Competive MC-5 125?

[Mike Lenz]

John, You can use either base gascut on a KTM bottom, they both fit.  KTM copied the Sachs top end pretty close except for thr KTM style porting. I would be curious what a Sachs Crank weighs. Chris, Its pretty easy.  I had to motify the intake to clear the KTM cases and accept the 30mm Bing from the 73 jackpiner I used.  I also had to cut the manifold attachment part from the 125 Sachs pipe and reangle it.  The other main mod could be done two ways, the easy way as I did it or the right way! There is much more room in the transfer area on the 175 engine.  I just opened up the Sachs ports and had to add (weld and machine) a little extra gasket sealing area onto the back of the Sachs base gasket area. I also stuffed the crank to try and reduce this area some the easy way(72 and 73 175's had full circle cranks with large holes in them).  I Still wound up with low compression and just milled the heads to compensate.  The right way to do it would be to weld up and remachine that extra transfer area. But mine runs pretty darn good. I can get even the 100 to "pop" a little as Ron Carbaugh puts it.  One other thing, you have to run the KTM 125 clutch complete, outer lightened hub and primary gearing and the all alum plates, and of course an internal ign, at least for MX.

This bike should be legal for the Sportsman class, However I dont think this bike would be top of the line competive for the Sportsman class, maybe if you welded up the transfer area it would run even better. I am also still running the stock 125 exhaust. It still revs a little slow from all the rotating mass of that engine. If you can shift and want to put up with the shifting maintiance on the Sachs engine, it would be a more competive bike for Sportsman class, I just couldnt and didnt want to. As a 100 it will out pull any modified Hodaka I have come up against but it wont hang with the later yellow PV 100 YZ's...now maybe a 76 KTM 125 with a 100 sleeve for the post vintage 100 class...yea!

Other good posts on these issues: 11-4-2002 Rickf22, 11-18-2004 Rob W and 3-31-2005 me.

Everybody thanks for all your help!
Mike

[john durrill]

Mike,
 We used a postal scale. The  A crank weighed 6 lbs and had the knife edge steel rod .B crank weighed 4 3/4 lbs had the I beam steel B rod.
John D.

[john durrill]

Mike,
 We are building a spare alloy 125 A engine to have on hand. Would you like me to weigh a KTM 175 , 1975 transmission with all gears and shafts and a 6 speed Sachs tranny to compare rotating mass? Can do the clutch pack and primary gears for the Sachs also if that would be helpful.
 If I ever decided to go with a Piner instead of the conversion we built I would have opted for the 125 KTM primary and clutch set up on a 175 engine. I always thought that the lower 1st that would have given me would be a better gear setup for trails in a Piner. I like a stump puller 1st gear for my woods bikes. Make life a lot easier when you have to stop and start in unexpected situation chuckle chuckle!
 John & Peter D.

[Mike Lenz]

Well the mystery is solved with the help of John.  The 76 125 KTM crank weighs 7 Lbs 1.5 OZ !!  No wonder why it reved so slow!  Anyone know of a good crank lightener and balancer? I think Teddy Landers knows one.  Not much I can do about the tranny except use the couple of lightened gears I found.  Clutch is all alum except for the outer hub and it is machined thin and drilled already.  I need to lose at least 2.5 pounds on my crank, dont know if that is possible and be able to keep it balanced??
Mike

[john durrill]

Mike.
 We used these folks
http://www.faliconcranks.com/faq.html
 The people Teddy used would be good one also and may be faster on turn around and less expensive. You may not be able to get it down to 4 and 3/4 lbs but anything you can loose would be worth it . 6 Lbs or less with an internal rotor ignition  ought to be good
 John D.

[Mike Lenz]

Thanks John, and all for your help. I will let you all know how she comes out.

[john durrill]

Mike,
 There is one thing i think we have both thought about but have no answers for. I think there is a point where you would get the crank too light to work on that engine. The differences between the reciprocating mass of the cank assembly  ( all parts turning on the crank shaft) and the transmission assembly ( both shafts with primary gear and clutch pack) probably has a sweet spot. Get the crank assembly too light and it would have problems spinning the tranny. The engine could be prone to stall.
 Wish i knew enough to figure that one out.[] Let us know how this project works out. What we learn will help on down the line on engine changes of any type i bet.
John D.

[Mike Lenz]

I always thought the clutch and tranny spinning mass would have just the same effect as the crank itself.  What makes it different??  Wouldnt total spinning mass weight be what matters from a stalling standpoint?  However I have NO engineering education.  What am I not realizing here?

[Ernie Phillips]

quote:Wouldnt total spinning mass weight be what matters from a stalling standpoint?

No.  Moment of Inertia (product of mass and radius squared which relates to rotor weight and geometry) and rotational velocity (RPM) are what control the Kinetic Energy (flywheel effect).  The Kinetic Energy of the gearbox is nil because it has low I (moment of Inertia)and little rotational speed.  Even if the gearbox had the same I of the crank, it spins at much, much slower speed (reduction via primary ratio).  And, since Kinetic Energy varies as the square of the speed you don't have anything.  Removing material from the largest diameter of the crank will produce the greatest effect in reducing I.

Another alternative: Assuming that the 125 MC-5 has a heavy external rotor flywheel you could replace it with an internal rotor PVL and add some zip as well as an improvement in reliability.  

Balancing:  There is always unbalance of a single cylinder crankshaft assembly (crank, rod, pins, piston, bearings).  You can counter balance to reduce the shaking forces produced by the reciprocating piston for a given speed.  The price you pay is increased lateral vibration (force) when the crank is either 90 degrees before or after TDC.  You can go for zero force at TDC/BDC (0/180) and have large lateral force at 90/270 degrees or go for zero force at 90/270 and have big force at 0/180 or go for a compromise balance.  Refer to your Penton Six-Day Maintenance Instruction Manual page 41 for an example of a "compromise" static balance procedure.  For the Sachs engine the manual recommends balancing for 80-83% effective weight of piston, pin, bearing.  You could set-up you KTM crank assembly as shown in the drawing and baseline you factory balance percentage.  After you lightened your crank, rebalance to that percentage and you should be OK.  Or not - I've never done it ... so good luck.



Ernie P.
Chattanooga, TN

[john durrill]

Thanks Ernie,
 Mike, Ernie helped a lot with that. I was thinking of the time span between braking and acceleration
The lash between the gears selected ( primary and tranny ). The mass on the tranny and the crank would be separated for the time span between throttle off brake on and throttle on. I was thinking low speed tight one track with twists. Maybe the engine would stall quicker in that situation .
 From what Ernie just posted its another one of those things i just dont know enough about chuckle chuckle! hope I can stick around a bit longer and learn a lot more about Dirt Bike engines.
 Thank you Ernie