I am building a 76 125 MC-5 for MX. It will primarily be a "fun" racer as I am WAY heavy for a 125. However 125's are so much fun I want to run one...for fun. I also want it to be a Penton and a PV bike. So that leaves me to try and make that heavy motor go. I have the lightened clutch basket and alum inner hub and plates that came on that engine, an internal roter Ign and I found some gears that were lightened. Im also going to run a smaller chain and 1980 125 wheels. What Im wondering about is the crank. Nobody wants a slow reving 125 for MX and from what I remember this engine revved pretty slow? Im wondering if I should try and have the crank lightened. Anybody out there had any experiance doing that type of thing, or have any other ideas?
Mike, The 76 125 motor was very hard to keep running. Lots of issues.
I know of a couple of the enduro bikes that were thrown into the dump with hardly more than 30 miles on them. Broken cases, Siezures, etc. Best to keep this motor as a collectors item.
Dwight
Do you know why it was hard to keep running? Same basic engine as all the others??
I did a bit of searching on the site and found a post that thought the stock compression was too high. That would maybe explain the top end problems. I have a set of 125 cases that are cracked. The cases are the same as all the others so mine may have been from improper assembly, however early 250 and 400 cases did crack on the mc-5 at the left engine strut before it was redesigned. Maybe the case story was do to that?
Mike,
I never tried to make the 125 KTM engine light, but I did ride the bike for some time, and never had any problems. No seizures, broken cases, etc. I don't know where Dwight found all that stuff. The biggest difficulty at that time, was making sure that the Lectron carburetor, was "jetted" right.
My brother still has my 76 ISDT bike that is a 125 now. (was a 175 for the Six Days)
Dane
I worked for Butler Motors in Jackson MS and know Don Burgess who owned Competition Cycle in Kenner, La. In fact down south I never saw one run more than a few miles. I have heard more stories of the 1976 models unreliability than of those who finished races. I heard of no problems with the 1977 models.
In fact didn't I see you with a KTM 125 top end on a Sachs bottom end ?
Dwight
I have to agree with Dwight on the 1976 125 (KTM engine)we had numerous problems with the 1976 model---- seized, blubbered, hard to start etc. It ran poorly compared to the earlier Penton/sachs 125 engines and was not reliable. We ended up getting rid of it after a short time (3 months)and moved on with the mc5 250 and never had problems with the KTM 250. Never paid much attention to the KTM 125 model after that to find out if the bugs got worked out, luckily I kept the 75/76 Penton 125 with the lay down suspension and the sachs D series motor, which still runs great.
thom coles, paw paw, michigan
I have seen a few 76' 125 KTM engines with Sachs top ends on them. In an old Keeping Track there is an artical about a Carl Cranke built Penton 125MC with a 100cc Sachs top end. I am interested in this thread because I currently have a 76 125 enduro. I finished restoring the bike but carb issues have kept me from running it.
Chris
Hi Mike, I say FINISH THE PROJECT. I respect the opinions of both Dwight having problems and Dane not having problems. The bottom line is your satisfaction with what you're doing, AND it would be a cool bike to have [8D] When you're finished if you don't like it, sell it and do another project!
This group is all about saving the brand of Penton motorcycles. Last weekend I picked up two 1976 YZ250 bikes and a 390 Husky. I look at the Husky brand like the Penton: Those two YZ's will get sold long before I part out any of my 5 Pentons or 5 Huskys. Now if only I could find the time to restore my own projects......:(
Chris,
Cannot be much help, the bike performed poorly and it was later sold. It had been taken back to dealer/shop several times for repairs but never had much luck with it in competition, just did not run right compared to the Penton 125 Sachs model. I did hear later the bike was just used for trail riding and ran okay? Good luck with your project...
thom coles paw paw, michigan
I have put a Sachs 100 and 125 top end on a 73 Jackpiner with success. Do any of you know the difference in the 125 KTM 76 top ends compared to the 78 top ends? They have different part numbers. Different pistons too. I am just going to rubber mount the carb. 34mm I guess, although they were using the big 36mm Bing on the mx model by 78!! Timing and deck hight were alot different between the two also. Thanks to all for all the input!
Oh, By the way anyone got any 54.5 pistons for this bugger?
Thanks
Mike
Oh, By By the way anyone got a spare 78 style cylinder for this bugger?
Thanks
Mike
Mike,
How does a Sachs base gasket fit the 125 KTM bottom end? the 76-77 175 base gasket fit the 125 KTM? should let you know what options are their to work with.
I don't know what class your looking at running but Dane said his had a Lectron.
Lectrons would be a good carb for a wild port, pipe set up. Using base gaskets to keep the cranking pressure down to 160 LBS max should be safe #. 150 to 155 might work better I would think. Thats just rule of thumb to keep a track or trail with long high speed straights from causing a heat soak issue. We can weigh a Sachs 125 B crank if you would like and post the reading if it would help.
Its a neat project. Let us know how you are doing.
John D.
Mike,
I had no idea you could put a Sachs top end on a Jackpiner bottom end. That is very interesting. What type of modification to the parts needed to be done? I have a Sachs GS cylinder and head off a Monarch...now the gears in my head are spinning.
Chris
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
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.
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.
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
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.
Thanks John, and all for your help. I will let you all know how she comes out.
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.[:p] 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.
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?
Quotequote: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
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