Hi all, i noticed on my 1979 dt250 that the piston is not flush with the exhaust port floor at BDC.
Would raising the cylinder so that it is flush with the exhaust port floor and then getting the top of the cylinder skimmed down so the piston sits at tdc correctly like stock make the bike run better? As i know modern 2 strokes have the piston flush at BDC with the exhaust port

Do you think someones been cleaning up the port & cleaned it more than desired on the lower side. I've seen then like that but not by so much. I'd leave it alone. I've seen on some models the rear of the two transfers on each side cut lower than the piston at bdc so you'd think the gas wold smack into the piston. But yamaha must know what they're doing--i guess. Ok that opposite to your problem i realize but lifting the cylinder & skimming the top raises other problems if you go as far as the step look in your pic looking in. See what others with experience on these things say. I'm an expert--at cocking things up lol. ps, piston looks nice, & wiseco?, did they give it good clearance.

Hope it's not a Chinese piston. Definitely looks aftermarket.

And yes, looks "ported". Make sure that when piston is at TDC, the lower edge of the piston skirt doesn't go higher than the bottom of the exhaust port.

Check clearance-squish to head at tdc. Looks about right with transfers at bdc. Maybe nothing worth worrying about.
I wouldn't recommend it--even though i might try it--is modify the piston at the port to advasnce it a bit. Can't do it with the L shape top yamaha ring. Na, better not go that way. Piston looks too nice.

Most I have seen are exactly the same.

Exhaust does not matter at BDC, X-fers use the edge of the piston to provide a soft transition and attachment of the flow.

This directs more flow horizontally to flow across the crown better.

Without this on a stock port, the air/fuel flow will be vertical for the most part, never flushing the crown.

cliff

Its a wossner piston which is forged (made in germany) the engineering shop bored the cylinder to suit the recommended clearance by wossner on the box the piston came in. 

I did clean up the ports myself and ported it to the yamaha mx250 spec but i never lowered the exhaust port floor or any other ports.
I cant remember when i rebuilt the engine if the stock old piston did protude above the exhaust port or not,
would anybody be able to clarify this if the piston sits above their bikes exhaust port?

"would anybody be able to clarify this if the piston sits above their bikes exhaust port? "

Automan just did in post #5.

 

I guess if the old piston "didn't" sit up like that, then new one is going to push the head off. So on that warped theory of mine i think you'll be ok. You'll soon know when turning it over before bolting the head on. Let's know the answer please.

Cheers just seen his post,
but would getting the piston flush with the exhaust port give the bike more flow and power from the expansion chambers  supercharging effect?
Just thinking as modern 2 strokes have theirs flush at BDC, and maybe this was one way yamaha detuned their DT lineup? or old two stroke technology that hasnt progressed yet to flush piston crown at BDC like later new models?

Im just thinking with the piston protruding like that it will disrupt flow of the exhaust gas leaving the cylinder after combustion by not allowing the exhaust port to be fully open and not allow all exhaust gases to escape and get a new full charge of fuel/air to combust when on the way back to TDC. And the exhaust gas/sound wave being disrupted as it reenters the cylinder by the expansion chambers "supercharging" effect not able to get more "charge" back into the cylinder as the exhaust port is partially blocked by the piston and isnt able to use the exhaust port to its full potential. And also hot exhaust gases hitting that protruding piston edge when they reenter the cylinder. Maybe im over thinking this...

Does anyone know if the yamaha yz's, it's or mx's of the same era have their pistons protrude a little like this?

Would any engine building gurus like RICHTAYLOR etc have any info whether its good to get the piston flush at the exhaust port at BDC?

​​​​​​RT325 when all bolted together the piston goes to TDC exactly the same as the stock piston removed, and does not hit the cylinder head, the wossner piston is the same size in height as the stock yamaha one
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I'd better not send you down the wrong path. See what others say that know a lot more than me about porting. I'm good at making hand grenades out of my motors--fast but not for long lol. Just bolt yours together & it'll be fine in my opinion.