Airhead Tech Pages


336 Cam Conversion

Tom Childers



I've always wondered at the allure of a 336 cam. A stock type 247 airhead engine is torquey, but does not rev all the way to the red line easily -- it feels like it runs out of breath over 5000 RPM. The 336 cam, a factory racing part from BMW, increases the cam lobe duration (the length of time the valves are open), without increasing valve lift. The result is better breathing at high RPM, with more torque and horsepower. The disadvantage of a 336 cam is that it reduces low-end torque in the 2000 to 3000 RPM range.

The classic way to fix this is to increase compression, which will increase power and torque throughout the RPM range. However, compression ratios over 9.5:1 in a stock BMW type 247 engine cause pinging with current gasoline formulations. Then you need to add a second spark plug to the cylinder head, and retard the timing advance curve to compensate. CC Products in San Jose, California has offered this modification for many years.

I'd heard stories for years about hot-rodded airheads, with CC Products piston kits, lightened wrist pins, higher compression, dual plugs, racing cams, Krauser heads, turbocharging. But I'd also heard lots of stories about engines self-destructing, thrashed bearings, burned pistons, wrist pins coming loose, Krauser heads with poor oil flow.

Could I bring together a 336 cam, higher compression and dual plugs, and make an engine that was powerful, reliable, and easy to ride on the street? Jim Roche (aka Dr. Curve), Kari Prager at California BMW and others all said yes.

In 1994, I bought a very tired 1979 R100RT for $1400, with 85K miles and no fairing. In 1995, the bike started burning lots of oil, and my long-anticipated engine rebuild begin. Here is the complete saga, as reported on the IBMWR list in 1996. Perhaps the questions I asked, and decisions I made will help others if they step into this journey.

March 3, 1996: The Rebuild Begins

Happiness is a camshaft in the hand. Today I completed the first stage of my R100 rebuild. The bits are scattered all over the garage, with neat little tags.

For the newer list members, I bought a 1979 R100RT about a year and a half ago. The bike has about 85K on the odometer, and needed LOTS of work. The fairing was off, and the bike had S-bars on it, with a brand new S fairing still in the factory wrapping. So I replaced the worn carb jets and gaskets, replaced the reed valve with the modern all-metal version to reduce the "popping" sound at idle, cleaned up all electrical connections and installed the "octopus" ground system upgrade. I also replaced all cables and ignition wires, installed Progressive springs and shocks, got the seat recovered, and did a full tune-up. And I converted it from the single-piston Brembo rear disk to a drum rear, so I've actually got a functional rear brake now (the early ATE rear disk brakes are nothing to write home about).

Oh, yeah - I replaced the rusted-out mufflers with Staintune racing pipes, giving the bike quite a presence. It is about as loud as a stock Harley, or Norton with a Combat engine. One of our fellow presidents nicknamed the bike "Mondo Decibels", and I guess that is her name. With the stock 40mm Bing carbs and large valves, she has a lot of potential.

The engine had lots of blow-by, causing the hideous "pop pop" sound through the reed valve when warm. So I knew that I was looking at a rebuild, and I started doing my homework to see how I could improve the power, without sacrificing reliability or rideability. About 5 months ago, the left cylinder started smoking a lot, so I tucked her away in the garage. Over the last couple of months, I've been accumulating parts, like a 336 cam, gasket set, and new timing chain. I also bought an exhaust nut wrench and alternator puller, the only custom tools I needed for the job.

So yesterday I pulled the cylinder heads, cylinders and pistons. This is a very easy job...I think it took 40 minutes. The exhaust nuts came right off, because I had soaked them with penetrating oil for a few days before work began. I also found out that the exhaust nuts have opposite threading on the left and right sides. I labeled all the parts carefully, so that the valve lifters, rockers, pushrods, etc. will all go back into their original configuration.

