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Keep 'em Flying:
For Want of a Pin
By Matthew Parkhouse
(December Owners News) |
December 11, 2006
Page 1 of 4 1234 |
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The Critical Pin - this tiny steel pin locks the forward main bearing to its carrier. Drilling the locating hole is a very precise procedure involving two drill bits and a reamer.
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One of our local riders had a 1978 R100RS, a very well ridden, high mileage machine. It was brought to me, shortly after a major top end rebuild, with the complaint of low oil pressure. There was a bit of a delay while I ordered replacement parts for my direct pressure oil gauge. This instrument screws into the engine block on the left side, right behind and next to the left cylinder. The bike's oil lamp was coming on at idle once it warmed up. If this happens, STOP operating the bike until the cause can be determined and repaired. An Airhead in good shape, even well-over the 100,000 mile mark, should show oil pressure around 50 to 60 PSI when warmed up. This will drop to 20 to 30 PSI at idle. The oil lamp sender is set up to open the switch and turn off the lamp at 4 to 7 PSI. So, if you're lamp flickers at idle when warm, get it looked at! Fortunately, the most common cause of this is a bad sender switch. That's an easy fix: buy a new one, unscrew the old one, using a 24mm deep socket and screw in the new one. Be ready for a tablespoon of oil to run down the side of the engine.
To verify that this gentleman had an oil pressure problem, I removed the oil lamp sender, screwed the gauge fitting into the engine block and rode the warmed up machine a few blocks. Yup; the oil would only get up to 20 PSI or so at 4,000 RPM. I gently rode it back to the shop and turned it off. On a high-mileage Airhead, low oil pressure is rare but not unknown. Usually, the area of loss is badly worn or damaged con rod bearings. They had just been replaced as part of the top end work, so I was pretty sure the problem lay deeper, like with the main bearings. I called the owner and got the go-ahead to tear all the way into the engine. A quick look at my old flat rate manual told me that "replacing a crankshaft" was estimated to be a 12 hour task. While we hopefully were not replacing the crankshaft, we were going to have it in our hands and would be installing new main bearings.
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BMW MOA
P.O. Box 3982
Ballwin, MO 63022
509A Old State Rd
Ellisville, MO 63021
ph:(636)394-7277
fx:(636)391-1811
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