KR-1S.co.uk Forum

Good morning

I bought the latest Mitaka piston.
a piston has a casting defect in the seat for the piston pin.
it looks like a trough. (left in the images)
perhaps 0.5cm wide and about 2 mm deep.
the piston pin itself fits perfectly.

what do you think?
just a cosmetic problem?
or it can weaken the piston?

otherwise the pistons are good.
Only the piston skirt has a few sharp edges that need to be reworked.

Greetings!

Did you buy two pistons Dirk, or just one?

If you bought two, does the other one have the same defect? Has each piston got the same defect on both sides?

It would be interesting to know whether anyone else's pistons have the same defect as well. I only have the older Mitaka pistons (modified Yamaha 1KT type) - I haven't bought any of the new ones yet.

Regards,
James

Hello James

I bought two.
only one piston has the casting defect

regards
Dirk

I would take both of them back to the supplier and get them checked/replaced. You don't want them letting go as a broken/shattered piston will completely wreck your rebuilt engine

mellorp wrote:I would take both of them back to the supplier...

Yes - if you bought two and only one has the defect, then something isn't right. Certainly return the defective one! If you're satisfied that the other one has no defects, there should be no problem, although you may like to ask the supplier anyway. Let us know how you get on.

Regards,
James

good morning

I wrote PJME a mail with pictures.
i have not yet received an answer.
unfortunately, I can not speak English.
call does not go ..

i naturally asked a couple of two-stroke specialists, what they think about the damaged.
it is 3: 1
three people say there is no problem.
Clairvoyance can of course none ...

regards
Dirk

Post them both back to PJE using registered post, and ask for replacements. Why risk it. PJE are notorious for not answering emails and phone calls.

good morning

answer by PJME:
"I have seen this before with cast pistons and I would have no problems with running that piston."

dirk wrote:....
i naturally asked a couple of two-stroke specialists, what they think about the damaged.
it is 3: 1...

now 7:2

i will install the piston.

regards
Dirk

dirk wrote:good morning

answer by PJME:
"I have seen this before with cast pistons and I would have no problems with running that piston."

dirk wrote:....
i naturally asked a couple of two-stroke specialists, what they think about the damaged.
it is 3: 1...

now 7:2

i will install the piston.

regards
Dirk

If they have never seen a piston like that before, surely you should be entitled to a replacement and that piston sent back to the manufacturer. If the reply had been, they are all like that, then that would be different.

Are PJE offering to repair your bike for free should the piston fail.

For the price of a piston I would buy another one (The Tuning Works can supply) and send the one you have back to Mitaka direct

Hi all,

It's a casting call misruns. It's due to a too cold mold die or entrapped
air.
In this case, it's due to too cold mold. When you start a production, you have
to preheat the mold quite close to the production temperature.
The right place for this piston is absolutely not an engine.
The second picture shows a shrinkaged area, near the piston axle bore, a very stressed area.

Might be useful info I'm not sure. I've got 2 sets of the "new" type Mitaka pistons now that have come out of the engine for one reason or another after less than 400 miles. Both sets, so 4 pistons, all have noticeable play on the gudgeon pin holes. Holding the piston with the pin in place and moving them you can feel it so I got a friend to set one up on a surface plate and measure it with a dial guage, turned out to be around 7 thou vertical movement and 3 thou across the piston. Also if you tip the pistons the gudgeon pin is so slack it just falls out. I've got some very old pistons (poss genuine Kawa) that have done a lot of miles and the pins are still a nice tight almost interference fit.

The problem pistons have come out of 2 different engines, one set the pistons had been coming VERY close to the head so I can understand if that had somehow put excess force on the little end mounts but the other engine was only stripped for a general overhaul and both sets have very equal wear. I'm going to send them all back to the supplier and see if Mitaka can suggest any reason, and have decided when I rebuild I'll probably use Wossner pistons next time.

If you want to see what I mean I can post a quick vid of the problem.

Cheers, Doug.

before you order woessner's, read this : viewtopic.php?t=4648&highlight=one+seen+kind+damage

if you still want them, I'd ask a written warranty that if the pin's aren't located exactly where they need to be they take them back.

Thanks for the info on the Wossner pistons, I had a read through the link.. I'd definitely make sure I can return them if they're not up to spec before I put an order in.

I'm really surprised at the Mitaka pistons having problems but I've got enough of them all with the same faulty to pretty much convince myself that it's a problem with the piston kit not the engine(s). I've been using Mitaka for years on other engines and never had an issue at all.

Other than the advice would you recommend an "off the shelf" piston that will fit the KR1s without any mods?

aparently I never posted the ending of my Woessner-story (or at least not in that topic) : Woessner denied they made a mistake and said they never had that problem, even though the importer send them back my cylinder, there 3 piston's which all had the pins slightly different and non matched the original piston that was also send to them.

the Belgian importer agreed with me the pin's were not located correctly and offered me 2 piston's of my own choice. as he is also the Wiseco importer and I lost faith in Woessner, I went for YZ125 '89 Wiseco's.

@bozbridge
thanks for the info.

i got on warranty (piston pin problem) new pistons.
installing or not installing is the question ...

Cheers
Dirk