Project Old School KTM 380: Rebuilding the Bottom End

Nov. 04, 2009 By Eric Tabb
Our donor transmission, in all its glory.

In our last installment, the 380 motor was back from getting the crank rebuilt and the cylinder porting cleaned up. With the motor being in the shop so long, we burnt some of the free time by doing more research on the different 380 models. The 380 was offered in the SX, MXC and the EXC variations. Our understanding was that all of the KTM EXC models had a transmission with a lower first and second gear than the MXC and SX models and taller top gears.

Our donor also had a broken shift spring, which seems to be common on the 380.

When it comes to the 380, however, this is not the case; in fact, the 380EXC shares first through third gear with the MXC/SX model, and then it has a taller fourth and fifth. Once we found out this fact, we began our search for an EXC transmission to drop into our project bike before assembling the cases.

Our quest was not an easy one, but eventually a used 380EXC motor with a blown top-end came up for sale online for $125. The seller had even agreed to throw in the top end, which we figured we could have replated and use as a spare. Since the seller was the same person we bought our spare set of WP 50mm forks from, we should have been weary from the start. When the donor motor arrived it was evident that the poor 380 that we scored the 50s and the motor from must have been ridden into the ground and left outside for years. The motor looked like hell from the outside and the cylinder that was “thrown in” was missing the skirt. To top it off, the oil drain plug to the transmission was missing, nice! “Buyer be warned” was for sure applied with this seller.

The kickstarter shaft spring should look more like this.

Despite our strong desire to toss the motor into the trash and not even waste a minute’s time splitting the cases, we decided to go ahead and dig in. It was ugly until we popped the clutch side cover off and were treated to clean and still oily internals. Once we split the cases things improved even more, and the EXC tranny was in great shape and ready for a transplant into our project motor. We carefully removed the shift forks, shift drum and then pulled each shaft with gears in place out and dropped them in place into our motor, making sure we didn’t leave behind any washers or springs. With both sets of gears and shafts in place it is just a matter of dropping in the two rods the shift forks slide on, the shift drum and then aligning the actual shift forks with their respective grooves in the drum.

The two cases pieced together - it only needed a little finessse to get them lined up correctly.

With the tranny finally in our motor we began the assembly of the rest of the bottom end. The center case gasket was installed and the crank held securely while the two cases were mated together. A little finesse is needed when getting the cases to line up, but we didn’t run into any real issues. The shift roller and shift shaft were installed next followed up by the kickstart shaft assembly. Before putting the kickstart shaft in place recall on our initial ride we were experiencing the kickstarter slipping once in a while when trying to start the bike. We took the opportunity to replace the return spring on the shaft in hopes of the ratcheting mechanism getting a better grip and not slipping once we get the motor together. Note the photo of a kickstarter shaft assembly from another bike with a broken pin that is supposed to hold the spring in place, a weak point in the 380 for sure.

Here's a look at the 380 shift shaft.

The clutch basket and primary gear were installed next and tightened up with an impact. When installing the primary gear on the end of the crank, be mindful that it is reverse threaded. We came across a used Rekluse auto-clutch and decided it would be a great addition to the 380 to help prevent any stalling and to make nasty singletrack disappear.

We went with the recommended Rekluse setup knowing there are other adjustment options available to try later if we want.

Installing the 380 isn’t overly difficult, but we recommend a good understanding of the install process before you start. Having feeler gauges ready to go before beginning the install will save the headache of going to the store and having to reread the instruction process when you get back. We went with the recommended Rekluse setup knowing there are other adjustment options available to try later if we want.

We buttoned up the clutch cover and moved over to the ignition side. Recall our 2K3 ignition was upgraded to direct the current into one lead instead of two. This lead will be fed into a regulator/rectifier where it is converted from AC current to DC current and then regulated so the voltage doesn’t spike too high. From the regulator/rectifier, the current is fed to the battery to keep it charged and from the battery it is run to any accessories we want to run on the 380. The 2K3 updated like this should be good for around 110 Watts of DC power. HID lighting needs DC power, so this mod will enable us to run the HID lighting and still have enough power left over for grip heaters, a tail light and a GPS unit.

The 2K3 updated like this should be good for around 110 Watts of DC power.

The Project 380 is now going to be more versatile with the EXC transmission. It will still retain the nicely spaced SX/MXC typical first and second gear while gaining the higher top speed useful for the desert and fire roads. Next up, our 380 will get the top-end installed and then final assembly can begin. We are keeping the 380 on a budget and trying to acquire used parts like the Rekluse when possible. There are a few interesting items that we have acquired new that should really help with one pitfall some 380s develop – vibration. In order to help prevent, or at least minimize, vibration check back with our Project 380 to learn about Flexx bars and anti-vibration bar inserts from Fasst Co. Newsletter
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