Also, I've got the timing chain cover off, and I've removed the chain and pulled the cam. This was a pretty easy job, too. You simply label all the wires, remove the points can, take out about a dozen bolts, then put the alternator puller on the end of the crank, tighten it down, and pop! The entire timing chain cover will come off with the alternator stator and rotor. The easiest way to remove the chain is to cut it with bolt cutters.

Rob Lentini made an interesting comment to me when we met at Death Valley in January. The 336 cam effectively lowers the compression ratio because it increases valve overlap. Therefore, if the engine tended to ping at 9.5 compression on our low-quality American gas, it will be less likely to ping with the hot cam. I don't want to reduce the durability of the engine, so I am reluctant to push the compression too much, even with dual plugs. But I do want to open up the top end, even if I lose a little torque at low RPMs. I tend to ride this bike at 4-6000 RPM, anyway.

So now the question is, how much do I want to push the compression ratio? If the cylinders are not too worn, I can just hone them, install new rings, and use the same pistons, keeping the stock 9.5 compression ratio.

If the cylinders need boring, then I'll go ahead and get higher compression pistons and dual-plug the heads. I won't know if boring is needed until the shop takes a micrometer to the cylinders.

So assuming I can keep the stock pistons:

Can anyone comment about the longevity of an engine with higher compression and dual plugs?

I got back some great comments from IBMWR list members. Kari Prager, of California BMW, wrote:

Great Project! I weigh in w/Dr. Curve on this one - if you are doing all this work, then go to dual plugs, raise the compression, limit the spark advance travel. No harmful effects that I can think of. But it won't hurt anything to just try it stock & see how you like it. You can "surface grind" the heads on a piece of plate glass & sandpaper to be sure that they are perfectly flat. BTW, if you are replacing the cam, it is a very good idea to also replace the followers. In my personal opinion there is not so much point to changing the wristpins, since you are not trying to build an 8500 rpm motor. I like the stock /7 aluminum pushrods just fine. Keep us posted!

Jeff Dunkle said:

If you're really feeling ambitious, volumetrically match the heads. Just takes a chemsitry class graduate, a Dremel grinder, and the hutzpah to grind a little here and there off the head interior till the smaller volume head matches the larger one.

And Nathan Meyer:

As to the light-weight parts; again I tend towards the factory bits for longevity; however do get your top-end balanced! This pays the greatest dividends in smooth running.

If you are interested in lightening the drive train for better revving; then I recommend having your fly-wheel lightend and balanced. The 79 and 80 model years had the cush-spring drive shaft with the older-style massive flywheel. I always thought they felt sluggish compared to the earlier no cush/big flywheel and 81 on cush/NO flywheel bikes. I'm in the process of putting Alice ('78 R100S) back together with a lightened flywheel and a cush drive; I think it's a nice combination.

March 11: Installing the 336 Cam

This weekend, I started reassembling her. My first problem was to move the cam sprocket onto the new 336 cam. Fortunately, my local shop (BMW of Marin) is very friendly (especially when I bring a 6-pack of Sierra Nevada beer!) and we did the job with their hydraulic press. Wow, jobs like this are easy when you have the right tools! In 15 minutes, with a torch and a little anti-seize compound, we had removed the sprocket and pressed it onto the 336 cam.

After picking up a new front crankshaft seal, new bolts for the alternator (had to drill one off!) and new diode board mounts, I headed home to start the reassembly. All of the other new parts, like gaskets and a timing chain, were already waiting for me. The cam went in beautifully, with good snug fit in the bearings and what looks like the right end float for the sprocket. A little blue Locktite on the front bearing bolts, hand-tighten them snug, and the cam is all done. Then I had to start the dreaded cam chain replacement project.

