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Old 01-03-2009, 10:12 AM   #1
bgoodsoil OP
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Balancing a BMW engine

I've read posts about people 'balancing' their airheads--removing the con-rods and pistons and making sure they're exactly the same weight and doing something to the flywheel.

Anybody have any experience with it?

My BMW runs pretty smooth right now, that's why I bough it instead of another thumper, but there's always room for improvement! And heck, pulling the pistons and con-rods on these engines takes no time.

What exactly is done to the flywheel? As far as I know the G/S has the lighter flywheel, right?

Oh, it's an '85 r80 G/S, US spec low compression pistons.
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Old 01-03-2009, 11:00 AM   #2
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When I rebuilt my 1981 R100CS engine I installed new European 9.5:1 CR pistons and new con rod bearings. Seeing that everything was new, I decided to measure all of the individual components and then mix and match to obtain the best balance. I did not separate the con rods. By just doing this I was able to balance each side to within 0.1 grams of each other. This produced an amazingly smooth engine and all it cost was some time with a decent scale.

I have found that BMW's are pretty well balanced from the factory, but if you have the opportunity to do this, I recommend it! If mixing and matching new components does not produce results like this then you can remove metal from the undersides of the pistons to achieve a better balance, though you have to make sure that you remove it from the correct spot. Also there are limits to what you can achieve.

From memory I think BMW specs allow about 8 grams of imbalance, but I never saw anything like that in practice. YMMV.

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Old 01-03-2009, 11:16 AM   #3
bgoodsoil OP
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thanks! I never even thought about mixing and matching parts, that's genius!

I've got a powder scale for measuring the weights of bullets that's incredibly accurate. Not sure if it goes up to enough to weigh a piston though

I rode an r80st when I was first looking for an airhead. The guy selling it had several airheads and had been rebuilding them for years so he really knew what he was doing. It was weird revving the engine. I'd hear the noise, the bike would lean to the right, but I couldn't feel ANY vibration. Seriously, it was hard to tell the bike was running. I know that kind of smoothness is possible in my G/S.

Any thoughts on the titanium con-rods that Siebenrock sells? waste of money? I definitely don't want to go to the high-comp pistons.
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bgoodsoil screwed with this post 01-03-2009 at 11:26 AM
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Old 01-03-2009, 11:22 AM   #4
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When checking the rods, weigh the ends separately. The wrist pin end, being much farther from the center of rotation, will have more of an effect for small differences.

The flywheel requires a fixture to mount it to. Would a wheel balancer be sufficient?
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Old 01-03-2009, 11:31 AM   #5
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taking minor amounts of material away from the underside of the wrist pin bosses with a drill press is how i heard to balance the pistons. I also undrstand that the whole thing- piston and conrod and wristpin- are weighed first to see which is heavier, relatively, overall.

If you are going to remove material fom the conrod, make sure any filing is done parallel to the long axis, as opposed to across the short distance.
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Old 01-03-2009, 12:08 PM   #6
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When I rebuild these engines I always balance the the piston and connecting rod as a unit relative to the other cylinder. BMW try there best withing a norm . If you want to do better then you must remove son mass on the heavier unit to balance with the lighter one .It is easier to remove some of the extra from the piston underside pin boss with a 3/8" end mill or drill bit.

Remember you are balancing a unit not just a piston .A piston can be off from the other but as (piston/con rod unit) they could balance out.The flywheels are very well balanced from the factory .If you are going to modify your flywheel (removing some mass) you should get it rebalanced by a professional shop that offer this service otherwise it could cause vibrations.
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Old 01-03-2009, 12:19 PM   #7
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Couple years back I put together an all together amazing airhead engine. Too many mods to list them all and the price was, needlesstosay, on the high side. The motor was simply fantastic. Smooth as silk. Rock solid power from the bottom all the way to somewhere above 8000 rpm. Never any sign of stress or fatigue from over-rev although I did unwind an alternator rotor.

