79 420 KTM

[Jeff D]

I'm not sure if or how this relates to the 420s, but last year we had a heckuva time trying to locate an overbore piston for Kevin Lavoie's 1981 KTM390 in time for the ISDT Reunion Ride.  I think he found one through Barry Higgins H & H KTM.  I'm not sure how the 390 relates to the 420 but my understanding is they were the same engine with top end mods for ISDT class displacement rules..possibly shorter stroke, I don't know.  In any event, the piston was supposed to be available through KTM but my local dealer found them to be on eternal backorder and Kevin was lucky to finally find what he needed at Barry's shop.  He had the bike at the Reunion Ride last year, it looked gorgeous and it ripped!  Hopefully he'll return with it again this October.
Jeff DeBell (who can't wait for the Reunion Ride!!!)

Jeff DeBell

[sachsmx]

The 420 was introduced in mid 1979 and built until 1984. From 1981 to 1984 it was primarily an enduro model since the 495 was released as an '81 model for MX use. The 390/420/495 had the same stroke, and a nonreed 495 top end will bolt onto the 390 and 420 bottom ends. I believe the cyl. stud spacing was altered to allow the use of a reed valve in '82 for the 495s. I too have found pistons in Europe, although they are getting hard to find there also.

 

[Heinz Raidel]

Probably the best thing about  the folks in Mattighoffen is that they never have been afraid to change anything at any time in the production schedule; the post-vintage big bore KTMs are an example. The 350/390/400/420 and 495 all shared lower ends and the 74mm stroke.

I remember my older bro's '83 420 as having a very very aggressive power delivery,  the 390s and my 400 seem very tame and torquey.

The December '81 reparaturanleitung lists the 390 as being 44HP;  the MC420 as being 52HP;  and the 495 as being 53HP.

This doesn't mean that Kevin L wasn't ripping on his 390, he probably could rip on anything except an Ossa.

[Patrick Hayes]

The 79 420 did indeed exist. I have a cylinder and head from one if any one needs it. I had a 1980 420 that was a fantastic bike.The problem with the old KTM big bores is getting pistons. I think the 420 is 85 mm. You can probably get something pretty close to it from wiseco. I got a 92 mm. piston from a watercraft for my 495 from them. The only draw back was pin hieght. A connecting rod from a CR500 combined with stroking it 4mm and lowering the port timing 1.5 mm. makes for a 520 that should be more than reasonably powerful. I would think that you should have better luck with the smaller piston. Good luck.