil which gets trapped between the 2 parts, you can see it on the first set of pictures in the original post.

You can just see the O ring groove in the outer bearing bush here


This is it all assembled on the bench, you can see it's all quite close to the sprocket


I'm trying to figure out a way of doing it using the points cover but with the original seal in place to stop any oil seepage.

As for overkill, as I said, I'm a marine engineer. If you've ever stood inside a liner with a 900mm bore and 2 metre-odd stroke grinding off the piston ring ridge with a 5" angle grinder you'll understand!!!

Edited by craig10

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This problem would not have happing if you had been change your oil evey 1000 miles and ocen year or if put 3000 miles on it then you need claen out the oil pump and centrifigal oil filter rotor as well as flush out the crankcase crank case, clutch cover and the stator cover.

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