As Don Eilenberger can tell you, this is one of the most frustrating jobs in the known universe. You have to wire the tensioner back to get clearance, line up the sprockets to get the right timing, and fit the master link into the ends of the chain. From past experience, I remembered one key parameter for reassembly:

* Do Not Oil The Chain Before Fitting the Master Link!!! *

However, it still isn't easy. Fortunately, I figured out a neat way to make the job easier. You can use the old master link from the old chain to hold the ends of the chain together! Fit the *old* master link into the chain ends from the outside, which is fairly easy. Then, you can worm the new master link into the chain ends from the inside, and pop the old link out. Piece o' cake.

Next, I had to spend an hour with a putty knife removing the old gasket from the timing cover. Then another hour with brake cleaner and a stiff brush removing the spooge from the timing cover, then a half-hour in the kitchen scrubbing the cover with various toothbrushes and pads to get it nice and clean. Back to the garage, one whack with a hammer to remove the old crankshaft seal, and a little gentle tapping to put the new seal in place.

With a little heating, the alternator support bearing on the front of the crankshaft popped right off. A little more heating of the new bearing, some more anti-seize, and the new alternator bearing snugged right onto the crank. I probably didn't need to replace the bearing, but it wasn't expensive, and I don't plan to take the cam chain cover off again any time soon. I replaced the chain tensioner and the chain guide on the right side of the engine, too. Now I think I'm ready to put the cover back on.

The BMW shops out here sell this wonderful gasket stuff called Dirko. It is a thick, clay-like paste that spreads nicely with your finger tips. It also gets all over the place if you aren't careful. With a few minutes of work, I had an even layer of Dirko on both sides of the gasket, the gasket was stuck in place onto the timing cover, and the two little gasket washers for the upper cover bolts were in place, too. I used the torch to heat up the middle of the cover (where the alternator bearing fits), squirted oil into the alternator bearing, timing chain and tensioners, then lined up the cover, threaded wiring bits in the right places, and pushed the cover into place. Voila! Just a little tapping required. So I replaced the 10-odd bolts holding the cover in place, started torquing them down, and realized that I had forgotten to replace the diode board mounts.

Oh, well, the starter cover comes off, doesn't it? I tighted the timing cover up, wiped off the little thread of Dirko that oozed out on the outside, and attacked the diode mounts. Another half-hour of removing the gas tank, starter cover, and fiddling the little 8mm bolts onto the diode mounts, and I had a stiff back and a bad attitude, but the mounts were in place.

Time for a beer, and a little more work. The alternator went on easily, the wiring connections all went into place okay (masking tape labels on the wires REALLY help!) and the diode board is back in place. The points-in-a-can popped right into place with a new o-ring to seal the hole in the timing cover, and the front of the engine was finished.

March 25: Machine Shop Reports...

...the cylinders are too worn to take the stock piston size. Oh, well.

So I had them bore and hone the cylinders back to spec, and I bought 1st-oversize stock pistons and rings. I'm going with stock parts, because I want the bike to last a long time. I've heard too many stories of R100 engines that self-destruct with the Veniola high-compression pistons some shops provide. I'm also keeping the stock wrist pins for the same reason.

I've decided to go most of the way on the performance mods. In between bad influences like Jim Roche, Kari Prager and Rob Lentini, I gotta admit: I want a hot rod. This bike will not be pushed to full race configuration, but 70 hp would be nice. So, the heads are off to California BMW, so they can mill 0.020-0.030 off to push the compression ratio from 9.5 up to about 10.2. I'm also getting the heads drilled for dual spark plugs, and will get the top end balanced. She's already got big carbs, big valves and a Staintune exhaust, so she should really fly...

One important detail: you have to change the timing advance curve when going to dual plugs, and get new ignition coils. There are several ways to change the advance curve, but one is to put stronger springs on the timing weights in the points cannister. California BMW had the springs, but replacing them requires small tools and lots of patience. There is an inspection window on the side of the cannister that gives access to the timing advance mechanism.

For the ignition, I got Dyna "green" coils, which bolt right up to the original mount points. And I fabricated the second set of ignition wires from standard automotive parts.

