Of Brakes and Valves on Classic Motorcycles


| 2/16/2017 12:00:00 AM


Last issue, I mentioned the poor performance of the front drum brake on my 1973 BMW R75/5. That column generated a surprising number of emails on the subject, with BMW owners generally arguing that the major issue isn’t so much that the brakes are a bad design, rather they just need to be set up properly.

I was getting ready to launch into a rebuild, replacing shoes, cables and anything else obviously necessary, when vintage suspension specialist Matt Wiley at Race Tech emailed me, asking if I knew about their drum and brake shoe arcing service. I didn’t, but I learned that back in the late 1970s, Race Tech's Paul Thede, fighting brake issues created by increasingly powerful MX bikes still running drum brakes, which couldn’t cope with the loads, designed and built his own machine for turning drums true and arc-matching the brake shoes to the turned drum.

Way back in the day, when everyone ran drum brakes, brake arcing was common. But the advent of disc brakes made the practice something of a lost art, with fewer and fewer people providing the service. After Matt's note, I decided to try out their service and check the results. Given that most of the load is up front, I’m opting for arcing the front drum/shoe set only. The rear already works better than the front, so I’m confident I’ll be happy with the results. Look for a full report in a future issue.

Last issue I also reported on top-end problems with my Laverda RGS, confirmed after pulling the engine and removing the cylinder head. Why the exhaust valves were tightening up wasn’t immediately apparent, but an about-to-blow head gasket sure was, a telltale combustion stain writ large on the center cylinder, with minor but no less concerning indications on the left and right cylinders, as well. That was a big surprise, as I replaced the head gasket back around 2010, when I also had the valves and seats recut. As it turns out, that was right about the time some bad head gaskets were circulating; the flame rings around the cylinder bores weren’t thick enough, and once compressed, they slowly let go. As mine was doing.

In an odd way, that made feel better. I wasn't convinced pulling the cylinder head was going to reveal the problem, but finding the failing head gasket confirmed it was the right decision. As a side benefit, the tear down inspired a road trip to New Mexico with son Charlie and daughter Madeline to go skiing and generally relax — and visit Laverda guru Scott Potter. I was already working with Scott on parts for the circa-1972 Laverda 750 twin that I pretend I’m building, and the visit was the perfect opportunity to collect the parts he'd rounded up for the 750, and show him the RGS head to get his take on why the valve clearances kept shrinking.



Scott’s assessment brought something approaching relief. The RGS has an aluminum cylinder head with a cast iron “skull” insert and hardened valve seats pressed into the skull. Fearing the worst, I figured the seats were bad, or something weird was going on with the skull, but Scott’s examination showed the problem to be nothing more than regular wear. The RGS has almost 75,000 miles on it, and a combination of worn exhaust valve guides and worn exhaust valve stems had resulted in the exhaust valves constantly shifting on the valve seats, enlarging the seating area, resulting in the valve dropping deeper in the head, resulting in diminished clearance at the camshaft.



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