Restoring a 1973 BMW R75/5 Motorcycle

July 29, 2007

GI Joe’s Kung Fu Grip

Filed under: /5, 1973, airhead, BMW, BMW motorcycle, motorcycle, motorcycle restoration, R75, R75/5, restoration — Penforhire @ 3:59 pm

Let me start off with a tip of my hat to Pete, a co-worker who read my blog and took pity on me. He just so happened to have a new 1/2 ton arbor press that he donated to the cause. With all the press fits in this bike I should have had one sooner. I won’t have to make do with standing on assemblies to crush them now.

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You know I like any tool that’s a pure force multiplier. I think Stanley Kubrick said it best, and I paraphrase, “This is my press. There are many like it but this one is mine. My press is my best friend. It is my life. I must master it as I must master my life. Without me, my press is useless. Without my press I am useless.” I think I’ll name her Charlene. Thanks Pete!

As promised, Motor Works sent me an expensive package this week.

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Here’s a view of some new valves and a head that’s had the carbon casserole blasted off. The valves are Black Diamond EV8 stainless steel, same as Bob’s BMW uses. I believe all other parts are OEM from BMW but I did not make that a condition of service.

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I am surprised by pitting in the combustion area (barely visible above) but it might be in the original casting. The carbon layer was so thick I don’t think combustion had a chance at damaging the original metal surface. I could not get good pictures of the intakes but the casting is so rough it could obviously use polishing. Oddly, I don’t see a lot of mention of typical hot rodding techniques like that in the /5 community. Maybe we owners settle for just keeping it running?

Here’s the output port on the left side. It cleaned up nicely.

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Why show you this? To contrast the appearance with the right side, whose output port required thread repair. Remember how I mashed ’em flat because I ignored my “pile of knowledge?” There are several accepted techniques for that repair and Motor Works used the weld-on-a-new-one.

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Not as sexy, is it? Let that be a lesson to all of you out there who have yet to smear their own output port threads. There is, coincidentally, a recent discussion on the Airheads Beemer Club mailing list about anti-seize pastes. I have a big jar of the plain grey Permatex goop carried by every auto parts store. While it claims to be suitable for this application, and it works for some, others recommend a more expensive high nickel-content paste. It is rated to even higher temperature and may be better for an aluminum and steel coupling. Regardless of what you use, it is highly recommended to make re-lubing the exhaust nuts an annual service item.

Check out this brand new piston.

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Sweet! It looks good enough to put in the living room as art. The “vorn” arrow must point toward the front of the bike (naturally it means “in front”). The pistons are not perfectly symmetrical. Three rings are already installed and new gudgeon pins or clips were provided (pre-installed on one side).

Here is the matching over-bored cylinder. Motor Works did a fine job of cleaning up the exterior and sealing surfaces. The overbore itself is almost not enough for the pitting it had. I can see some black spots in a few areas but I can’t feel them so they probably won’t affect anything. We’ll see.

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Bob had an interesting suggestion for assembly lubrication. He recommends using a two-stroke motor oil to wet cylinder and piston parts so the very first start-up will not smoke so badly. We are going to burn whatever oil is inside the combustion chamber so that sounds like a useful idea.

Motor Works sent along some souvenirs.

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Nice to get the old parts back, on principle, but I don’t know what I’d want these for. I used to have a valve from a big ocean-going ship engine as a curio. The thing weighed five pounds and the base was six inches across! I kept in on my desk several jobs ago and I’d wave it at people as a device of persuasion. These parts just look grungy and who’d be intimidated if I waved these at ’em?

Here is a view of me trying to get the gudgeon clip on one piston.

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The wrist pin is already through the connecting rod. One repair manual suggested putting moly grease inside the rod first so I did that. The wrist pin is an amazingly snug fit anyway. I did not detect any slop, per the manual, indicating a worn connecting rod. In the picture white towels are supporting the piston and preventing any contact with the cylinder guide studs or engine case. I just about blew a head gasket (my own head) getting the gudgeon clips installed. You can see some light scratching around the hole from my many failed attempts. At first I figured I should be able to do it with my bare hands but the clip tension was just a bit too much. It didn’t stop me from abusing my fingers trying.

