Year, Size, Model Identification

     Every work day I receive calls from someone looking for parts to restore or repair one of the many models of Penton and KTM motorcycles that were sold from 1968 thru 1982. I am pretty good at knowing what some of the parts many people are calling about, especially if I know what year, model, and size of motor the bike is. It saves a lot of time if the caller knows this information before calling. It also helps if the caller has a part number to identify exactly what part that they are looking for.
     To identify what year, size, and model bike you have, write down the serial number off of the frame (stamped on the steering head). On the KTM motors, write down the number stamped on the ignition side of the center case (right under the cylinder fins).

               Penton/ KTM Frame Identification

     1974 frame                1976 MC-5          1978 GS-6 frame

     The frame serial numbers contain a date code that identifies what month and year the bike was built. These number are stamped on or around by the steering head. In most cases (from 1972+) the first three numbers are the date code. The first number is the year, the next two are the month (eg. 403 = March of 1974).

           KTM Motor Identification

                 1976- 400                                          1982- 420                                       1988- 350

The KTM motor serial number also contain a code that identifies what year and what size the motor is. On the 1972-80 motors these numbers can be found on the ignition side - just below the cylinder fins. On 1978+ motors the number can be found on the clutch side - on the bottom front. The first three numbers are the code. The first number is the year, the next two are the code for the motor size. The following is a list of the different codes used from 1972 thru 1983:
51 = 125                                  52 = 175

54 = 250                                  55 = 400  
500 = 125/II RV or II LC       501 = 125 MX/MXC/GS (LC)
541 = 250II GS(RV)              542 = 250 MC

543 = 250 LC                         555 = 350                      

560 = 420                               561 = 350

562 = 390 GS                         563 = 495

LC = Liquid Cooled                GS = Enduro
RV = Reed Valve                    MC = Motocross

     These codes are not only used to identify models on the motor cases, but are also used in the parts books to identity parts for these motors. In most cases, the first two or three numbers in a part number identify what size motor it goes to.

     There is a date code cast into every motor case around the drive sprocket area towards the lower rear part of the motor. It looks like a small clock with a triangular arrow in the center. The arrow points to a number on the clock indicating the month. There are two numbers under the arrow which indicate the year.

There are also part numbers cast into the outside of the center cases. The first two or three numbers are a way of identifying what size the motor is.

     It is important for everyone to be able to identify what year and size bike that they have. Most used bikes have the original motors still in the frames, however I have run across situations where the motors have been replaced with a dissimilar year. (e.g.. a 1979 frame with a 1976 motor). Although many of the early KTM motor parts are the same and are interchangeable (1974 thru 1979) there are differences in some of the parts especially the clutches, push rods, and bearing plates.

     Parts lists are available showing every part on the bike and motors and identify everything with a part number. The illustrations help in determining if something is missing, what order the parts go in assembly, and how many there should be. The part numbers help you to obtain the exact part that you need.
     For the 1972 thru 1973 they are sold by model size. For 1975 thru 1977 they are sold by model year and include the parts for the 125/175/250/400 in the same list. Go to the Repair Manuals - KTM page.

KTM Motor Tips

Do not remove the stop bolt for the kick starter, on the bottom of the motor, to drain the oil. Follow the directions in the owners manual to do oil changes. When doing an overhaul of the motor, remove the stop bolt after removing the kick starter shaft. Check the bolt for wear. If it is worn flat, replace it.

On the black motors, check for cracks around the hole in the case for the kick-start stop bolt. Cracks and breaks can be welded. ALL KTM MOTORS ARE MADE OF MAGNESIUM. Take it to a certified welder with experience. They use magnesium welding rods. Small cracks can be repaired with JB Weld or any high temp. epoxy to seal oil leaks.

On 250, 400, 420, and 495 motors, check and tighten any loose nuts and bolts after every race day. Pay attention to the motor mounting bolts on all big bore bikes. Loose bolts will allow the vibration of the motors to wear away the magnesium mounting areas.

Casting mark under exhaust port

250 casting mark above intake

400 casting mark above intake

KTM Cylinder Re-sleeving

All KTM cylinders can be re-sleeved, however in some cases there are more than one style of sleeve for a size of cylinder - in particular the 250 and 400 cylinders.

The 250 cylinders could use a sleeve with a standard bore size of 71 or 72mm. Look for casting mark "ELKO EZ 71" or "ELKO EZ 72"

The 400 cylinders could use a sleeve with a standard bore size of 81 or 82mm. Look for casting mark "ELKO EZ 81" or "ELKO EZ 82"

Double check the casting marks on your cylinder (see above photos for locations) to determine what size cylinder that you have before calling to order a new sleeve.

The 1972-75 175 cylinders were throw-a-way cylinders that did not have replacement sleeves. I have sleeves for these in stock. The cylinder must be over-bored to install the sleeve.

Do not take your cylinder to just any shop for re-sleeving or boring. Make sure that they have the experience of working on vintage 2-stoke cylinders. Shops doing work on the new motors can screw up your cylinder because there is a difference in setting them up.

I recommend the services of Bore-Tech in Ohio or Forest Stahl in Indiana. They have the experience and know how to do the job right the first time. Check the "professional services" listing on the General Information page.

KTM Clutches

For clutch slippage and sticking the following steps should be followed in order:

1. Check the clutch activator (the bearing plate) on the right side of the motor. Refer to the repair manual for the minimum clearance for the clutch activator arm. If your measurement is less than the minimum – remove the arm from the actuator and check for wear on the small pin inside the barrel connected to the arm which presses against the clutch rods. If the pin has a flat spot (after 25 years of use, most of these are) replace the pin.

2. If the clutch actuator arm is wobbly and loose inside the clutch actuator (it should be a snug fit) replace the clutch actuator (bearing plate) – part #51-32-013-344.

The clutch actuator and arm must be within specifications before dis-assembling the clutch. A loose or worn arm assembly will not allow enough pull against the clutch pressure plate to allow the plates to release.

3. Before removing the clutch nuts, observe how many threads are showing through each nut. Remove the clutch plates and check for uneven wear, bent or cupped plates. Hold the plates together in a stack and see if there are any visible gaps between the plates. Replace any plates that are bent or causing gaps.

4. Check all the clutch springs. They must be of the same wire thickness and same length. Check the parts list and repair manual for the spring specifications. Replace any mismatched springs.

5. When re-installing the clutch nuts, they must have uniform pressure when tightened. Check the repair manual for specifications. In most cases, they should not have more than 1-1/2 threads showing above the top of the nuts.

6. Use one continuous safety wire through all of the clutch nuts. You do not have to use individual cotter pins. The purpose of the cotter pins is to prevent the nuts from backing out and reaming a hole through your clutch case.

NOTE: Clutch plates can stick together if the motor has been sitting unused for a length of time (more than 1 month). Although I have not been able to prove it, it is my theory that the type of oil (gear lube) used in the bottom end has an effect on the clutch plates causing them to bond together. Sometimes the plates can be broken loose by starting the engine, pulling the clutch lever in and putting the bike in gear. Or while the bike is moving (at low speed) pull the clutch lever in and apply the brakes.

In worst cases, when a motor has been sitting for 10-20 years, it is best to open the clutch assembly and pry the clutch plates apart.