Much of the credit for the way she ran goes to the balancing. For sure it was the best money spent. Cost me $200 at Gromm Racing in San Jose. They not only balanced the pistons, rods and wrist pins, which is simple to do at home, they balanced and lightened the crank. Through some kind of calculation beyond my simple mind, they determine an optimum counter weight to the reciprocating mass and shave material from the lobes by a combination of drilling and grinding. It worked, that's all there is to it. No other airhead I've ridden has even come close. I suspect balancing just the pistons and rods will take out some of the normal shake (assuming yours rough to begin with). Much better to go all the way if you can see yourself pulling the crank.

Here are the threads documenting my fun...

Old school power play" (Jinx wrote some really good shit in this one)

Yesterday I pulled out the engine (lots of pics)
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Old 01-03-2009, 01:10 PM   #8
Max Headroom
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Balancing engine components is a little more involved than simply matching piston and conrod weights.

What is at stake here is the ratio between the reciprocating bits and the rotating bits. This ratio varies according to the number of cylinders and the configuration, so for example a horizontally opposed six cylinder engine will have a different balance factor compared to a V6.

The process begins by matching the reciprocating items, obviously the pistons and conrods. Careful accurate measurements are taken to find the lightest item, and the others are then lightened to match. As pointed out by another poster, the conrods are not only matched by their total weight, they are also matched by their end-for-end weight. This is because for balancing purposes 1/3 of the length of the conrod (at the crank end) is counted as rotating weight.

Once the pistons and conrods are match-weighted (including the ring sets and wristpins), the crankshaft is then assessed to establish the weight ratio or "balance factor" between the reciprocating and rotating components. This figure is a compromise based on the fact that there will be vibration somewhere in the rev range due to harmonics and other influences. The major obstacle is that the weight of the piston/conrod assembly can't be counterweighted directly since the crank wouldn't be able to rotate 360 degrees. So the counterweight is offset either side and this results in a compromise. Add to this the "rocking couple" or offset bigend journals in the airhead engine as compared to a "knife'n'fork" bigend arrangement found on some twins, and there'll be vibration somewhere as sure as lil' green apples. The challenge is to put the vibration somewhere on the rev range where it won't matter so much. For example, a vibration at 7500rpm won't matter if the safe working redline is 7200rpm. Alternatively a vibration at 2500rpm is OK if that's not where the rider is likely to be running at a constant speed on the highway.

Anybody can match the weights of reciprocating components. The skill and experience of a balancing specialist comes into play by knowing where the weight can be safely removed from without compromising the structural integrity of the component, and having the equipment to check and (if necessary) correct the balance factor. A specialist will also know whether a particular engine must have the front pulley or flywheel matched for counterweighting purposes, such as some V8 configurations.

The ideal for an airhead engine is to leave the balancing process until last. Everything should be ready for final assembly, with all machining done, damaged threads repaired and worn seal areas tidied up. The list should include pistons, rings, wristpins & circlips, conrods, bigend shells, bigend bolts, crankshaft, alternator rotor, flywheel & bolts, and complete clutch. The balancing specialist will mark the flywheel and clutch for reassembly purposes to ensure they are assembled in the correct position along with the pistons and conrods.

The guy I went to had done the balancing on the Britten racebikes amongst other things, and is very highly regarded. The theoretically ideal balance factor for an airhead is supposed to be around 73%, but once my guy had finished my R90S it ended up at 69%. To get any better was going to involve using a lot of *expensive* heavy metal in the crank or excessive lightening on the piston assemblies with no guarantee of a significant improvement. We chose to leave it there, and in practice I have found it to be very smooth, with much of the shaking below 3000rpm minimised and the vibration at 4100rpm almost eliminated.

To conclude, it has been found that good attention to balancing can extend engine life, as well as making a bike more pleasant to ride and helping to reduce fatigue.

YMMV etc
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Old 01-03-2009, 03:21 PM   #9
R-dubb
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Quote:
Originally Posted by Max Headroom
Balancing engine components is a little more involved than simply matching piston and conrod weights.
You certainly understand the process better than I do.

Seems to jive with what Gromm did to my rig.

