MZ/MuZ Riders

Hi all,

My 03 baggie has finally ben stripped and the piston has cracked and split. My mechanic says she needs a new piston, rod , rod, big and small ends and a rebore. Can anyone advised me of my options please? I can't afford to spend loads, I just want to put her back on the road. Where can I get the bits, are there any good/cheaper alternative parts to standard, what's my best option and what are the approximate costs please? Can anyone also tell me what the yamaha part numbers are for the bits I need please, and are there any bits from alternative bikes that fit? Any help would be really appreciated. I'm near Reading in the UK.

Cheers Mud

Not much to go by!
How abou a little history leading up to the disaster?
A couple of photos would help, too.

I had a broken piston once, but that was 12,5:1 forged piston in a racing engine.


Since this was a 102mm piston, the cylinder wall was very thin. Yours will not be crushed like this. or even really damaged beyound scoring.
This one is beyond help. I threw it away. You could shrink a new liner in, of course, but that costs as much or more than a decent cylinder used.
There was no damage to the bottom end at all and it is still going strong in my present engine. Same big and small ends and crank bearing.

OK it is a Carillo rod and one-off crank but it was perfectly OK despite many engine blowups. Has to be expected when you approach 80hp.

It definitely will need to be rebored from 100mm to 100.5 if you go for stock oversized piston.
Or for a 101mm Wiseco flat top W4607M10100 which will be no more expensive than the OEM item, probably less in fact.
Don't be mislead by it supposedly being for a Grizzly; it works fine and I use it almost exclusively in the 660 engines unless allout HP is the target.
I have see damaged rods. Look for yourself, if the small end is chewed out, try the piston pin for fit.
They are rare, however, and where they are bad, the small end can be bushed with bronze. The rod itself is almost indestrucible.
chewed up big ends are more common and alway the result of:
1) no oil, or
2) blown head gasket.
So regardless, the head will have to milled and a slightly thicker cylinder base gasket used.
The crank will have to be rebuilt if such is the case, a costly item: here in Germany at least 300€ which would be very good price.
Again, don't take anybody's word for it, look for yourself. It is easy to check the big end. If the small end is OK, the rod can be reused usually.
Instead of rebuilding, you might look thru eBay for a XTZ/SZR/Skorpion crank (3YF) which you ought to be able to garner for 150€ at most.

So collect your parts, clean and assort them on the table and have a close look, taking pictures.
DO NOT LET THE MECHANIC THROW ANYTHING AWAY!
If you know a good toolmaker, have him look as well. They are familiar with tolerances and surface finishes.
Above all, take the oilpump apart and clean it. It may be too far gone to be reclaimed if it picked up any real debris and/or was pumping more water than oil.

Hi Bill,

Thanks very much for the advice. I haven't seen the engine as it's down at the mechanics workshop. He is trustworthy. He reckoned it would need a new rod as at matter of course as that's what he does with his racing bikes, but I'll get him to check it thoroughy as it would save a heap of cash! I bought the bike with a rattle off a mate who's had it for 3 + years and looked after it. It's not been raced or tortured. It's got 35000 KLM's on the clock and was riding fine until the rattle started. My mechanic said he'd never seen another piston like it as it's cracked all the way through the skirt at the rear. The bore is scored, so it's going to need a rebore. Is there an optimum size to go for and a decent piston to fit? I've seen loads of pistons on Ebay etc, but I'm a bit lost without a part number as very few list the size? Is there a good online retailer that anyone can recommend? He reckons the crank is fine, but suggested putting in new bearings to be safe. Are there any other bearings worth searching out or should I stick with standard yammie items? I'm a bit out of my league at this stage, so once again, thanks very much for the help!

Cheers Mud

Forgot to add, I need a gasket set as well. What sort of price should I be paying, and where is the best place to get one? Do all yammie 660 variant gasket sets fit, ie SZR / SDR etc as I've seen a set for a 96 SZR which was cheap?
Cheers Jim