Balancing the Crank

from the POG Message Board

After seeing this posting (made on  09/07/2013) on the POG message board (under Penton Racing Talk), and seeing the response to it, I decided to use it for this issue's Tech Tips to share it with our members who do not use computers or use the internet. This is good information as to what problems you can run into if you use big hammers and not the proper pullers and tools when working on engines. Also when buying that non-running bike or engine, this is one of the problems you can encounter when someone else did not use the proper tools. Thanks to the POG members for sharing this information.

Alan Buehner

I have two penton 125s and one of them, my race bike, I recently put a new forged Wiesco piston in it to replace the Mahle that was there.

I noticed that the other 125, though low in cylinder compression (since it is still in it's standard bore), is a much smoother sounding motor than the other with the new piston in it. To describe the motor with the new Wiesco piston; it has these cyclical or eccentric motion vibrations in it. Hum……??

POSSIBLE SOLUTION: If you look on page 41 of the Sachs powered workshop manuals provided by Beuhner's Supply, you will note the 4 steps in "Balancing The Crank". On the first step, you will see that the piston is included when determining the weight of the assembly. Therefore, if the Wiesco piston is lighter, as it feels, the assembly is no longer balanced, thus a source of vibration when the motor is running despite other good functioning parts of the motor….

QUESTION: Those of you who restore motors, do you re-balance the crank assemblies if another brand piston is installed and the weight is different???

Let us know your comments.

Peter Klinck

After reading page 41 in the Sachs manual, this is a simple & practical way to balance the crank shaft. This is also be a very time consuming process. I am sure the engine may run slightly smoother with this done but usually the different piston weighs only grams apart.

When rebuilding\restoring an engine I am mostly concerned with the runout between the 2 crank halves and not the "balance" of the crankshaft. The closer to 0 runout the less vibration the engine will have. This is usually the main source of engine vibration.

Jerry Birky

You can not really "balance" a single pin crankshaft in the literal sense. A Boxer opposed twin pin crank is the easiest because the reciprocating mass (rods/piston) completely offset and counterbalance each other and that allows you to "Zero" balance the rotating mass (crank wheels/snout). The single pin crank (whether a single cylinder or a single pin twin like a Harley) can not have the rotating mass of the crank zero balanced because the reciprocating mass is constantly in a state of imbalance, not being offset by an equal and opposite mass. What you can do is "tune" the single pin assembly so that you get the least amount of harmonics (vibration) in the most often used RPM range. That is why a Harley engine vibrates so much more at idle than at highway speed, the crank is balanced to run smoothest at that speed. In the Penton/Sachs example, the small difference in piston weights will have minimal effect and what Jerry Birky mentioned, runout of the two halves, is the most important thing to fine tune.

Brian Kirby


You have hit on a topic that is quite interesting, please excuse my long reply..

"Back in the day", the local Penton dealership initially was owned by Kupec Cycle owned by Fran Kupec, Fran eventually sold the dealership and it was picked up by an engine building and machine shop, J & D Engineering in Valencia, PA. The reason they picked up the dealership was to supply their own needs for motorcycles, as they were all weekend riders, as well as to sell some machines, like many single brand shops of that era. What they didn't realize at the time is that I went along with the sale….

The guys at J & D did tons of balancing of cranks for every sort of engine. The guy who actually did the cranks was Dunk Pacdozi, if any of you were into dirt late model racing "back in the day" you probably remember the team of Dunk Pacdozi and Bob Wearing Sr., their accomplishments are legendary in the tri-state area. The other specialty at the shop was cylinder head modification, they really understood the connection between cylinder head flow, proper engine balancing, compression, etc, etc, to get maximum engine performance along with durability. The link below will take you to a very nice write up on Bob wearing Sr and Dunk Pacdozi.

It didn't take long for these guys to turn their talents to my Sachs powered Penton, just for the enjoyment of doing it, as well as a way to promote the shop as a Penton Dealership. Their opinion of the balance job that was done on the Sachs engine from the factory was not very complimentary.

Turn the clock forward a few years to the ISDT Qualifiers in Oregon, between the Trask event and Bad Rock, we parked the Cycleliner out in the boonies along a little used road to finish prepping our machines and sort of kick back for the day. Dane and I were working side by side finishing minor details on our Pentons, soon we were both done and we fired our bikes up for a few "test runs". Well, boys will be boys, and soon we were lining them up for a blast down the road, Dane was on a 100 and I was on a 125, but Dane knew how to make a 100 run and knew he should give a "normal" 125 Penton a run for it's money, he was surprised at how my 125 consistently pulled away from him. I let Dane ride the bike and soon he had lots of questions.

