Replace BMW Airhead Twin Pushrod Tube Seals


| 12/27/2012 11:52:44 AM


Tags: how-to, bmw, classic german motorcycles, race to rebuild,

 

12-14-12-pushrod-01

Introduced in 1969 for the 1970 model year, BMW's "Slash 5" line of air-cooled twins was the Bavarian maker's most successful new offering to date. Well made and incredibly durable, the /5 and subsequent /6 and /7 models in all their various guises were built in record numbers, with almost a quarter million churned out from the BMW factory between 1969 and 1980.

Today, more than 40 years after the series' introduction, there are still tens of thousands of these venerable "airheads" plying the roads. Legendary for their ability to run forever, they are rolling testimony to BMW's excellent design. But good as they are, they're not immune to the sort of common problems that can afflict just about any machine. Eventually, any motorcycle that's used will develop leaks of one sort or another at critical sealing points, and with these BMWs the pushrod tube seals — the focus of this How-To — are a common leak area.

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Visible below the cylinder, the pushrod tubes, two on each side, house the valve pushrods. The tubes are pressed into the cylinder at their outer end for a permanent seat and are sealed at the engine block with pliable rubber "pillow" seals. Over time, these seals harden and crack, with sometimes extensive oil leaks the result. Replacing the seals requires cylinder removal, so it's not surprising that many owners let them fester for years before finally replacing them.

However, thanks to these engines' relatively simple and extremely accessible design, replacing pushrod tube seals is within the scope of a competent weekend warrior. Specialized tools are limited to a good torque wrench, and if you do it our way you won't even need a piston ring compressor. As always, a good shop manual for reference and needed torque specs is critical. We suggest ordering the parts you know you'll need before you begin, including new pushrod tube seals, cylinder base gaskets, cylinder head gaskets and, if they're suspect, new valve cover gaskets. You'll also want four new piston pin clips (never reuse old ones if you can help it), and make sure to have a tube of Permatex Ultra Grey for the base gaskets. Before you begin, consult your manual for exhaust and carburetor removal, which we won't cover here. Remember to clean each and every part as well as you can.

Although a seasoned mechanic can do this job in around four hours, we'd suggest budgeting two weekend days; one for disassembly, an intervening week for cleanup and parts ordering, and a following weekend day for reassembly. Importantly, this How-To assumes your engine is otherwise in good running order, with good compression and no oil burning that would require fitting new piston rings or refinishing the cylinder barrels. If you discover serious issues inside, you'll want to reassess your options.

monty brown
2/28/2013 3:07:37 PM

What great timing as I'm doing this currently. More information is better in my case. Really enjoy reading "how to's" with regards to our classic bikes. Can't afford to have the work done and doing it yourself really gives you a sense of accomplishment. Great article - thank you! As a side note - thanks to those who post with more insight as those below.


tony carlos
2/18/2013 2:15:41 PM

This is one of the many easy jobs that can be done on an airhead. Decades ago it was my first intro to wrenching these bikes. Weekenders, have at it. But be warned that the trickiest job here might well be getting the exhaust off the head. Without frequent removal, or a good coating of never-seize, the fine threads on the head lock into the finned exhaust nut. Force them off and they strip. The trick is if you sense they are stripping, cut the nut off rather than continuing to unthread it. You'll have to buy a new nut, but it's cheaper than rethreading the head. Problem is most first-timers don't have that feel, until its too late.


monte miller
2/1/2013 8:23:46 PM

A fairly helpful article with excellent photography. I'll print this out for my service binder. Some base gaskets will benefit with just the lightest smear of YamaBond -or similar- on both sides. Use caution as excess material can clog oil passages to the rocker assembly.


jack hawley
1/31/2013 2:39:38 PM

The early /5 or maybe even /6 tubes did not have the collar that presses on the seal soldered to the tube. Therefore even with newish seals, they would leak when the collar slipped up the tube. So if the tubes are early and original, they must be replaced to do a proper job.





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