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Replace BMW Airhead Twin Pushrod Tube Seals
12/27/2012 11:52:44 AM
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.
This engine is from our Race to Rebuild 1974 R90/6, removed to refinish the frame. The steps are the same otherwise, starting with removing the exhaust and carburetors, followed by the valve cover, which is held by three nuts, one on the face and two behind, as shown.
Removing the valve cover shows the four studs with nuts that secure the rocker arm assemblies and the cylinder head and cylinder to the block. There are two more studs at 12 and 6 o'clock. Remove the spark plug, then loosen the nuts a quarter turn at a time, working in a cross pattern. Back them off completely.
Here's what the cylinder head looks like with one rocker arm assembly removed, the studs at 12 and 6 o'clock just barely visible. Note the pushrod just visible to the left of the lower right cylinder stud.
Noting their original position, remove the rocker arms, followed by the pushrods. Mark the pushrods for their orientation in the engine and keep the rocker arm assemblies in order of intake/exhaust so that everything goes back together just as it came apart.
The cylinder head and cylinder can now be removed. Tap the head and cylinder with a rubber-faced mallet to break their seal. They should separate fairly easily. Remove the head followed by the cylinder. Here is a pushrod tube and its seal releasing from the block.
If your engine is like ours, you'll find the pistons caked with carbon, which is quite normal. We removed and cleaned our pistons, which actually makes reassembly easier as you don't need a ring compressor for assembly. Inspect the piston and rings carefully for wear.
Our cylinder head (in the background) was similarly carboned up, so we had Doc at our local machine shop clean it and check the valve seats and guides, which were fine. We're showing the cylinder only to note the two chamfered locating dowels visible here at roughly 11 and 5 o'clock. These must be removed before reassembly.
Here are the two dowels removed from the cylinder barrel. Ours came out fairly easily using simply a pair of pliers. Why remove them? BMW decided they were unnecessary and has since changed the head gasket to suit. New head gaskets will not fit over the dowels, hence their removal.
Piston removal requires prying out the wrist pin clips so you can remove the wrist pin from the connecting rod and piston. Gently heat the piston with a hair dryer to aid wrist pin removal and assembly. Before piston and cylinder reassembly, install one new clip in the piston. It doesn't matter which one. This will facilitate installation of the piston/cylinder assembly, as we'll see shortly.
At reassembly, lightly oil the cylinder bore. Ensure the piston rings are staggered (no end gaps lining up) per the manual, and insert the piston into the cylinder bore from the bottom, which has a pronounced chamfer.
Ensure the arrow stamped in the piston crown will face forward on final installation. Using your fingers, compress the rings and gently work the piston into the cylinder until the rings are completely in the barrel. Leave the wrist pin area of the piston visible.
Rotate the crankshaft until the connecting rod is extended, then gently pack a rag around the connecting rod at the block. Lightly coat the base gasket with Permatex Ultra Grey sealant and install it over the studs. Note: The wrist pin is installed here only to hold the connecting rod up from the engine case.
Carefully slide the cylinder barrel with piston over the studs until the connecting rod lines up with the piston. Install the wrist pin until it passes through the piston and connecting rod and butts up against the clip already installed. Install the other wrist pin clip to secure the wrist pin in place.
Remove the rag around the connecting rod and gently push the cylinder over the piston. Note the pushrod tube seal orientation, ribbed side toward the engine with its offset at bottom. You can install the seal at this juncture, but it's easier prior to the previous step.
With the cylinder pushed home, place the new cylinder head gasket in place. The factory gasket comes with sealant already applied. Before going any further, make sure both pushrod tubes and their seals are properly positioned in their respective seats.
Oil their ends and install the pushrods, ensuring they're properly located. Make sure the threaded valve adjusters are backed off. Install the rocker arms in their respective places, followed by the six stud washers and nuts. Following a cross pattern, slowly work the nuts down until they just seat.
Check that the pushrod tube seals are properly seated, then torque the six cylinder head stud nuts in two stages to their specified torque. In our case 26lb/ft, with the first torque at 15lb/ft.
Adjust the valves per the manual. Turn the engine over several times and recheck the valve adjustment. Install the valve cover gasket and valve cover. Assuming your engine is in the bike, it's now time to reinstall the carburetors and exhaust system. If already drained, refill the engine with clean oil. It's time to start it up, free of oil leaks!
