A friend of mine named Bud Provin- a name not unknown to some of you who ride Airhead BMW motorcycles- has an oldtime motorcycle repair garage in Vermont. He works on a mix of mostly older "Airhead" BMW motorcycles and some of the old British motorcycles. A few days ago, Bud called to pick my brain about a problem he ran into. Bud is rebuilding the 5 speed transmission on a customer's BMW Airhead bike. Fifth gear has clutch dogs on one face, and otherwise free-spins on the output shaft of the transmission. There is no bushing or bearing in the hub of this gear. It is a helical gear, none too wide. The problem Bud encountered is the bore of the gear and the portion of the output shaft (journal) it turns on have become worn with some scoring. The problem of lubrication may have been a factor in that the shaft the gear is on has a spline immediately next to the journal. My belief is any oil film is probably none too good and prone to collapse via the grooves in the spline. However, there is no real load on this gear when the dogs on it are disengaged and the shaft spins free in it.
The problem is these motorcycles are around 40 years old. Some new parts are available, but a new shaft and new gear would be around 800-900 bucks. Add the cost of bearings and seals to go thru the transmission and labor, and the bill to put the transmission to rights would be up around $2,000.00.
Bud called me to ask about salvaging the fifth gear and output shaft. We kicked a few ideas around. One, which seemed the easiest and least costly, is to send the shaft out to have the journal ground under and hard chrome plated, then finish ground to slightly larger than original diameter. Bud has a good assortment of Sunnen Hones and a honing machine, so he could then re-size the bore of the gear to run on the hard-chrome plated journal.
I have no experience with hard chrome plating other than on journals which had spun in roller bearings. My own concern is the hard chrome plating might not hold up as a "running journal". My other question is how thick a deposit of hard chrome plating is needed to get a good solid journal with an allowance for finish grinding.
Bud is going to get back to me as to whether the gear is through-hardened or hard all over (surface hardened, but same difference from my perspective). Bud asked my thoughts on putting a bushing into the hub of the gear. My first thought was that if the gear is hardened, there is no boring it out for a bushing. If the gear hub is not hardened (which I think is unlikely), I had thought of boring and shrinking in a hard bronze bushing, something like an aluminum bronze and possibly pinning it. The ideal bushing would be a "floating bushing" as is used on heavier gears is this same transmission, but this gear hub design may not lend itself to that.
The bushing idea struck me as having more potential problems than the hard chrome plating of the journal/honing the bore of the hub to suit. I pretty much ruled it out at this point.
The easier and surer fix, at least to my way of thinking, is the hard chrome plating of the journal. Having no experience with hard chroming a running journal, let alone in a hard-steel on hard-steel application, I would appreciate any insights members of this board could give us.
The shaft journal is not all that large, perhaps 11/16" diameter, and the shaft can go in a USPS flat rate box, so it is not a big job. If anyone can recommend a good hard chrome plating shop who also does the precision grinding, please let us know as well.
Thanking you-
Joe Michaels
If you are looking for more details, kindly visit Hard Chrome Plating Equipment.
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