Will KX420 barrel fit on KDX400?

[John Orchard]

Does anyone know if the barrel & piston from a KX420 will bolt onto a '79 KDX400 bottom end?

[JohnnyO]

Rusty could tell you..

[Trent Farlow]

Yes it will, the KX420 has a larger bore. You need to change to the KX420 head as well, as the bore diameter and head squish bands are different.

[holeshot buddy]

ha ha trents onto it same cases

[GMC]

Come on John, what did you buy the cross reference CD for?

Crank cases are the same…
Part # = 14001-5044
Part Description = CRANKCASE SET,FLAT BLACK
Model Count = 3
KDX400-A1 79 390 KDX400 OFF ROAD
KDX400-A2 80 390 KDX400 OFF ROAD
KX420-A1 80 420 KX420 MOTOCROSS

Right hand crank is the same…
 Part # = 13037-1035
Part Description = CRANKSHAFT,RH
Model Count = 5
KDX400-A1 79 390 KDX400 OFF ROAD
KDX400-A2 80 390 KDX400 OFF ROAD
KDX420-B1 81 420 KDX420 MOTOCROSS
KX420-A1 80 420 KX420 MOTOCROSS
KX420-A2 81 420 KX420 MOTOCROSS

Con rod is the same…
Part # = 13044-5011
Part Description = CONNECTING ROD ASSEMBLY
Model Count = 6
KDX400-A1 79 390 KDX400 OFF ROAD
KDX400-A2 80 390 KDX400 OFF ROAD
KDX420-B1 81 420 KDX420 MOTOCROSS
KX420-A1 80 420 KX420 MOTOCROSS
KX420-A2 81 420 KX420 MOTOCROSS
KDX450-A1 82 420 KDX450 OFF ROAD

Base gasket is the same…
Part # = 11060-1452
Part Description = CYLINDER BASE GASKET
Model Count = 6
KDX400-A1 79 390 KDX400 OFF ROAD
KDX400-A2 80 390 KDX400 OFF ROAD
KDX420-B1 81 420 KDX420 MOTOCROSS
KX420-A1 80 420 KX420 MOTOCROSS
KX420-A2 81 420 KX420 MOTOCROSS
KDX450-A1 82 420 KDX450 OFF ROAD

Head gasket is the same…
Part # = 11004-1015
Part Description = CYLINDER HEAD GASKET
Model Count = 6
KDX400-A1 79 390 KDX400 OFF ROAD
KDX400-A2 80 390 KDX400 OFF ROAD
KDX420-B1 81 420 KDX420 MOTOCROSS
KX420-A1 80 420 KX420 MOTOCROSS
KX420-A2 81 420 KX420 MOTOCROSS
KDX450-A1 82 420 KDX450 OFF ROAD

Even the cylinder head is the same…
Part # = 11001-1030
Part Description = CYLINDER HEAD,FLAT BLACK
Model Count = 5
KDX400-A2 80 390 KDX400 OFF ROAD
KDX420-B1 81 420 KDX420 MOTOCROSS
KX420-A1 80 420 KX420 MOTOCROSS
KX420-A2 81 420 KX420 MOTOCROSS
KDX450-A1 82 420 KDX450 OFF ROAD

I reckon the only difference would be the porting specs and bore size, KDX is 390cc and the KX is 420cc

[bishboy]

So if the cases etc are the same, would it follow that the engines would be interchangeable between them?

[John Orchard]

Hmmm thanks Geoff, I thought it'd be quicker & easier if someone had done the swap.  I remember someone posting that there was a difference between the cases on the '79 A1 and the '80 A2 but your cross-reference info doesn't reflect that, I'll see what I can determine.

I guess the KX420 engine would fit into the frame where a KX400 engine came out of.  Interesting.

[JohnnyO]

Go with what Trent says, he's owned more and knows more about green machines than anyone..

[HVA61]

Trents explanation of the Kwaka hop up , sounds very similiar to the 1980 390 husky hop up 420 kit.

[Trent Farlow]

I stand to be corrected on this, but I think that the 79 KDX400 cases are different to the cases on the 1980 KDX400 - in 79 the KDX400 cases were designed much like enlarged KX250A5 cases and used a bracket to mount the rear of the motor to the swingarm pivot bolt. For 1980 the KDX was upgraded to use KX420 cases which had the swingarm pivot bolt running through the back of the crankcase. In addition the 1980 KDX400 used different gear ratios. The cylinder heads are interchangeable but they are different - the KDX400 has a bore of 80mm and a compression ratio of 6.8:1,  while the KX420 is 83mm with a slightly higher compression ratio of 6.9:1 - at first glance the heads are externally the same , but the squish bands differ, and the KX squish is 3mm wider to allow for the increased bore size.

For works bike fans and green tech buffs, Brad Lackeys open class SR works bike that he campaigned in the GP's in 1979 actually used 79 KDX400 cases with the back of the cases cut down and then rewelded up with a large billet alloy lug with a collar  to take the swingarm bolt pivot of the then all new works Unitrak.  The cases were modified to take cylinders and cranks with capacities from 390cc to 476cc with many in between, through the season.

As a footnote -  Brads 1980 works GP bike displaced the full 499cc , and had special sandcast cases with the swingarm pivot point integrated into the cases of what was Kawasakis first full capacity SR500.  it was a very special one off hand made bike built just for Brad, and only two were built. However the overall design, including the engine, was successful and the following year, modified and improved versions of these engines with sandcast cases then found their way into the 1981 SR500 works bikes which were made in very small numbers and in Oz as many will know, were ridden by Anthony Gunter and Trevor Williams in 1981 to great effect.  These then directly gave rise to the design of the engines used in the first production KX500's in 1983.  So the humble KDX400 has some heritage and influence on history.

[John Orchard]

That is awesome Trent !  :-)  Any further info relating to KDX400 would be great!

My KDX400 is the '79 model and the rear engine mount plate is (as you say) is identical to the KX250A5, making it easier to transplant the engine.  Apparently I may need to make new front engine mount plates though.

It looks like I've found a KDX400 piston so I will get the cylinder resleeved to suit that rather than waiting for a KX420 piston to appear, checking its compatability, and then resleeving to suit the 3mm larger size.