The sachs clutch is a somewhat complicated issue. It is often blamed for the general distaste of sachs mopeds, and while the sachs clutch is definitely a weak point, it can be addressed and made quite usable. If you want to tackle it it can be quite rewarding but the TLDR general recommendation is to get familiar with rebuilding the clutch (see video for setting clutch preload), double nut and loctite the clutch nut, and keep the kit simple - 15mm carb on airsal 70 for example.
First, familiarize yourself with Setting Clutch Preload on a Sachs. This also includes a video showing proper removal and reassembly.
The clutch was designed to handle the 50cc powerplant using a small pad and plate cage type design like motorcycles, but in miniature. The engagement is enacted by a ring of weights on a spring, referred to regularly as the donut, which spins out agains a small ramp plate as the engine revs and engages the clutches. This has a few main drawbacks:
By far the most common Sachs motors have the 1 speed Automatic sachs clutch (504,505-1A,B,C,D).
These consist of:
- 2x pads [#17] 3.2mm thick
- 2x plates [#15] 2.6mm thick
- 1 plate 1.7m thick [#15]
- 2x spring washers [#16]
- Donut spring [#20] with hub  and ramp plate 
- Shims (see Setting Clutch Preload on a Sachs)
in the clutch bell
The pads have tabs which fit in the clutch bell  which is engaged to the internal gears and moves with the chain drive and pedals. The plates are engaged on a toothed/splined hub piece and move with the engine crankshaft. At rest they are separated by the spring washers , so when the engine is off or idling the rear wheel and drive spin freely from the engine. When the starter clutch is pulled, the engagement bell  presses the pads and plates together, so that the clutch bell and pads which turn with the pedals now engage the plates and turns the crankshaft to start the engine. When the starter lever is released the pads and plates again separate allowing the motor to idle without the rear wheel turning. As gas is given and the motor accelerates, the donut spring  expands due to centrifugal force. As it expands is it presses out against the ramp plate  and is forced against the inner clutch plate. This pushes the plates and pads together and allows the motor to drive the wheel. Upon stopping the donut again retracts so the bike can idle when stopped.
Automatic clutch performance
The automatic clutches are a weak point in the Sachs motors.
Clutch slipping & glazing
The biggest issue is it has very limited clutch surface area and limited engagement pressure so they slip under load. Over time the pads overheat and "glaze" or lose friction and the problem consecutively gets worse. Most people wont recommend going more than the airsal 70cc kit because it's about the most the stock clutches can handle without slipping wildly and glazing over quickly. You can sometimes make the clutches work with the big 80 kits, but you've gotta be prepared to take the clutches apart and service them about monthly. See Sachs 505 1a performance
In order to alleviate this problem it's important to improve the friction between the pads and plates, which may be achieved by increasing contact area. A 7 piece (4 plate 3 pads) version (instead of 3 plate 2 pad stock "5 piece") exists from some late model sachs engines, but theyre not really available anywhere at this time. It makes the clutches last much better but still needs pretty regular attention.
Parts from two standard 2-pad/3-plate clutches may be milled to fit 3 pads and 4 plates for more friction area and less slip. The center plates are the thinner 1.7mm (usually one in each clutchpack is this size), outer 2 plates are standard 2.6mm thickness. The pads can be milled down to 2.3 from 3.4mm. When doing so it's good to recut the oil channels in the pads for proper lubrication. I milled the pads on a bench disk sander and checked regularly with a micrometer that it was very even. Parallel surfaces is key to this working. I left +/-0.05mm tolerances, it's not hard, though somewhat tedious. The thicknesses can vary between the different pads, as long as theyre flat and parallel. After that, add shims to make sure the center toothed piece [#19] has proper clearance and grabs only as the donut springs out or the starter bell is pulled in. See also: Setting Clutch Preload on a Sachs. There was an "uber" 9 piece clutch (see below in manual clutch tuning section) but you cant get those anywhere anymore.
There have also been attempts at aftermarket clutch pads but theyre not really that much different and also require reshimming.
Lots of efferts have been made to affect performance with transmission fluid changes and coolers and splashers etc but gains are modest at best. An option used by Nick K with his 3 speed Sachs is to maintain cool oil temperatures with a simple pump and radiator cooler set up to circulate the transmission oil and dump int back into the top of the clutches. This was reported to also reduce problems with slipping. It is also worth noting that the 3 speed motors have a 4 plate 3 pad clutch with greater surface area. Transmission oil also has an effect on the clutch performance. Sachs recommends 90w gear oil for it’s transmission but users have tried everything down to ATF with no noticeable affect.
Its also been suggested that it could help to put a little whirly piece to whip the oil around and keep everything lubed and cool. That's potentially equally as important as friction area. This could be a cut piece of tin can with bent out tabs bolted in there, or purpose made pieces have at times been available on ebay Germany. Get creative.
