Is it worth it to balance the con rods?

[DonW]

Hi, I have the jugs off my '92 R100GS (83K miles). It will have a fresh valve job, and new rings. I checked the weight on the piston assemblies, and they are dead even, right to left. I'm wondering if it's worth it to pull the connecting rods and balance them. I understand I'd have to balance them not only on total weight, but also make sure the big and small ends also match.

The extra labor, the con rod bolt wrench I need to buy, the new con rod bolts (I read they stretch)... I just wonder if it will make a difference.

The right con rod has had material removed on the tip, and I wonder if that is the factory's balance job from 1992. The bike vibrated above average, but it had bad rings and valves (90 compression on one side, 120 on the other). I'd hate to pull it apart and find out that they are already balanced or so close it was a lot of wasted time and effort.

Thoughts?

Don

[moore84rs]

I balanced mine and at the same time put in light wrist pins. I think the two
things combined made a noticeable difference in smoothness. I can't tell you
how much the balancing alone contributed.

[George Ryals]

At least you would know that it's not the rods causing the above average vibration. I assume you mean above average vibration for an R100 engine. Of course if they are perfectly balanced, you will continue to wonder what is causing it.

I think the R100s are more prone to vibrate because of their heavier piston assemblies and the same "rocking couple" set up with the offset (front to back) rob journals on the crankshaft.

[DonW]

Thanks guys. I'm going to do it I guess. Yes, above average vibration for a R100. My other bike (the ST) actually has nearly the same engine (an R100 motor out of a '92 GS) and it is MUCH smoother. When the ST was still an R80, it was smooooooottthhhh as glass, but I like the extra power of the 1000cc's.

Don

[vanzen]

R100s experienced more vibration primarily due to the fact
that one balance factor was used for every size production engine –
Or so the story goes ... Hence the light wrist-pin theory of old.
IMO, the theory is junk –
there's more to balancing an engine than changing pins or merely balancing the rods ...

IME, some vibration in many R100's can be tuned out,
ie. much of the vibration that I have witnessed in R100s
is the result of poor tuning skills and negligent maintenance
rather than the balance of factory parts.

Beyond that, more vibration can be eliminated, too,
and this will simply be a matter of weighing the cost of investment
vs the real world return on that investment.

The production machines in good nick actually do quite well.

[Chuey]

When I rebuilt my R90S engine, I took the rods to Riddle Machine who is highly regarded in the VW engine world. Actually, I took two pair of rods and had them balanced and they do them end for end as well as matching them. The pistons were done too, with the pins.

Both pair of rods had had some substantial material removed from one rod. That makes me think there was some discrepancy that made it worth having it done.

I really wanted light weight piston pins but Ted Porter said he didn't trust them. Just from the heft of the originals (which I used) I would guess that perfectly reliable ones could be made that are a lot lighter. I don't know what makes me think I would know such a thing but it seems that they are ultra heavy. Next engine, I'll probably go with lighter pins and see if they work OK. Ted Porter was pretty forthright in saying that his unwillingness to recommend lighter pins goes back to some from a long time ago that failed.

Chuey

[Major Softie]

I really wanted light weight piston pins but Ted Porter said he didn't trust them. Just from the heft of the originals (which I used) I would guess that perfectly reliable ones could be made that are a lot lighter. I don't know what makes me think I would know such a thing but it seems that they are ultra heavy. Next engine, I'll probably go with lighter pins and see if they work OK. Ted Porter was pretty forthright in saying that his unwillingness to recommend lighter pins goes back to some from a long time ago that failed.

Chuey

Yeah, I've heard some stories about light wrist-pin failures too. No personal or direct experience, but stories.

A superior wrist-pin could be fabbed from titanium, and be both lighter AND stronger. If such a thing already exists, I think it would be a worthwhile upgrade. Not cheap, but still worthwhile.

As far as the balancing, I wouldn't spend any money at all on balancing unless I was going to have the entire crank, rod, piston assembly balanced. As Vanzen says, the stock motors can do very well if really well tuned. That doesn't mean that the entire assembly can't be balanced to a finer degree than stock, but, if you don't do the whole thing, I don't think you can be certain of any real improvement.

With one cylinder pumping 33% more compression than the other, ANY work to fix that is going to be such a huge improvement that everything else will be minuscule by comparison.

[chasbmw]

There are some leightweight pistons for a variety airhead engines being produced in germany at at the moment, they claim effective increases in horsepower and reductions in weight, W/O having to replace cylinders as well. I had also heard that the later lightweight gudgeon pins had had too many failures.

Might be more expensive than lightweight gudgeon pins, but have 9.5 compression ratios. Motoren Israel website has some stuff listed.

[mattcfish]

I really wanted light weight piston pins but Ted Porter said he didn't trust them. Just from the heft of the originals (which I used) I would guess that perfectly reliable ones could be made that are a lot lighter. I don't know what makes me think I would know such a thing but it seems that they are ultra heavy. Next engine, I'll probably go with lighter pins and see if they work OK. Ted Porter was pretty forthright in saying that his unwillingness to recommend lighter pins goes back to some from a long time ago that failed.

Chuey

Yeah, I've heard some stories about light wrist-pin failures too. No personal or direct experience, but stories.

A superior wrist-pin could be fabbed from titanium, and be both lighter AND stronger. If such a thing already exists, I think it would be a worthwhile upgrade. Not cheap, but still worthwhile.

As far as the balancing, I wouldn't spend any money at all on balancing unless I was going to have the entire crank, rod, piston assembly balanced. As Vanzen says, the stock motors can do very well if really well tuned. That doesn't mean that the entire assembly can't be balanced to a finer degree than stock, but, if you don't do the whole thing, I don't think you can be certain of any real improvement.

With one cylinder pumping 33% more compression than the other, ANY work to fix that is going to be such a huge improvement that everything else will be minuscule by comparison.

How about this source?
http://www.tricktitanium.com/wrist-pins ... wrist-pins

[Deleted User 287]

How about this source?
http://www.tricktitanium.com/wrist-pins ... wrist-pins

A group-buy looks good!