1979 gs 420 motor?

[sportyrider1]

I bought a pair of 1979 KTM 420's ( 1 motocrosser, 1 enduro) from a friend of mine that raced them in the late 70's early 80's in doing some research i found somewhere that the motors might be different in the 2 bikes, is this true? The mx is up and running and I am working on the enduro, it had a frozen bottom end but it appeared it was just varnish as I was able to free it by poring some kerosene into the bottom end other than that it is complete just need some time to put it all back together. Anyway any Info on this bike is welcome.

[sachsmx]

I believe that the engines are identical with the execption of the "deck height", or the distance from the top edge of the piston to the top edge of the cylinder. This is controlled by using various combinations of bas gaskets to get a particular deck height. this affects the compression ratio and also the port timing.
There also may be a difference in the primary drive gear ratio and the (usually) 1st and 5th gear ratios.
I will check my manuals and get back to you.

[sportyrider1]

Thanks, for some reason I was thinking the GS acualy had a left over penton 400 engine and the MC was a 420 I guess I could measure the pistions and stoke to check. Also is there any diffrence between the 1979 and 1980 plastic I see DC plastics has a 1980 gs rear fender will this work? thanks

[rob w]

Both Type 55 (400cc), and Type 56 (420cc) engines were found in the 1979 model KTM's. The 400 was in the earlier made bikes, then the 420 was released later in the year.

[454MRW]

Sportyrider1,
The GS rear fender should be the same for 1979-1980. Phil Ketchum actually has one on his 1979, and had sent in an original one for Don to copy. The MC fender changed slightly from 1979 to the 1980 version, which uses aluminum spacers where it attaches to the side of the rear frame rails. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[sachsmx]

Yes, Rob, you are correct that both the "old" 400 and the "new" 420 were available in 1979. I assumed he knew they were both the 420 engine. The 420 was primarily available in MX trim, but GS kits were available for all KTMs in those days.
It is real easy to tell the difference, even from a distance. The 400 engine will have more cylinder fins that are spaced more closely together than the 420. The 400 has a radial finned cyl head and the 420 head has just short vertical fins. Also, the 420's rear engine mount is shared with the swingarm pivot bolt and the 400 uses a seperate bolt with a bracket, I believe. Due to the rear engine mount differences, the engines are not interchangeable.

[sachsmx]

I checked my manuals last night and yes, there were both GS and MX versions of the 420 available in 1979. The engine differences are:

deck height setting: MX: 1.0-1.1mm; GS: 1.5-1.6mm
Primary drive ratio
1st gear ratio
5th gear ratio
carb jetting

The gearbox differences allow for a close ratio (MX) and wide ratio (GS) transmissions.

Hope this helps