Clutch Nut Loosening
Another problem people have with the sachs auto clutches is the nut backing off the end of the crankshaft. The reason for that is that the clutch attaches to the crank via the toothed piece . That piece is only attached to the crank by the nut on the end; whereas most moped clutches are taper fit. Thusly, jerky starts and stops essentially impact the nut loose and then your clutch isnt engaged anymore and suffering ensues until you open it up and reassemble. This can be corrected by replacing the clutch nut with 2 thinner nuts and locking them against one another, and lots of red loctite. Clean with brake cleaner to remove oil before using loctite.
On very high power setups and lots of coasterbrake skid stops, you can look into drilling and tapping a setscrew through the clutch hub spline piece. What I did was to take the toothed piece [#18]and drilled and tapped a 1/8 in. hole through the side of it and then got a short allen set screw for it.
-somethin like that. I lined it up on the crank and drilled a tiny divit into the crank where it would sit normally, making sure that's alligned right. Then drilled a big hole in the top of the clutch case to get to the screw and tighten it down through the slits in the side of the bell once the bell and spring and everything behind #18 was in place. Then i just put a cover over the top hole, theres no pressure on it, just to hold the oil in. Assembled the rest as usual. Red loctite on the end nut is still a good idea. clean it with brake parts cleaner 1st to get any oil off.
Tuning clutches refers to adjusting the RPM of the engine at which they engage. Typically with aftermarket performance the power is shifted to higher RPM to go faster and make more power, while sacrificing low RPM power. As a result your stock clutches are engaging well below where the motor is making power. A tuned clutch allows you to increase gearing for top speed without sacrificing much off-the-line speed. Unfortunately, due to the donut/ramp-plate setup which Sachs 505 motors employ, this is very difficult to achieve. Other moped brands' clutches there are springs, set screws or parts that can be easily lightened to engage at higher RPM. Sachs clutches are too small to replace with a tunable spring-based centrifugal clutch, which leaves as the only option stiffening the existing donut spring. I only know of Mike Bartell doing this and doing it sucessfully, and I don't know the full details, but what you need to do is to remove the donut and split the spring inside. You want to remove some small length of the spring (or replace it with a similar length stiffer tension spring) and then replace all the donut segments and very firmly reconnect the spring. This will hold the donut from expanding until the engine reaches higher RPM at which point it will press out and go. Stiffer aftermarket springs are available at Treats and a tutorial is on Youtube but reports are of only modest improvement for high effort.
Many Sachs performance enthusiasts will opt for the manual shift, 2-gang or 3-gang sachs/hercules motors (505-2B(or D) or 505-3B) ordered from ebay Germany and occasionally available from US parts suppliers like doscycles or treatland.tv. Helpful search terms include: "2-gang" or "2 (zwei) lamellen" sachs/hercules "Prima 5" and "kupplung" meaning clutch. Unfortunately, even the 2 speed manual motors have trouble with clutches slipping and the nut backing off (see solutions above).
- There was a limited run of 504/2B engines produced by Sachs in the Portuguese market in the 80s. They only differ from their 505 brethren by using the normal large taper 504 crank.
- The 3-gang Sachs is a beast! You can spot these engines by their over-sized, black clutch cover. The cases are slightly different as well as the driveshaft extends out the side of the case more than the normal autos and 2-gangs. These engines are super rare especially in the US; however, a few dudes in the Sachs community have them. The 3-gang is perfect for the addition of the Athena 80 Reed kit. Basically, bolt and go!
The standard clutches consist of:
- 2x pads [#18] 3.2mm thick
- 3x plates [#16] 1.7m thick (on an standard 2 pad automatic clutch you instead have one 1.7mm plate and 2 2.6mm plates)
- spacer [#15] 4.5mm
for a total 16mm thickness
Additionally the manual clutches use a spacer at the bottom of the bell and a spring washer to press the clutch together when engaged, and three 15.8mm long x 3mm diam pins [#14] which go through the plates and push back the bottom plate against the spring to disengage the clutches when the shift lever is pulled.
Manual clutch performance
The above options detailed in the Automatic clutch performance section above also apply to the manual clutches.
One option to prevent slipping is the so-called "uber clutches". I don't believe these can fit in the automatic clutches with the donut, but the manuals have more room to remove spacers and washers to fit the larger clutch pack. These have been occasionally available through ebay Germany, however the cost is 150+ Euro, plus shipping, and the seller doesn't ship to the US, so you need a contact in Europe. These have been tried and provide good action without unwanted slip.
The "uber-clutch" consists of:
- 4x New "uber" pads [#18], 2.15mmx4
- 5x clutch plates [#16] 1.7mm
- no spacer
for a total 17.1mm (meas 17.3) total thickness It also includes a stiffer base spring [#20]
The second option is to make these uber clutches yourself. If you find any second clutch pack even from an automatic, grinding down the clutch pads is all you'd need to do. 1.7mm plates should be thin enough as-is. The manual engines already have 3 and the automatics usually have 1 plate of desired 1.7mm thickness. Grind the pads on the side where the oil channels don't go all the way out the teeth, this should be apparent when looking at them. This is so that you dont loose oil turbulence. It's also very important when grinding to keep the 2 sides absolutely parallel.
