Aluminum Journal Bearings and Chromed Shafts 2

[CB450EK1]

I am restoring a 1968 Honda DOHC motorcycle that has aluminum journals for the steel camshafts. The journal i.d. is 22mm. I am thinking about hard-chroming the male part of the new camshafts, and polishing the aluminum journal bearing. The original design is problematic and has produced an expensive aftermarket solution which installs needle bearings in the aluminum journals, and a sleeve on the camshaft mating part. Is this combination of hard crome and polished aluminum advisable? Thanks, John Boettger P.Eng.

 

[evelrod]

There is the alternative of machining for bearing inserts that are fairly common in other applications. The aluminum saddles will work, quite well, actually. I only have a problem with this setup from asthetic/longevity points only. As to hard chroming the cam journals---probably a bit of overkill unless you need it to increase/renew the original diameter. Theoretically there is no metal to metal contact so a suitably polished journal should work. If the original surface treatment or "Parkerizing" has been removed then the chrome treatment or re-nitriding the journals is in order. Be certain of the 'line bore', drill the camshaft at each journal(assuming camshaft is gundrilled) for oil, machine an anular grove in each saddle for oil distribution and, hold clearance to 0.001/0.0015" and it should be ok.

Rod

 

[CB450EK1]

Rod, that is very helpful information. The Honda spec is .001 to .0015" clearance as you suggested. I find the fact it curious the steel shaft has damage given the aluminum is much softer. The damage is over about 1/3 of the shaft approximately opposite the cam lobe, i.e. in the thrust area. This is why I wondered about chroming. I will look into hard surfacing. The aluminum saddles are gun-drilled by Honda and I will use new ones of course. After the engine was produced Honda increased the oil pump capacity by 30%, and I have an uprated pump. Also since 1968 the lubricating oils have really improved particularly in boundary lubrication additives and performance.

 

[Metalguy]

CB450EK1,

It's probable that you have a condition where the steel is really riding on alum. oxide, which is far harder than steel. The oxide will be very thin (microns), but it's still extremely hard and tough. But if something manages to wear thru it, the alum. underneath is soft, of course.

 

[CB450EK1]

Thanks Metalguy. This is beginning to make sense to me. Aluminum Oxide is indeed very hard and the conditions of heat and aerated oil would cause the reaction on the aluminum bearings. Back in the 1950s GE introduced aluminum shells for steam turbines that had steel shafts. They devised the first 'chemical lubricants' which had long chain molecules that chemically bonded to the unreacted aluminum when microscopic-level contact produced 'welds' that were subsequently torn away by rotation. The bonded long chain molecules protected the aluminum. Because this - and most - motorcycle engines use wet clutches lubricated by the engine oil there are real limitations to the additives that can be used.

 

[GregLocock]

I like metalguy's theory - but if you have a steel shaft running in nylon bushes often the steel will wear first. Cheers

Greg Locock

 

[strokersix]

When a soft and hard material combination is running together, the harder material often wears more. This can be caused by particulates in the oil partially embedding in the softer material. This leaves the harder material sliding on the embedded particulates and explains the wear seen on the harder material. This is, in fact, the way a copper lap works in a machine shop environment. In the presence of an abrasive slurry a soft copper element can be used to remove material from a harder surface. Automotive bearing inserts have a characteristic called "embedability" which I believe means that the soft surface of the bearing is so soft that any particulates in the oil fully embed in the bearing and don't stick out above the surface to cause wear of the mating part.

 

[patprimmer]

I think strokersix is spot on.

Also. has the head been surface ground.

If the head is a little bit warped, it is not always desirable to machine it flat, as the design and material of the head is not nearly so rigid as the cylinder block and crankcase, so on tightening down the head, it will conform to the deck of the block and straighten. This will pull the original cam bearing bores straight also. If you machine the head, you also need to line bore the cam bearing bores. Regards
pat