Quote:
Originally Posted by R-dubb from "Yesterday I pulled the engine out"

Crankshaft lost 176 grams of metal through balancing and matching to the lighter rods and pistons that are going back in. The balancing shop first measures all the reciprocating mass, down to the shell bearings and wrist clips. If necessary those items are matched by subtracting metal from the heavier of the two. In my case, the Carrillo rods and Venolio pistons were perfectly balanced by the makers. Then the magic begins. They spin the crank on a fancy-ass machine balancing one lobe to the other by removing metal from the heavier lobe. Then they calculate the reciprocating mass on the crank shaft and subtract metal equally from both sides to equal the mass of the rods and pistons, etc. Simple concept, I have no idea how this really works, but the cost was reasonable. $200, and it took less than a week.



You can see the holes drilled on this side to reduce mass toward the outside of the lobe.
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Old 01-03-2009, 06:15 PM   #10
bimmerboy
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Blueprinting

I am new to the BMW rebuilding world but have been doing old car motors for many years.

Does anyone blueprint their BMW motors during rebuild? I know it really helps a v-8.
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Old 01-03-2009, 08:20 PM   #11
Max Headroom
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Quote:
Originally Posted by bimmerboy
I am new to the BMW rebuilding world but have been doing old car motors for many years.

Does anyone blueprint their BMW motors during rebuild? I know it really helps a v-8.

Good question, sir. The term "balance & blueprint" is commonly used but often misunderstood.

To blueprint an engine means to check the dimensions of all engine components and all running clearances with accurate micrometers & dial guages, and ensure that all clearances and components conform to manufacturer's specifications. And no, even a digital vernier caliper is not suitable for this task.....

Often engines are assembled without a micrometer anywhere in the surrounding neighbourhood. The assembler relies on suppliers handing over the correct bearings etc, along with a certain amount of luck. It's a bit like taking a photo with a film camera: there's no provision for reviewing the shot, and one can only wait until the film is developed to see if the photo "came out ok". Blueprinting/use of micrometers etc helps remove the uncertainty and guesswork, and helps prevent unexpected and unwelcome meltdowns.

Ultimately, the responsibility for a rebuilt engine lies entirely with the assembler. Faulty components, incorrect clearances and poor machining can perhaps be blamed on someone else, but these issues can and should be identified, rejected or rectified by a diligent and methodical assembler.

One shop I worked in many years ago had a sign on the wall listing the six golden rules of engine assembly:

1. Clean.
2. Clean.
3. Clean.
4. Check.
5. Check.
6. Check.

YMMV etc
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Old 01-03-2009, 08:29 PM   #12
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Balancing

Blueprinting has as many definitions as people who do it. In the auto world, this used to mean line honing the crank journals, squaring deck heights, equalising combustion chamber volume, etc.

Align honing a boxer motor would be tricky-I don't know where you'd get a larger O.D. main bearing. One thing I like do is to optimise crank to bearing fit, something BMW stopped doing about 1977. This requires some educated guess work to obtain the right bearing and often several bearing changes and trial assemblies. I find this results in a very sweet running and durable motor.
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Old 01-03-2009, 09:27 PM   #13
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This is a fascinating thread and very pertinent information. I think we are onto something here that may focus a great deal of related information.

Thanks for the info.
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Old 01-03-2009, 10:31 PM   #14
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Great info here, but, I have found that most "lumpy, unbalanced, or vibrating" BMW's are because of unbalanced Carbs.
bgoodsoil says his bike is pretty smooth now, so the carbs are probably balanced.
But I would work on the basic stuff first, like valve adjustment and carbs before I pulled the jugs.
If it ain't broke don't fix it.
just sayin
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Old 01-03-2009, 10:43 PM   #15
Max Headroom
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Quote:
Originally Posted by rtw02
Great info here, but, I have found that most "lumpy, unbalanced, or vibrating" BMW's are because of unbalanced Carbs.
bgoodsoil says his bike is pretty smooth now, so the carbs are probably balanced.
But I would work on the basic stuff first, like valve adjustment and carbs before I pulled the jugs.
If it ain't broke don't fix it.
just sayin

And I agree completely. I'm not advocating that folk strip their engines to cure a vibration. Basic maintenance & carb balance has a big impact on how an engine runs.

However, if an engine is already apart for refurbishment, a little extra attention to detail can return big dividends. It's the details like correct running clearances and well-balanced components that differentiate between an "OK" engine and a sweet machine that purrs right through the rev range.

As always, YMMV etc
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