April 12: Top End Assembly

The parts are back from the shop. The heads have new valve guides, are drilled and tapped for dual plugs, and were milled to raise compression. The shop attempted to match combustion chamber size in the milling process, and also did some porting and polishing on the intakes. And I had them install new pushrod tubes. The later bikes have pushrod tubes with brazed seal brackets, so the pushrod tube seals stay tight against the engine.

Here are the assembly tricks I've found out so far:

April 15: Final Assembly

So tonight, the final bits were completed. Headers installed, front cover replaced, battery reconnected, valve covers on, and oil in the engine. After cranking the engine to get oil pressure, I put in the plugs (connecting only the two stock plugs to the ignition system), replaced the gas tank, and turned the taps. Flip the choke lever. Crank for about 10 seconds, and...

An incredible noise fills the garage, as a cloud of oil smoke erupts from the Staintunes. Very deep, throaty, and smooth. Loud. Yes, even louder than before the rebuild. Smoother, too. Small throttle movements keep the RPMs cycling between 1000 and 2000, as the engine warms up. Wow.

No pinging, no excessive valve noise...at least, none that I could hear over the exhaust. Once again, I am back to the combination of boyish enjoyment of unbridled engine noise and horror at the noise pollution I've introduced into the environment.

There is no doubt that the 336 cam changes the engine characteristics. The exhaust note is different...throaty smoothness is the only way I can describe it. Even before break-in, the engine is running much better than it was before the rebuild. The idle is even (thanks to a new cam chain), the crankcase vent noise is gone (new rings), and the valves sound quiet (new guides).

When the engine was warm, I shut it down with a big, idiotic grin. The left cylinder is hotter than the right, so some carb adjustment may be in order. Or, it may just be break-in pains. Actually, I haven't even ridden her yet, so I'm sure that there will be some of carburetor tweaking ahead.

June 10: Connecting Rod Bearings

Throughout the rebuild, I left the connecting rods in the bike, thinking that the bearings were okay...

After the first few hundred miles, I started pushing the engine a bit, and heard a tapping sound from the right cylinder whenever I ran over 4000 rpm. Uh-oh. Pulling the head, I found that the piston and cylinder head make slight contact in the squish band area on the right side.

So I called Cal BMW, and they preferred to take the engine apart themselves to diagnose the problem. It turns out that my conrod bearings were pretty far gone, especially on the right side, with the big ends of the rods somewhat ovalized over time. The elongated rods and worn bearings were allowing the piston to contact the head. Oddly, they found two small pits on the crankshaft conrod journal for the right side, too. The pits are less than a millimeter wide, but are about 0.25 millimeter deep, enough to go through the hard facing. The bearing shell was badly scored about 3 mm from one side. Clearly, a hard particle got in there and toasted the journal and bearing.

Crankshaft replacement on a type 247 engine is a big deal. The book says 18 hours of labor, and the procedure is tricky enough that I didn't want to do it myself. And I can't afford to spend that much additional money...she's already cost me about $4500, and she isn't even painted yet.

So I went and talked to Kari. I can't tell you his entertaining stories of what these engines can survive, but I can tell you he reassured me that the engine would be okay with two little pits in the conrod journal. He says it is easy to hear a bearing going bad, so I'll get lots of warning if I know what to listen for. I can live with that. So they are going to lightly hone the edges of the pits in the crank journal, recondition the conrods (mill 0.005 off the big end cap mating surface, then line-bore the big end round again), balance the conrods, and button the bike back together.

So I guess there are some morals to the story.

I figure this could have cost me about $1400...and I'm getting away (with new conrod bearings, balanced rods, etc.) for under $400.

BTW, the bike has turned into an amazing beast as the engine breaks in. The power band with the 336 cam is wide and strong, and just keeps building. I can't wait to get it up to 6000 rpm...