My engine was worked over, including some very nice port work and a little extra compression, but the thing that intrigued Dane , and very shortly afterward John and Ted Penton, was the balancing of the crank. After some calls and information was exchanged between the folks at Penton and the guys at J & D, along with some inquires from Penton to the folks at Sachs, John hand carried one of the balanced cranks from J & D back to the factory so that they could see the difference for themselves. Those engines with the balanced crank just seemed to rev out farther, and were so smooth it was incredible. If I remember right, the balancing removed some mass that allowed for some quicker revs as well.

In 1973 we did my ISDT engine ourselves, and I have always felt is was as good, if not better than the "D" engine that came in my bike from the factory in 1974. Interestingly enough, at the ISDTRR a few weeks back, Jack put me on his Carl Cranke built 250 GS 6 on Sunday, before I rode the bike I figured I might have my hands full starting the bike and keeping it running at a leisurely pace, man was I wrong…. Just jab the kicker and it lit right up, and pick just about any gear and it would cruise along, it could almost fool you as to the power it had until you put it thru it's paces, but it was very usable power…not just a big hit that would bring on wheel spin, it reminded me of how my Sachs engines ran many years ago….smooth and strong.

Sorry to ramble, google dynamic balancing of crankshafts for some good reading, and even watch them do some on youtube, and if you get someone to dynamic balance that Sachs crank I will be curious to see what you think. Keep talking to shops about what you want to do until you find one that is experienced in dynamic balancing of single cylinder cranks, you will know the right guy when you find him.

I would also like to say that I personally am not an expert on these matters as far as doing the work and being able to provide details of the operations, but I was very fortunate to have been the recipient of the efforts of these very talented and ingenious folks and to whom I will always be deeply indebted…I just loved to ride, and they just loved to tinker…

Paul Danik

I finally found out the problem and subject issue associated with single cylinder crank vibration on my 1973 penton 125 with a Sachs B motor.

1.) I started to disassemble the motor. I was just about to separate the case halves, tilted the bottom end on it's side and heard a sound… I grabbed onto the crank spindle and found lots of lateral play, say .030 approx. although there was all new bearings in the motor on a recent rebuild, this was too loose and was a cause of the vibration. Lucky the bearings were new and so did not have enough movement up and down and thus did not ruin my new MZB ignition. THIS LOOSENESS MAY HAVE BEEN CAUSED BY POINT 2 BELOW.

2.) I then broke the cases down and took out the crank assembly. A couple of weeks later, I took the crank assembly over to a garage and put the crank on a metal lathe. In fact, he had 2 lathes so we began to test and check the crank spindle straightness or run out, etc. The following results were;

aa) Mag side spindle (from the more true lathe)

  • measured on end of crank next to threads: .0085 thousands out of round
  • mes. just on the inside of the flywheel: .007 thousands out
  • mes. on the inner crank bearing race: .004 thousands out of round

bb) Clutch gear side

  • .00050 thousands out. this was ok

Conclusions: I think the crank spindle on the mag side has or had been bent. SO I believe it is now not of worth?

Question: Is it not a basic bench test when a new rod assembly is replaced to check on a simple lathe whether the crank assembly is straight? I suspect so….

Let me know of any comments on this subject.

Peter Klinck

I've seen a great deal of crankshaft spindles ruined by improper use of a large hammer when pulling a flywheel, even with the correct tool without the spindle nut installed during the process. Without the nut installed loosely, the shaft tends to deform and bend. The problem with Penton and KTM pullers, is that there is no room for the nut to remain in place.

Michael R. Winter

My three cents worth on the subject! Typically each crank half has two center drilled blind holes, one on the shaft end and one on the flywheel end. Very frequently the one on the shaft ends are damaged by puller tools, hammers!, … If the crank is being trued up by holding and spinning the crank on these end holes between centers on a lathe or truing stand, and one or both are damaged, the run out readings will be false, leading to a crooked crankshaft rebuild. When the crankshaft I am working on is pressed apart, I always check each half between centers for trueness, which will uncover damaged center drilled holes. I can't say I have ever found the actual shaft portion to be bent, it is almost always a damaged end hole giving false readings. I have developed a technique using an eight or ten flute, 60 degree carbide counter sinking tool in a drill press, and holding the crank half in my hand to "fudge" the end hole back straight and centered, and yes, I said by holding in my hands!! Sometimes it takes many, many tries back and forth between the drill press and the truing stand to get it right: 30+ minutes once!, but the results are worth it, and the only way to fix these old dirt bike parts.

The first Penton 125 6B motor I built, with a oem piston vibrated like a washing machine, so bad I asked a racer from the day, and he commented that they were like that. The second motor I built used a much lighter, modern Wiseco piston, and the engine was much smoother. Since single cylinder cranks can't be 100% balanced, engine with heavier cranks are smoother than those with light cranks, 6B being the lightest Sachs crank! The modern made forged Wiseco pistons from Al Buehner are much lighter than the oem Mahle pistons. Hope this is of some use to Poggers!

JP Morgen