R80 Piston Question – motor with different sized pistons

Hi all

I’m an airhead noob and have acquired a project bike that was once quite nice but was left by the ocean for 7 years which took its toll. I am in the process of building a café racer from it as a restoration wasn’t realistic. The bike wasn’t running when I got it and I’m now up to the engine rebuild stage.





I have stripped the motor right down and have found the usual wear and tear, however I have a slightly unusual situation with the pistons and barrels, namely they are different sizes! The two barrels and pistons fitted to the bike are as follows:

LHS: 84.775mm piston with a “B” stamped barrel

RHS: 84.765mm piston with a “A” stamped barrel

I’m told by the previous owner that the bike ran beautifully when last running (he had actually spent a considerable amount on it prior to importing it to Australia) and never mentioned a vibration etc. which I believe.

I haven’t come across this before and maybe this is something considered acceptable, I don’t know, but I guess my options are:

(a) run them as they are;

(b) have the pistons matched by machining the heavier one to match the lighter one; or

(c) buying another barrel and piston the same size as one of the existing ones to make a matched set.

Bear in mind that this project is running on a shoestring budget and I don’t have cash to burn…

Any advice/guidance is greatly appreciated!

Charlie

The difference of 0.4thou is negligible.

I would simply weigh the pistons with pins and see whether there actually is a difference in weight between the 2 assemblies.
If the pistons are the same form the difference in mass should be about 0,02%... As also the difference in displacement.
I would not worry about this...
Nice build you are doing! Looking forward to more pictures of the complete build.

Hi guys, thanks for the advice and suggestions.

I have weighed the two pistons and got the flowing:


LHS – 411.9gms


RHS – 409.0gms

Therefore a difference of 2.9gms or 0.704%. AlpineRAM, this is more than the 0.02% you suggested as being acceptable, in that case should I consider lightening the heavier piston by ~2.9gms to bring them a little closer?

Thanks again

Charlie

I would weight balance them to less than a gram. Do this with the pins in and the new circlips just sitting down inside.

Balance the rods to within a gram too...but weigh the rods before you remove metal from the pistons. You want both sides within a gram and at this point you can't go switching parts. Doing the balancing on the rods is a LOT easier. just grind on the parting lines. Lotta extra metal there and steel weighs more so you remove less. Replace the bolts every time. Plastigauge the bearings (Cheap, use the old bolts) I wouldn't go for tighter balance than this, carbon buildup will throw it off pretty quickly. It's a touring bike, not a (real) race bike.

Strip the head, measure the valve stems and eyeball the sealing surfaces. Mix up nothing. If it's pretty good and you're low on cash, lap the faces lightly and put it back together. If anything is weak, this is a place to spend some $$.

Inspect the cam followers. Mix up nothing. Check the push rods for straightness, mix up nothing, including end for end.

Replace the pushrod tube seals (treat yourself)

As far as some go to go with the show, dual plug it and bump the compression. Even if you don't have cash for the ignition stuff right away (you can make up the wires cheap, but you need coils) get the drilling done (12mm) while the head is in hand.

Then check clutch and oil pump (block the crank) and crank endfloat.

Leave the timing chain alone unless it was really noisy. Easy to do some other time engine in.

Clean up the starter, esp. the elec. connection. Cheap insurance.

Check the alternator brushes. You can run them low, but not out. Clean the rotor commutator (crocus cloth and maybe a dental pick in the grooves). Reinforce it if you are planning on a lot of ye-haw on the thing. Thread in tips & tricks on that (by me).

Transmission inspection next, and the rest of the driveline.

Plan on a fork rebuild if it was sitting long, possible some brake work too. Put it all together then evaluate. Clean and grease wheel bearings, steering head bearings and swing arm bearings. Grease is cheap. Get all the preloads set right.

I suggest reading up on balancing rods. You'll find there is a lot more to it than just matching up the weight of the rods. After you read up on it and understand how it is done right, you can make the equipment to do the job but it takes a real good setup. Even with a real good setup the job is very subjective. With a shaky setup the job is about impossible. BTDT.

