Crank balancing

[Lew Mayer]

I have a shop question not asked before and I'm sure Mr. Doug can probably help out here. Is the page on crank balancing in the Six-Day repair manual still applicable today? I may try the procedure. You're never too old to learn new tricks.

Lew Mayer

[Merlin]

.......I'll guess you are referring to truing rather than balancing,I have not reviewed the service page you are asking about however I have rebuilt hundreds of single and multi cylinder 2 stroke crankshafts with success. The truing method of putting a crankshaft between centers is flawed,the reason is when installed the crank assembly does not run on the centers. The best way to true a crank is by using the main bearing journals,this is where the crank rides when installed. You may use a simple double "V" block setup or clamp one end in a lathe chuck to check for runout............

[tlanders]

I have dynamically balanced the engines on my mint 400 to reduce vibration. It works.

Teddy

[Lew Mayer]

Merlin, the section is on balancing the crankshaft. It's on page 41 in the section "working on individual parts".

Lew Mayer

[Big Mac]

Ted, who did you have balance your 400 crank? Cost?
Haven't found anyone who works on old bikes who believed in it.
Thanks,

Jon McLean
Lake Grove, OR

[Merlin]

.......O.K.,I reviewed page 41(actually 40) and it is static balancing,while useful to get an engine severely out of whack not to vibrate like a supercharged washing machine on 3 legs in the spin cycle, it is not dynamic balance as refered to by TLanders. Static is just that,at rest,it does not take in to account the speed of operation as dynamic balance will however be cautioned,in a single cylinder engine all balance techniques are a compromise due to the single power power pulse per revolution..............multis narrow the pulse width of compromise........

[tomale]

So what is... dynamic balancing... what I mean is how is it done? How do you manage to spin the crank rod fast enough with out damaging the rod or with out the rod effecting the results?

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (finished project bike)
72'sixday (project bike)

[Merlin]

........static = stationary, dynamic = in motion........dynamic balance in a 2 stroke is possible but can be problematic and pricey..............

[Ernie Phillips]

I have access to a dynamic balance stand (hard bearing).  If someone can provide guidance on how to dynamically balance a crank with a connecting rod attached ... I'll give it a shot.

Ernie P.
Chattanooga, TN

[Kip Kern]

Pack it up, add a rod kit, send it to Forest Stahl in Yorktown, IN. and it will be rebuilt and balanced correctly and mailed back.  He can't be beat!;)  Advance Tool, Forest Stahl, 765 284 7653 CIAO

[dkwkid]

I have an old Cycle News article about engine porting by E.C.Birt.In the article he states that while all engines run a little rough Sachs engines are a little rougher than most.At the time he balanced the motor(exactly how he did not state)and he also welded the center of the Bosch magneto to the outer part. They had a nasty habit of breaking loose over time because they were swaged together. I actually have a late model Bosch mag that is riveted together.
 Edlebrock used to balance 2 stroke motors back in the day.I wonder if the Sachs factory did anything special for the 6 Days motors(the event-not the bike).Did they run the GS or stock? another trick I have come across over the years was to run a KTM rod which is wider at the big end by machining the crank halfs inner.

[john durrill]

Lew , Merlin ,
 Here is what we found out on Static balance on a 2 stroke when we went with the 175 kit. It answers the question that might come to mind in dkwkids post .  This is a repost on a 2002 POG message thread.

 Mark,
I don't think you would need to worry if the rod is a little heaver than the NOS ones.
Here is what we found out from the 175 conversion on the 125 Sachs lower end.
The Sachs and Penton engine repair manuals have a balance factor of 80 to 83 % listed as the thing to shoot for.
Knowing the 175 Piston was 1 and 1/2 ounces heavier than a Sachs 125 piston we thought that weight would have to be added to the A crank we used. We contacted Falicon and went through what we had and what we needed. They recommended that a 60 to 70 % be used when building our a crank. That is a very big difference so we double checked with Martijn in the Netherlands on what his 125 Sachs Road Racer was balanced at. He confirmed what Falicon had told us. He is using a 66 to 70 % balance factor.
We went with the balance factor Falicon recommended. Over 2 and 1/2 years the crank has worked fine. No difference in vibration over the stock 125 Sachs or the 175 KTM engine in my sons Piner.
Anyone else that has more experience with cranks or some better idea's Please offer what you know here. Doug , Ron, Bobby , Kip , anyone please chime in here. This is an important Bit of knowledge that everyone will gain from.
John D.

[Merlin]

..........I think some of the responses posted here may be confusing  truing and balancing.........

[Lew Mayer]

John,
now that's what I was looking for. I saw that the SixDay manual called for the 80 to 83% balance factor. I appreciate the info on Falicon using the different balance factor. They are certainly experts in this field. You answered my questions. Thanks John.

Lew Mayer

[tlanders]

Sorry I didn't come back to this post. I have been welding freeze out plugs in the big holes on the counterbalance webs of the crank to "stuff" the crankcase a little. Then I take it to a race car machine shop here in West Plains, MO and he takes the crank apart, weighs the rod and piston with rings (with a little oil on it to simulate real conditions) and clamps on a weight of the exact same weight on the journal. He spins it on his dynamic balance machine at whatever rpm I say should be max/optimal and drill holes in the counterbalance webs and fills them with material that is either heavier or lighter than steel to get the whole assembly dynamically balanced. Hood Performance Machine, Allen Hood does it for me and he charges $175 for the work. Phone (417) 255-9950.

Teddy