KR-1S.co.uk Forum

Hi guys I've done a search but I just want to confirm.

The mitaka pistons sold for the kr1s are just re-badged dt 125 pistons with 4 holes drilled in them is this correct?

Because the holes have been drilled in them this would be why kr1 mitaka pistons are more expensive than mitaka pistons for the dt 125 ?

If this is the case I will buy dt 125 pistons and drill the holes myself.

Also I have noticed that PJME stock a vertex piston for the dt 125 too:

http://pjme.co.uk/acatalog/Yamaha_DT125 ... p_End.html

Now I would much rather run a vertex than a mitaka as I know they are a quality piston, I rang up to ask if they had experience running the vertex in a kr1 and they told me that the piston height is different and it won't fit, they didn't tell me by how much or anything.

I don't believe that it won't fit, if a mitaka dt 125 piston fits, why won't a vertex one?

Also would the vertex pistons rings be ok running on a plated bore when the the dt 125 has a cast iron liner?

Cheers,

Dan

if I'm not mistaken, Vertex piston's are made by Woessner ...

Mitaka dimensions can be found here : http://www.mitaka.co.uk/PISTON%20DATABA ... Page3.html

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some pics here (if you havent seen)
http://kr-1s.co.uk/forum/viewtopic.php?p=55490

KDX125 vs KR-1


DT125... seems better alternative than KDX (longer skirt, drill some lube holes)


Mitaka...

Vertex...

...looks to be cast therefore not made by Woss?

the Mitakas have that galvanised/eutectic, dull as cast, look - the Vertex reflects light so the skirt machining might be better for lubrication (and wear appraisal - tiny ridges)

Yep the vertex piston is a cast piston with a high silicon content. They are a very good piston. Ktm use them as their oem piston in their bikes.

I have seen the troubles of ring pig location with the wossners so i would like to avoid them.

I will order 2 vertex's and give them a try,i have a dt125 prox piston here that i can compare with.

Dan

Like the idea of using the dt125 vertex piston.
Excellent piston and you can get hold of them !

Good luck with the trial Dan

Hi guys,

Just spoke to TWS GB who are the UK suppliers of vertex pistons, they can get the vertex piston for the DT 125 in a 56mm size, 0.5 oversize, 0.75 oversize and 1.00mm oversize.

They are currently out of stock on the standard bore 56mm piston but he usually keeps them in stock and will have more in 2 weeks.

I asked him what price they are and he said 74.95 but that was a quick calculation he did there and then, by the sounds of things I think if we ordered a very large number the price would come down.

I will happily be guinea pig for these pistons and give them a go in my kr1 barrels.

Here are a few close up pics of the piston stolen from the tdr forum:









This is a 0.75mm piston but the 56mm is the same design just smaller. I asked the man at TWS if the rings will be happy running on a plated bore instead of a cast iron bore (that the DT has) and he said yep it will be fine.

I need to find out whether they need to run a 19.5mm wide small end bearing or a 17.5mm small end bearing, both sizes are easily available but just need to know which one is needed.

Information on the tdr forum suggests that a 17.5mm wide small end is needed whereas info on the PJME website suggests that a 19.5mm wide small end bearing will be fine.

Only one way to find out......

Also i remember reading that 3ma pistons fit the kr1 too, I have 3 3ma pistons and a dt125 mitaka so I will be able to compare all of those plus the vertex and the original kr1 and the later kr1s piston. I will join all 5 of them by the pin so we will be able to see the differences.

Dan

Vertex ordered, plus 2 X 19.5mm small ends and 2 X 17.5mm small ends.

Will update this thread when they arrive.

Dan

Dug out a 3ma piston and the prox/mitaka dt 125 and marked the ring peg locations on the crown and put them into a worked barrel:

Prox


3ma


The rear port has been widened on this particular barrel but the rings are still just about ok, single rings on this barrel would be better though I think.

Anyway so the vertex ring peg position should be fine, will just have to compare compression height and check squish.

Dan

If it helps i had a kr that had been sleeved and ran TZR 2MA pistons so yeah it should work fine. I suppose if you needed a replate you could use 2.5 oversize to increase capacity too although im not sure.

Let us know how you get on with them as i have some barrels that need a replate so i might try the over size option which would be about 270 at a guess i think?

maccas wrote:... spoke to TWS GB who are the UK suppliers of vertex pistons, and he said 74.95 but that was a quick calculation he did there and then

so how come PJME sell them for £56.95 + VAT (£68.34)

I'm not sure danny to be honest,i bought them from pj because they had them in stock too.

John,

When your bike ran 2ma pistons did anything need to be done about the increased compression height of the piston or was it the case that on a stock bike the tolerances are so wide that it was ok?

Dan

Well got the vertex pistons, they look very good quality indeed.

However they do need the 17.5mm wide little end bearings and guess what, they've sent me four 19.5mm small ends!

Typical on bank holiday weekend grrrrrrrr

Dan

Well got the vertex pistons, they look very good quality indeed.

However they do need the 17.5mm wide little end bearings and guess what, they've sent me four 19.5mm small ends!

Typical on bank holiday weekend grrrrrrrr

Dan

Pics to follow tomorrow, quick look suggests that dome radius is larger than kr piston but will see tomorrow (i.e height of dome is less)

Compression height is slightly more approx 0.5mm

Piston is 2-3mm longer in total too

Dan

The cut aways around the gudgeon pin look like they will cause some bad short circuiting between the boost and transfer ports.