I didn't say that .02% is the max. acceptable difference, I just calculated the displacement and volume difference due to the different bore.
Please do weigh the pistons with their pins and the c-clips. You can usually remove enough material from the inside of the pin with sandpaper on a dremel.
(But maybe the guy building the engine before you already did this?)
As it was stated a difference of less than 1g would be nice.
As supershaft said the balancing of the rods is a good bit more involved, basically it is about mass distribution and inertial moment that should be matched...
As for the rest that Plaka said- yes to all, clean and inspect, new seals and gaskets, maybe new rings..

Cheers!

Mass displacement? Inertial moment?

Uh...no. The rod only moves through a very small angle at the big end and this decreases to no angle at the small end. It's essentially a reciprocating mechanism. If you're removing say 2 grams (which I did recently to one of my rods, the pistons were already as perfect as I could measure) there is no place on the rod where you can remove so little and have a measurable inertial force except in line with the rod---and for that it doesn't matter where it is*. The overall inertia of the rods/pistons system should match side to side for the best smoothness. You still end up with a rocking couple but it's built into the engine design, nothing to be done. Anyway, balancing the rods is that simple. Grind on the parting lines, evenly end to end, and on both sides. This minimizes how deep you have to go anywhere.

I was figuring the slight difference in displacement would be taken up with carb tuning. You do that anyway as carbon deposits. And then you need to match the combustion chamber volumes to account for variance there. Not worth it except for a very high performance (and frequently overhauled) engine.

If you replace one I think B is the popular size. But matching wear is as important as volume. You want the weights to balance well and the compression to balance well. This leave the minimum to do with the carbs.---ideally you want the same jetting side to side.



I also recently did some mods to my advance weights. I drilled lightening holes in them. These things move through a huge angle so the radius of the holes from the center of rotation is critical to the change in inertia of the weights. I chose to do all holes (2 each) at matched radii. There are some advantages to not doing that but I wanted them balanced for the moment. They wee well matched for weight and well enough made for the inertial/mass profile (essentially the section at any given radius) to be halfway matched as well. I only took off a gram per, but I took it off out near the tip where it will matter the most. Dunno what it will do, should stretch the curve somewhat.






*if it isn't obvious, the bit about moving through arcs is about angular momentum, or acceleration.

Imagine a long metal rod sliding back and forth in a long well oiled tube. It's inertia is a product of it's mass and it's momentum is a product of it's mass and it's acceleration. Starting it moving, then stopping it and getting it going the other way requires a lot of energy to overcome that momentum. Making it light reduces the inertia and thus the momentum and thus the energy required. But internal combustion engines are still very inefficient because of this. The wankel was an attempt to get around this. Had seal problems. Turbines do get around it at the cost of being rather thirsty. Inefficient use of heat. Anyway drill a hole in one end of your reciprocating rod. Less inertia because it's lighter so in turn less momentum. But it doesn't matter where along the length of the rod you drill your hole. It gets lighter by the same amount anywhere you drill. it's momentum is strictly a matter of weight and the energy required to turn it around is strictly a matter of the accelerations involved.


Now take the same rod and mount it on a pivot pin on one end and swing it around in a circle. Unlike moving in a strait line it is constantly accelerating, even if it is swinging around at a constant speed. because of this is is constantly exerting a force on the pivot pin (so-called centrifugal force, actually it centripetal..), has to be constantly restrained to keep it from flying away and thus requires a constant energy input. THEN if you stop it and get it spinning the other way it takes even more energy to do the reciprocating just as the rod in a strait line does.

Now start drilling holes in it. Drill a hole near the pivot point and the weight decreases along with the momentum. However the distance of that now missing bit of weight from the center of rotation determines how much you decrease the centripetal force. Drill the same hole farther out and the centipetal force is changed far more. That hole is moving a lot faster farther out!



On a connecting rod the big end rotates first one way and then the other on the crank journal. But the angle of rotation is very small. Drill a hole in it near the crank or 2" away and the change in angular momentum is going to near unmeasurable. The farther from the crank you get the smaller the angle the rod turns through and the effect goes from unmeasurable to REALLY unmeasurable. However no matter where you grind, the rod does get lighter and its momentum decreases. This is what you are after.