There is more work required to make it fit into your clutch. Even when milled down to the uber clutch sizes, it still doesnt quite fit. You can mill the bottom spacer [#21], to leave room for more clutch to fit in the bell. You may also remove the spacer [#15] on the end. Another option instead of milling down #21 is to space out the cover. If you just flip the inner spacer  and get rid of the outer spacer  it buys you enough room to fit it all in the bell, but it pushes the engagement bell [#11] too far out so the clutch cover causes the clutch to disengage when the cover is tightened on. A few extra gaskets or a custom spacer works to alleviate the problem. So either you gotta mill down #21 about .5mm or flip it and add some extra gaskets on the clutch cover. I did the latter and have 3 clutch cover gaskets.
The second problem number 2, the new uber clutch pack is too thick for the old pins. The clutch disengages when you pull it by pressing the set of 3 pins  thru to push the back plate and spring [16,20]. The pins are only 15.8mm which works because the normal outer spacer  is recessed. With the new setup at 17mm thick, you need longer pins. These can be found (3mm diameter 18mm long) from McMaster Carr. They work, I have a ton extra, if you want some, they’d be free. PM wdaloz.
Converting manual to automatic
The 2 speed manual is very similar, someone a while ago put up a comparison. You can't easily make the automatic into a manual but you could do the reverse, though that'd be pretty worthless. This is not something that should be considered, sourcing the parts will already be more work and money than just buying a complete 2 speed motor. For the very industrious, details follow.
The cases are exactly the same casting, but vary by which holes are drilled. For the shifter gear slector on the manual there's a hole in the top of the cases; the spot is there for it on the auto cases, but it's not drilled out. Likewise the hole for the starter cable on the automatics is there on the manuals, but not drilled out.
Note: In Portugal, a limited run of Sachs 504/2B engines were produced. They utilize the same 2-speed gears and clutch as the 505/2 but use the normal 504 large taper crank.
The manual 2 and 3 speeds use the same clutch bell and same size main gear, but have either 2 or 3 sub gears on the main driveshaft, instead of onlyone on the automatics. These are selected by a gear shifter fork (not shown). The 1 speed only has one gear ratio permanently engaged.
To convert to a manual shift, you'd need to drill the holes in the cases mentioned above, then find and install a multi-speed main gear & shaft, secondary gearshaft as well as the shifter gear selector. Then perform the clutch conversion as described below. The cranks, pedal shaft, ignition, and main clutch components will still work.
Clutches- Auto vs Manual
The clutches are very similar and use the same pads and plates for the automatics and manual shifters. They also use the same bell housing and engage the same with the final drive gears.
To convert an automatic cltuch to a manual clutch you'd need manual parts in the left image #21, 20, 16 and 14. From the automatic (right image now) you would remove the #16 springs, and replace the 20 and 21 donut spring and ramp plate with the permanent fan spring and it's holder (20, 21 from the left image). The pads, toothed gear and plates (15, 17, 18) would remain. Pins #14L pass through the holes in the first 2 plates, and the last plate is rotated so they butt up against it. Then the spacer (16L) and washer 13 and nut 12 hold it all together. Pushing the outer bell 11 should force the rods (14L) to push the pads apart. You wont need the spacers 14R. The rod assembly (1-10) is different but should fit the same in either cases. Hooking up a clutch cable to the top and to a lever on the bars should complete the function. It may be possible that the automatic starter clutch would activate the manual bell and ins properly as-is with no modifications. This would have to be finished by modifying the cases and installing the multi-speed gears and selector.
You could very easily convert a 2 speed manual to a 1 speed automatic. Since the manuals sit at rest in 2nd gear, you could swap in an automatic donut spring, remove the pins, adjust the clutch lever as a starter so it presses everything together (instead of apart) and run it as an automatic. But that's worthless.
The clutch parts that are common between the manuals and automatics (nuts, spacers, pads, plates and bell) are totally interchangeable. If you had a 3pad/4plate clutch (some bikes did, treats had some up a little while back also) you could swap that into either setup, youd just have the springs pushing them apart for the automatic or the pins to push them apart on the manual.
Original Thread: http://www.mopedarmy.com/forums/read.php?7,2841027
And some other good threads for reference
- old thread on the manual with measurements http://www.mopedarmy.com/forums/read.php?7,1983164,1983164
- discussion of trans fluid and sachs autoclutch operation http://www.mopedarmy.com/forums/read.php?6,1796493,1796493
- thread deatailing removing and reassembling sachs clutches http://www.mopedarmy.com/forums/read.php?6,1777073,1777073
- discussion of weaknesses with manual sachs clutches http://www.mopedarmy.com/forums/read.php?7,1970001,1970001