250-500cc Another Piston Thread

I used this break in process the first time I rebuilt my 450... then it blew a rod
View: https://www.youtube.com/watch?v=G8Ms6JDLl-8

Now I use the "hard break in" method in the article above...

I used heat cycles do two or three then do an enduro.
The main reason to let it do heat cycles is to bed the rings in, on a two smoke if you give er the beans straight out you will glaze your bore and the rings will be sealing against glass.
Ever wondered why when you get a fresh cylinder back its honed with cross hatching?
Thats to trap oil and fuel mix so the rings have something to seal against.

I assume the heat cycles just harden the hatching.. I know its nikasil its hard as hell but it works.

Great advice from everybody, thanks. I'll do the manual's method, too.

hey Mike, glad to hear you finally got the piston. Good luck with the rebuild!

So many of us did replace the pistons over the last weeks- did nobody realise the transfer ports are not fully opened when piston is on lower dead center?
or is it just my bike beeing different to yours?

Transfer ports are closed about 3 mm from bottom to top of piston crown on most bikes.
This isnt really a problem as when the piston rises it forces fuel mix down to the lower end breifly to lube the mains then piston returns the reeds shut the fuel mix is accelerated up the transfers into the cylinder above piston out exhaust and then gets pushed back into the chamber by the expansion shock wave from previous explosion.
Piston cycles goes through it again.
you can change port duration with base gaskets but compression ratio and squish will suffer.
Personally if your bikes not down on power I wouldn't worry, well i dont
Suppose i could of raised my cylinder and machined more off my head to get squish back but then the stroke will be affected.

summary who cares, just ride.

So it's done! Broken in and running well.

I think I averted a near disaster.

It started on 2nd kick for the 1st break-in warm up.

Had choke on. It started revving to like WFO for a few seconds! Took a few stabs of the kill switch to turn it off. Turns out my throttle cable was pulled tight out of the carb from when I put the tank back on. That actually happened to me on a normal ride day about a year ago. Took a while to figure it out then.

Once I adjusted the cable everything was back to normal. So it turns out maybe I inadvertently did the let'er rip break in. Hopefully, no damage was done. I proceeded to follow the manual's break-in instructions. Still have to do the last 15 minute trip but I think I'm good now anyway.

Thanks everybody for the help throughout this thread.

Great news Mike!

Most important thing in the whole break-in procedure is the cool down...let the motor cool down to stone cold between each break-in cycle is super important!!

Thanks. Just about did that. Not quite stone cold but cold/cool I'd say for the 1st 2 cycles. Definitely stone cold for the 10 minute ride.

Finally putting it back together. Our Enduro is in 2 weeks so hopefully no additional complications View attachment 65535

Stopping point...

Looking good joedints, i normally get reed block on while the cylinders out more access and less fiddle, you doing squish clearance now?
Lever the rings off the piston there not needed for the squishy testing its quicker changing base gaskets before the proper assembly.

Love a fresh rebuild.

How did you get that cylinder so clean, inside and out? Especially in the power valve linkage areas.

I spent some me time on mine with a brush, simple green and carb cleaner but it looks nothing like yours.

Im going to say thats been replated so the company would of cleaned it.

Cylinder was replated and cleaned by them, I cleaned everything else myself.....

Now... If I can figure out the power valve... The actuator arm coming from the bottom end where it hooks up, the "d" shape is reverse of the arm it attaches too...

These photos help?

Yeah thats a 250 and 300 pv set up whats wrong then?