I had to stop, take a few breaths, and reconsider. I figured Joe would laugh at me if I had to call him up to cry about the darn gudgeon clips. Here’s a technique that finally worked for me. I didn’t bother to bust out a tripod and this is a two-hand thing, so just imagine the upper clip end in the picture is seated in the piston opening.

2185-technique-for-gudgeon-pin.jpg

The screwdriver keeps the upper end of the pin seated in the groove while the needle-nose pliers curls the other end into the piston groove. If you don’t capture the starting end, as I did with that screwdriver, it comes flying out of the piston and bounces randomly around the garage… every time. Once it flew dangerously close to getting into the engine opening so I blocked that with a rag (having visions of taking off the oil pan yet again).

Here’s a view of the piston just entering the cylinder. There is a piston-support tool described in the repair manual but it seems just as easy to insert the piston without it. Some rocking is required anyway and there was not much of a problem keeping the piston straight when needed. The job would be easier with three hands. While compressing the outermost ring with the finger tips of both hands you have to press the cylinder along the guide studs. The cylinder has a chamfer at its base to theoretically allow a person to get the rings inserted without an actual ring compressor.

Yeah, maybe if you’ve got GI Joe’s Kung Fu Grip. For us mere mortals it is too hard on the flesh. The ends of the rings nearly touch when properly compressed and that’s a LOT of spring tension. The recommendation is to rock the piston slightly to start one edge into the cylinder and work your way around to load the rest of the ring. I could never quite get it done. I got it barely into the cylinder chamfer but not far enough in to just press-and-continue. My fingers started to feel like ground beef and I wasn’t getting anywhere.

I’ve got a thin metal ring compressor but it was made for loading pistons into cylinders from the outer end. It doesn’t completely unwrap so I couldn’t place it around the piston after it was on the con rod. The manuals clearly indicate the piston should go on the con rod first. So after my hands felt like clubs and nothing good was happening I cast my eyes around the room to consider what might help me in this task.

Did you know that the driveshaft rubber boot clamps fit almost perfectly around the piston?

2183-makeshift-ring-compressor.jpg

Neither did I. But they do! I wrapped it and pinched one side in by hand. The band is wide enough to just cover two rings at a time. With everything wet with oil the piston slid over the compressed ring nicely and pushed the clamp out of the way without damaging anything. After getting the bottom ring started I released & moved the strap to cover the middle and top ring. Then just the top wide ring. I had to use the clamp’s screw on the other piston to close its rings. Maybe my hands were not gripping well enough by then.

The cylinder base gasket is a solid metal piece and sealant is specified. That was a dirty job, getting sealant on both sides of the gasket without getting any inside the cylinder or engine case. I had to use the heads to torque the cylinder completely into position. The rubber pushrod guide seals prevent you from just pressing cylinders flat against the engine by hand. Those seals need to be forced into the lifter openings and used the head to do it. The threads on the end of the cylinder studs require the head in place for the nuts to press on. Those same nuts-on-studs hold the rocker arms in place. Seems peculiar but simple that the whole cylinder is held down by those rocker arm nuts. I put new gaskets under the heads and finished the assembly.

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At this point the valves might as well be adjusted before mounting the valve covers. The first step is to rotate the engine to top dead center. There is a mark visible on the flyweel through the timing hole. It requires a flashlight to look in there. The camera’s flash lit up the German “OT” mark, see the arrow below.

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There are several ways to rotate the engine. I started by using just the rotor mounting bolt at the front of the engine (6 mm Allen drive) to get each connecting rod in place to mount the pistons. But after the pistons were in the cylinders that took enough torque to move that I feared for the bolt. The kick starter is another technique but I couldn’t get that to turn the engine, perhaps because of a gummy tranny. I have not yet hooked up the drive shaft so I couldn’t use the rear wheel to turn the engine either. I ended up pressing directly on the teeth of the flywheel to rotate it clockwise (seen from the front), just as the starter would.

Once that OT mark is in the window one head is ready for valve adjustment. The pushrod lifters will be completely down on that side. In theory the rocker arms should have some free wiggle on that side. All four of mine were all tight (!) so the only way I could easily tell which side was free was to rotate the push rods, feeling for the free side.