June 24: Conclusion

Today I finally got to experience the results of my 1979 R100 hot-rod project. I am pretty stunned by the results. Both emotionally and financially :-)

The connecting rod work eliminated all clearance problems between the piston and head, but the shop confirmed what I suspected - my carburetor diaphragms were shot. So today I pulled the tops off of the carbs, and replaced the (perforated!) diaphragms. After a quick carb balancing job, I took her out for a spin.

Yoicks. The carburetor fix put 20 hp into the engine. It was already stronger than stock. All of a sudden, the bike pulls like V8 all the way up to the top end without the slighted hesitation. I think my 0-to-60 mph time is under 4 seconds. Mondo is now considerably faster than my K100RS (insert sh*t-eating grin here!)

Unfortunately, if I nail the throttle at 5000 rpm, the clutch slips. So now I have a good excuse to upgrade to the lightweight flywheel and clutch assembly from a later model R-bike :-)

So what did it cost? I haven't added this up before, and it's a good thing that the expenses stretched over the last two years. Keep in mind the fact that I went for mostly new factory parts, and rebuilt everything that I touched, because I intend to keep the bike a long time. Here goes, with me doing all the labor:

Engine work:
        rebuild heads, port and polish heads,
                machine for dual plugs, 
                mill 0.030 to boost compression   454
        bore and hone cylinders to 1st oversize   111
        recondition and balance conrods            72
        R & R pushrod tubes                        37

Parts:
        pistons, 1st oversize                     389
        valve guides, springs, clips               68
        new style pushrod tubes                    58
        conrod bolts and bearings                  45
        head gaskets, pushrod seals, oil & filter,
                o-rings, timing cover gasket       62
        New tappets, chain tensioner and rail,
                cam chain, crank end bearing      130
        336 cam                                   184
        Dyna coils, mounts, plugs, plug wires     165
        Dyna booster                               58

Grand Total:                                    $1833

For less than $2000, I've got an engine that is mostly new. The bike originally cost me $1400, so I now have $3200 (plus new exhausts, shocks, springs, saddlebags, center stand, old-style valve covers, mirrors, cables, steering bearings, triple clamp, carb rebuild, seat rebuild, tires, brake parts...make that $5000) invested in a bike that still has a slipping clutch, and hasn't even been painted. Add the cost of paint, wheel powdercoating, and conversion to the lightweight flywheel and clutch, and I'll have about $6600 (and over 60 hours of labor) invested in her by the time I'm done.

Yeah, I think it's been worth it. I'd rather have this bike than anything I can buy. Special thanks go to Kari Prager, for all his advice, and to Jim Roche, for his information about 336 cam conversion and compression changes.

2005 Follow Up Report

I now have about 30K miles on my baby, and there have been no problems whatsoever with the engine. I did have to increase the size of the pilot jet in the carbs to get a smooth idle, and I ultimately upgraded to the latest Boyer-Bransden Microdigital ignition for dual-plugged bikes. The Boyer-Bransden has been wonderful -- it immediately gave the bike a noticable smoothness throughout the RPM range, and eliminated a slight amount of pinging when I accelerate full-throttle.

In late 1996, I swapped in a 1983 clutch, flywheel and transmission, which made a big improvement. The clutch pull is much easier with the later design, and the flywheel is much lighter, giving the bike awesome acceleration!

I also found a deep oil pan and oil pickup for the bike, and installed them with a modern silicone rubber gasket. This lets me run synthetic oil without any weeping around the sump. I find that the bike runs just fine with the deep sump and without an oil cooler -- it never seems to run too hot, even when idling in traffic.

In 1997, I completely upgraded the ATE disk brakes with a handlebar-mounted master cylinder and stainless steel lines. You can read about those modifications here.

Just this year, the original Bosch starter died. So I upgraded to the new lightweight Nissan starter conversion available from Motorrad Electrik. That trimmed a few pounds off the bike, and makes her start up very easily.

Total investment? About $7500. And she is still the fastest bike in my stable :-)

(PS - I have a web page up with photos and links to the resources I used for the site. You can view the web page here.)


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Last Update: 27 November 2012