Kx 250 1975-76 piston and rings

[oldfart]

Ok I'm looking for a bit of help from the brain trust in the green corner.

What other piston and ring kits have you fitted to these  STD chrome ( 70mm ) bore's ?

[bazza]

stu some genuine kwaka ones on evilbay last week at reasonable price

[oldfart]

Saw those and enquired, but were for a 74
     74 model had a 69.5 bore x 64.9 stroke    246 cc
75/76 model had a 70.0 bore x 64.9 stroke    249 cc  electro fushion bore

[John Orchard]

78 & 79 have two large transfer/intake holes in the intake side of the skirt as the 78/79 went to reed valve.

I'm also looking to see if I can get a 74/75/76 (70mm) piston and cut the intake holes to use in a 78/79.  I think it was the 73 model that ran the 69.4mm bore?

I've just ordered all combinations I could find of KX250 (74 to 82) pistons & rings and KDX250 80~84 and KXT250 to determine what will go with what; jeez, when ever I get a bike I have to know it all inside & out!

Apparently Wiseco piston & rings can be used in Electrofusion & steel (resleeved) bores, I'm going to call Serco tomorrow to see if stock 78/79 piston & rings (Electrofusion bores) will work in their steel sleeve.

[oldfart]

John .... spoke with allan a few weeks ago with regards to piston choices. Wiseco and both Wosner cater for 1978 models and above so you should be ok for piston choices.
They list the rings as CD   = alloy steel ( chrome faced internally notched )
Wiseco piston kits for 78 - 86  STD bore 70mm part # is 439MO7000
                                                          70.5
                                                          71
                                                          71.5
                                                          72.
                                                                                                                             
                                                                                     
                                                                                 

[bigk]

You need Wiswco CDM rings for chrome bore. I've used CR250M pistons in '74 KX250's (steel bore standard) many times, you may be able to get CDM rings to suit & use an Elsinore piston.
K

[John Orchard]

Are chrome bores (Honda) the same as Kawasaki's 'Electrofusion'?  I guess a similar process but if it is a different material then the needs of the ring material may be different?

I think with KX's we are lucky; use the std 74/75/76 rings in a steel bore and the 78/79 rings in the Electrofusion bore, that should save any issues.

[oldfart]

From what I can make out the 74 had a steel bore and 75 -76 had a chrome bore.
This is where you need to be carefull buying off flea bay ....  they mention in most of thier speils that they fit 74 -75 -76  yes and no.
Yes.... a 74 .050 piston will fit a 75-76 Std bore  , and no.... the rings are no good.
I have a Honda Cr piston  and a Ts/tm piston  both 70mm with 18mm  pins of which both look close to the KX piston but was unsure which to try.
Michael ... has come back and confirmed he has used CR pistons without any issues in the 74 , So I will go down that path and see where it leads to.
 

[TM BILL]

From what I can make out the 74 had a steel bore and 75 -76 had a chrome bore.
This is where you need to be carefull buying off flea bay ....  they mention in most of thier speils that they fit 74 -75 -76  yes and no.
Yes.... a 74 .050 piston will fit a 75-76 Std bore  , and no.... the rings are no good.
I have a Honda Cr piston  and a Ts/tm piston  both 70mm with 18mm  pins of which both look close to the KX piston but was unsure which to try.
Michael ... has come back and confirmed he has used CR pistons without any issues in the 74 , So I will go down that path and see where it leads to.
 

I think you will find that 74 KX 250 has electrofusion bore 

[bigk]

I beg to differ Bill, I've had a few 74 KX250's & all have had cast bores. I actually thought '75 did as well with '76 being the first electrofusion bore. Who knows for real?
K

[TM BILL]

All my 74 , 75 and 76 250 motors have been plated but it could be that cyls have been swapped over the years . But in the 74 parts book there is no listing for oversize pistons or rings, and magazine test of the 74 250 mention this new plated cylinder being different to the YZ A chrome bore .

My 74 and 76 KX 125s are plated as well.

I never had a new one though so i cant say for sure  was the 1974 KX 450 cylinder plated ?

[John Orchard]

Bill your plated 74,75 & 76 cylinders, are they reed valve applicable?  I'm thinking that the piston port cyl's are steel bores and the reed valve cyl's are Electrofusion?

[oldfart]

Bill ... 1974 parts book ( printed 20-8 -1973) on page 6 list
  
13001-070  piston std
13029-069  piston 0.020' 01S
13008-051  ring set std
13025-055  ring set  0.020' 01S

I have all three workshop manuals 74-75-76  and it only refer to   " Electro Fuse Cylinders on the 75 -76 models

I have no shop manuals on the KX 450 ole mate ....    

[GMC]

Bill your plated 74,75 & 76 cylinders, are they reed valve applicable?  I'm thinking that the piston port cyl's are steel bores and the reed valve cyl's are Electrofusion?

No John, I had a 75 / 76 KX 250 (non reed) in 78 and it had the electrofusion barrel then.

Are chrome bores (Honda) the same as Kawasaki's 'Electrofusion'?  I guess a similar process but if it is a different material then the needs of the ring material may be different?

I think with KX's we are lucky; use the std 74/75/76 rings in a steel bore and the 78/79 rings in the Electrofusion bore, that should save any issues.

Electrofusion is a Kawasaki thing.
A lot of guys refer to it as “chrome bore” but that just goes to confuse everything.

I think I read that Kawasaki hang some carbon rods in the cylinder and explode them with 5000 odd volts to line the cylinder.
This is a different process to Nikasil and chrome.

As far as I’m aware you can run the same rings and pistons in Electrofusion, Cast iron and Nickasil cylinders.

Chrome bores are the only odd ones that you need to be careful with.




I concur with oldfart
Zedder also lists a 0.02 oversize piston and ring (unfortunately it also lists the 74, 75 & 76 cylinders as all the same!!)

Part No’s for pistons are…

74 piston is for one model only

75 / 76 pistons have their own piston

and then…
Part # = 13001-1004
Part Description = PISTON,STD
Model Count = 5
KX250-A4 78 250 KX250 MOTOCROSS
KX250-A5 79 250 KX250 MOTOCROSS
KX250-C1 83 250 KX250 MOTOCROSS
KX250-C2 84 250 KX250 MOTOCROSS
KX250-D1 85 250 KX250 MOTOCROSS

[John Orchard]

Ah yes, after studting many parts catalogues I agree with you guys, the 74 KX had a steel bore and 75 onwards had Electrofusion.

I also agree with you Geoff in that I think the Electrofusion bore is not like Honda's chrome bore in that it can use the same rings as a steel bore.

I'm interested in comparing the TM & CR pistons, just not the CR rings?