I not only had the two repair manuals describing a valve adjustment but I also had articles by Mr. Parkhouse and Mr. Ausherman. Duane’s article is probably the most complete in describing the nuances of the job. While you’re in there looking at the valves you can check your pushrod placement. When the lifters are all the way down the push rods should appear above-centerline in the heads. See the arrow in the photo below.

2189-pushrod-spacing.jpg

I know nothing about this but apparently there is some sort of need to sometimes make a tricky adjustment if the pushrods are riding too low. Otherwise the light you see at the end of the tunnel will be an oncoming train (rods wear and bend or else wear all the way through the guides!).

Before you adjust the actual rocker arm-to-valve gaps you should take up any slop in each rocker arm assembly.

2190-taking-up-rocker-arm-slop.jpg

You have to use sockets to fit over the post-ends of the rocker arm. The clamp pushes the rocker arm assembly together while the upper and lower nuts are loose. I did not have a big enough C-clamp, you’d need one about 6″, so I used that wood clamp. You don’t need much force. This clamp could (and did) easily apply too much force, felt as binding in the rocker arm rotation. I temporarily opened the valve adjustment quite a ways so I could feel more rotation angle. At the same time you are taking out slop you are also moving the whole assembly up or down to center the rocker arm on the valve end. There is more motion available than I thought there’d be. After you’re in the right position, with the right tension in the clamp, you can tighten the upper and lower nuts. Use a torque wrench. Duane warns us not to overtighten these.

Now you’re ready to actually adjust the valves. There seems to be some change in the intake-side gap specification over time. I think it was originally 0.10 mm but all the literature now says 0.15 mm. That seems like an awfully big change but what do I know? Note Mr. Parkhouse’s article in the BMW Owner’s newsletter quoted incorrect values, there was a correction the following month. The exhaust side is 0.20 mm. Obviously you need the right feeler gauges but you also need to get a feel for the dirt-simple adjuster system. The adjustment thread is too coarse in my opinion. A small rotation causes a big change in the gap. And the locknut tends to close up the gap a bit when tightened. So you need to find your own technique for ending up at the right gap. You also need to decide for yourself how much drag you want to feel on the gauge as you work it in the gap. It took me a while to get it right.

Okay, now that you’re done on one side you have to rotate the flywheel 360 degrees, one whole turn, to get top dead center and free up the other head. Repeat the process. All in all, I’d say the valve adjustment job is cake (easy as pie?). There are only three nuts on the valve covers and a little oil drip to deal with. Amazingly quick and easy to do compared to my FJR or any other typical Japanese inline four with an overhead cam. Man, I’d hate to have to do the Honda VFR. That has a VTEC cam system and I think you have to adjust the gap twice for each valve (on & off VTEC)! On the flip side, my FJR only requires a valve check every 26K miles, not every 4K (if I remember correctly for the BMW), and most reports suggest FJR valves rarely need ANY adjustment at the first check.

Here we are at the end of the day. Looking down over our domain.

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Geez my hands are sore! I need to work on that Kung Fu Grip.

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3 Comments »

  1. Your photos are great. My pushrod seals are on their way out, so I am surfing for info. 1974 R75/6 160K miles, original cylinders and valves.

    I just had Bing rebuild my carbs. They did a great job.

    Comment by Gary Stevens — September 14, 2007 @ 9:05 am

  2. Hi;
    Great article and very useful to me!
    I am essentially doing the same job on my 73 R75. Recently received all my parts from Capital Cycle and had my cylinders bored and new valves put together by a local machinist that works on older bikes including airheads. So, I in the midst of reassembly. Con rod bearings replaced and rods back in place. Installed the new pistons/wrist pins and next will be base gaskets and the cylinders…getting excited to see how the old girl runs when I get it back together…
    Anyway, thanks for taking the time to put together a great site, it is much appreciated!
    Cheers,
    Don
    Victoria, BC

    Comment by Don — June 13, 2008 @ 9:20 pm

  3. Hi! I was surfing and found your blog post… nice! I love your blog. 🙂 Cheers! Sandra. R.

    Comment by sandrar — September 10, 2009 @ 11:33 am


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