I advocate grinding the parting line because there is excess metal there and it's easy and to go evenly end to end and on both sides partially to avoid creating notches (stress risers) and partly just as a matter of craftsmanship.

The bottom end isn't splash lubricated so the only real reason to mirror polish them is for strength.

The amount of bad information here is amazing:eek1

Just take some material off of the heavy piston until the weights are even, or very close.

All the other stuff involves going down a longer, complicated road.

Instead, get your bike running.

Agreed. :eek1:huh

:huh

Um... no. That's just not right.

Big ends rotate continuously in the same direction in relation to the crank journal. Are you thinking of the small end and it's rotation on the wrist pin?

What you were describing in your rod balancing explanation is actually inertial moment aka moment of inertia - not centripetal force.

+1 :eek1

Hmmm... did you read what I wrote before you answered? I don't think so.
I stated "mass distribution" and meant on the rod. And the rod moves through the same angle over it's length.. hopefully. The length of the arc differs with the radius.
And surprisingly some folks do balance their crankshafts, and then do also balance their conrods- simply put - Mass difference on the big end=Crankshaft imbalance, Mass difference on the small end=Piston imbalance. So that is why it does matter where you grind.


Ehmmm and your understanding of inertia and energy consumption by inertia is rather spectacular.


Please keep me informed on how your build performs with the modified ignition. (You go precisely the other way than everybody else on the ignition advance curve with your lightening of the flyweights)- Did you also increase the maximum advance?


Cheers

Yeah, inertia consumes energy. One of my greater engineering failures was a bicycle trailer I built myself. I sized it to carry a VW engine or a stack of plywood. I used lots of fancy geometry so it was very light. I put lower gears on my bicycle anticipating the power required. What I found out was that pulling the thing with a full load up any sort of grade took a whole lot more than a bicycle rear wheel was capable of, the spokes simply snapped. I could have gone to tandem spokes but it still took so much power the bicycle simply stood up (zero speed wheelie) rather than going anywhere. I shoulda computed it for real before building. In the end I had fun but it went to a landscaping guy that really just needed the capacity for bags of leaves.

Inertia can take a whole lot of energy---as soon as you move it.

You explanation BTW is completely superior to my very sloppy and long winded one. But does it pass the does-it-matter test? Look at the counterweights on your crankshaft (which is pretty well balanced, stock) and then look at the big end of your rod....Maybe if you take a couple grams off the low shoulder of the rod and the cap you could affect crank balance but again the radius of rotation is a lot less than the counterweights and the mass is significantly less as well. How many grams of oil do you have in/on there at any time? If you are building a super high RPM race engine you spit all the hares. For a low RPM pushrod twin???? It gets lost in the noise.



No idea what the curve will look like on the reworked ignition. I don't know about "everybody" but I do know that people that know what they are doing lighten the weights so if I want to seem like I know what I am doing....

I did the heavy springs routine on my /5 for the same effect.

The idea is that at a given RPM the lighter weights will not exert as much force against their springs so will not move outwards as much, so less advance. This extends the advance curve. You need more RPM to get the same movement out of the weights. Essentially longer and flatter compared to stock, not as much advance at any given RPM in the stock range and then not reaching full advance until a higher RPM.

If I don't want as much full advance whenever the thing tops out then I turn in the stop screws to limit the max weight travel. It's set at stock for the moment. You get to the stop screws through the service window.

There is no provision to extend the max. past stock. That's doable but tricky. What I could do already is limited by the clearance inside the bean can for my stop screws (when backed all the way out). however I can't see why I would want more max. advance, the setup of the engine (dual plugs) is to use less.


I have a stock ignition installed and I'll map it when the engine is running. Then I'll put in the modified and map that. My suspicion is that I have not lightened the weights enough. But what I did I kept in the realm of the easily reversible. I can drill some more small holes but past that I have to make them thinner, and that is complex to reverse. At that point I would probably just build new weights out of lighter materials or construction.

I'll post results in the build thread for the ignition.

I thought about answering Plaka, but then I thought a little more.... and won't do it.
To the OP-

did you weigh the piston pins too?
The weight of the complete assembly (piston,rings,pin,c-clips) is what you want to be the same on both sides. Take care that if you have to adjust the ring gap you remove a little bit of material. (so do this first and weigh the assembly before adjusting the weight)

Cheers!

