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Tech III, Part I: Case Repair PDF Print E-mail
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Saturday, 05 April 2008

Tech III, Part I: Case repair

What follows can apply to nearly any BMW case. The success of these kinds of repairs depends on a number of things such as the quality of the metal in the case, the severity of the damage, the metallurgical knowledge of those implementing the repairs, etc.

This particular bike had been wailed upon by someone bent on venting a great deal of anger. There was evidence of hammer indentations indicating that a ball peen hammer weighing approximately one pound was used.  Why someone would do this is left to speculation. I do not have much in the way of a history of the bike, other than that it was built in March, 1953, did not sell in Germany, was subsequently sent back to the factory to be updated to later specs with full alloy hubs and Weinmann alloy rims, etc. The bike arrived at Butler and Smith’s in April of 1954. I found it sitting in a shed in pieces near Waterford, Michigan.

My reasons for saving the case were manifold. The serial numbers matched, it was an R68, I enjoy these kinds of challenges, I am seriously mentally challenged...pick the reasons you like.

Jeff Borer handed me a severely damaged R69S case that had the needed parts for the R68 case.

I have some friends up in Toledo at Diversified Welding who are mighty good at what they do. These are the guys who advised that the case be pre-heated before welding. Molten aluminum shrinks about eight percent during cooling. Pre-heating the case reduces the difference between the shrinkage of the welds and the stability of the case. The parts are still going to move, and allowances need to be made when fitting the replacement pieces.

Once the case was welded up, those welds needed to be trimmed off. I used air tools, chisels, files, and a Dremel tool with an assortment of carbide burrs. Under some of those welds were gas cavities, which needed to be drilled out and rewelded. These gas cavities occur from impurities found in the metal. The alloys the factory used varied somewhat over the years. Silicone is present in these alloys to reduce the part’s tendency to expand and contract from heating up and cooling off. Silicone also makes the alloy porous. Porous engine cases are full of minute pockets where oil can seep into and cause problems in these kinds of repairs. I’ve seen older engines that were so porous that oil would seep right through the case, even though there were no cracks.

Once the case was cleaned up, it went into the blast cabinet and was sprayed with .035” diameter steel shot. The pattern this shot leaves on the surface of the metal appears similar to the original sand casting. Following the steel shot, it gets sprayed with #11 glass beads.

After this kind of treatment, the case requires a thorough cleaning, including running pipe cleaners (available at your local tobacconist shop) with solvent through all the oil passages, then blown out with high pressure air, followed by a second and third cleaning. I’ve been told that glass beads have a way of imbedding themselves into the metal surface and are very difficult to remove, yet manage to find a way into the inside of the engine to cause damage later on. (After I get the engine running for a few minutes, I drain the oil and pull the pan and examine for evidence of glass bead residue). If you prefer not to use glass beads on your cases, then don’t. There are alternative ways to clean these cases. One of those ways involves the use of coarse sandpaper and a hammer.

  

The engine has since been assembled. During all the heating and cooling, the forward camshaft opening dropped by about .004”, causing the timing gears to be too tight. I carefully ‘dressed’ the upper part of where the forward camshaft bearing carrier fits, applied ‘JB Weld’ to the lower insides, and inserted the camshaft/bearing carrier assembly and tightened everything down. I used a thin strip of paper between the timing gears as a temporary shim until the epoxy hardened.

The engine runs quietly. The amount of time and money spent on the repair is much less than I had thought it would be. I have about fourteen hours in repairing the case plus fifty dollars for the welders.

RS  
Last Updated ( Friday, 03 October 2008 )
 
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