Possible race piston?

dave32 · Post by **dave32** » Sat Sep 01, 2012 10:07 pm

Has anyone ever fitted pistons off a 90 KX125?
Ive had a look at a 91 which is a flat top design whereas the 90 was domed like the KR,gudgeon pin is same size,ring peg wont foul any ports (providing there not excessively wide) and the pint to crown edge is the same.
oNly difference i can see is the recess around the pin boss is different,not sure how much of a noticeable effect this will have but seeing as the Mitaka TZR pistons that most of us use arent the same in this area as the OE im hoping not much.
Cheers

KR-1R · Post by **KR-1R** » Sat Sep 01, 2012 11:07 pm

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the H1 125/250 were ground breaking with perimeter style frames
the 1990 H1 kx125 was only ever dished piston (the only NEW bike I have owned = thanks student loan)
it was bridged exhaust = lube holes centreline
flat top / dished (needs shallower heads for compression) see F3 heads for those pistons
shorter skirt (same stroke but shorter barrels, cylinder also lower in the crankcases?)

std KR head.......... vs.......... F3 head for dished pistons

oem parts are No Longer Available

SOME VERSIONS of F/G 88/89 will match but there was domed and dished options in those years (88 89) AS WELL AS SINGLE RING some will be undrilled
...what was the race option upgrade mod of that year became base spec the following year
maybe wiseco 566M05600 for single ring undrilled dished
578M05600 dual ring dished

the H1 didnt arrive in the USA at beginning of 1990 so the G was sold as well through 1990 leading to some doubt if ordering parts for a 90 from USA

there is no cheap supply of New old stock as Usa has pretty much dried up of these years and if you get an OEM pistons what rings will you put on them (you would think the stock of rings would far outnumber pistons)

KR-1..... KX125 dual ring dished..... Wiseco dished

pretty much Wiseco prolites are the only supplier of the pre 90 KX stuff
and the Wossner versions and compatible donor models that may fit (MItaka, for post 90 theres vertex, pro-x)

wossner make a piston specifically for the kr can someone definitively say if they are reliable or not - not needing to pull a working motor apart to try them

MARTIN....TEEREX ??? used mitaka ?

JanBros · Post by **JanBros** » Sat Sep 01, 2012 11:56 pm

Dave, I've done all that research last year : viewtopic.php?t=7562

I believe the YZ '89 is a far better option

KR-1R · Post by **KR-1R** » Mon Sep 03, 2012 11:50 am

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perhaps SATARN can share some main dimensions before selling his oversize Yz items

http://www.ebay.com/itm/WISECO-PISTONS- ... 2321c234ce

dave32 · Post by **dave32** » Mon Sep 03, 2012 2:45 pm

OK my bad,i read concave as convexed

I suppose though there's no reason you couldnt reprofile the head to run either the 91 flat top or 90 dished?
Jan,good research there,well done

Did you have to alter barrel lengths or re machine the head to run the YZ parts?
Any idea what the combustion chamber volume was that you ended up with?
If you go the wiseco route with the YZ parts dont you need to run extra piston to bore clearance as there Forged not cast so have a different expansion rate than what the bores were set for?
cheers

JanBros · Post by **JanBros** » Mon Sep 03, 2012 9:31 pm

I shortened the barrels on the top : you don't loose strenght , it raises the ports a bit and it's much easier (and almost impossible to ruin the flow of the transfer ports) to match ports to the bottom of the piston.

made inserts into an original KR1-head and designed my own combustion chamber (a conical-toroidal

)

The Wiseco importer said there is no need to change the bore to match forged/diecast piston's : the piston's are made for the specific bore, so the clearance is calculated within the piston. die-cast 56mm piston and forged 56mm piston use the same bore, the diameter of the piston's differs

There is absolutly no reason to go for the KX piston, unless you want to keep it "Kawa-parts" only. Every manufacturer has piston's for the YZ '89 and will be making them for a long time and they are in many oversizes available.
The KX piston is an oddity, not many make it so supply for the future is a question mark.

everything is in this topic : viewtopic.php?t=7529&start=0

JanBros · Post by **JanBros** » Mon Sep 03, 2012 9:50 pm

don't know why anymore but '89 YZ was the only good option.

if you want piston dimensions :

http://www.mitaka.co.uk/PISTON%20DATABASE/PAGE.html

http://www.mitaka.co.uk/pistons.htm

http://www.gpservice.de/html/racing.html

http://www.pistonigol.com/catalogo.asp#top