I've been following this but sorry I have not read some of the longer posts. Home static balancing is a worth while endevour. It's mostly straight forward stuff. You can get down to a balance factor that is silly and when you can't take off a small enough bit of metal with out swinging the scale the other way it is time to quit.

About balancing rods; There may be more involved methods used by professional shops that do this but there is a traditional way to do it at home. Sorry I don't have a picture of this but the idea should suffice. The rod has two ends, of course. The rods should be balanced to each other, the same. And their small ends should match and their large ends should match. This can be done with string but it is easier to make a board with a peg set in it to hold one one end of the rod. The small end is set on the peg and the large end is rested on the scale. Then the large end of the rod is set on the peg and the small end on the scale. It helps if the peg is moveable so the set up can position the rod parallel or level to the ground. When the two small ends match and the two large ends match the whole rods should also then match, but check.

There are so many other things a professional shop can do for you including sizing the large ends, they do get out of round. And they can also do dynamic balancing which is maybe controversial. There is a shop not far from me that does this kind of work but I have not had the money to try it.

This thread is way to anual - Clean it up pay close attention to the heads / valves, new rod bearings, push rod tube seals and run it!!!


OR - are you going for 100+ horsepower???

If going for 100 horsepower please heed all the advise in this thread and then seek help!!!

Wait a minute. This is the bike with one A cylinder and one B cylinder?

Whereas home static balancing is ordinarily worth while. In this instance I wouldn't bother. Unless you did something about the imbalance from different displacement. Since that is likely to be more involved or more money than the old bike deserves I don't think I would try to balance this engine.

Sorry I was caught in the fervor of the moment.

methinks that won't work. With the big end on the scale and the little end on a free pivot. you will know the force on the scale but you won't know where along the rod the excess mass is. It could be a whole lot down by the little end doing the damage or a small amount down at the big end.

You could lay the rod flat across a knife edge pivot (a utility knife blade in a clamp will work) and then find the balance point of the rod. Mark it and do the other. Measure the marks from a Piston pin inserted in the small end. Look at this measurement on the heavier rod to see which side of mass center the extra weight is on. if you find there is no identifiable heavy end, then assume the weight is completely distributed. Grind the whole parting line shoulder to shoulder and repeat the weight and balance check. if the extra weight is towards one end, grind that half only.

You can also check just the caps+bolts against each other and balance those, then go to work on the rest of the rod.

What you are doing is splitting differences to get under what matters for the application.


It is worth bearing in mind that our OP (without wanting to offer too much insult but somebody got to call him a bonehead*) got on a badly crippled bike and rode it around for fun, and is now looking at a pretty screwed up motor. Seeing as how the pistons are pre-knurled and given it's hard to wear out rings in 40k, even with some cylinder corrosion, I'd put it back together and run it. The finer points of balancing are not of concert to this particular user. I warrent the grosser points aren't of concern either.

There were a bunch of things worth doing to evaluate and resolve the compression issue without ever tearing into it. These were not done. The compression was so grossly imbalanced that wasn't likely to be any sort of ordinary wear. The cleanliness of the engine certainly pointed to anything else. I'd be looking at all the artifacts that affect a compression test; carb tuning, gauge technique, gasket leakage, valve timing, battery, etc. etc.. if all those check out I'd run a solvent strait into the cylinder to clear a possible stuck ring and then change the oil. All that kind of stuff BEFORE tearing it down.

The mismatched cylinders speak of a checkered past. Maybe an accident damaged one. Maybe the mismatch was a good exchange for getting a single replacement with similar mileage, maybe it was just sloppiness and expedience and maybe it was ignorance. But what is known is one side doesn't have the same mileage as the other, and it could be either one.






*that said, I've done it myself. My first bike was a T500 and with a dead battery you could kick start it and it would run on one cylinder only (of 2). Dead battery one morning, had to get to work and it was all I had. Started it and went. But it was an oil injected 2 stroke and I knew it would actually be getting better lubrication on the dead cylinder.