Need some help with piston to port alignment

[INTMD8]

I took my 89 250r apart to replace the piston since I only had 130psi compresson.

It has an LA sleeve and the new piston is 72.95mm bore, part number W5032P4. The engine has a hot rod if that matters.

Anyway at top dead center the flat squish band of the piston is perfectly even with the deck of the cylinder.

At bottom dead center however, the top of the piston is approx .080 to .090 above the bottom of the side ports and exhaust port. I know nothing about 2-strokes but this does not seem right to me as it looks like the top of the piston would block flow through the ports. (I would think the piston should stop even or even go below the bottom of the side and exhaust ports)

It seems as if to fix this problem I would need a .090 spacer between the cylinder and crankcase and then I would need to mill the top of the cylinder .090 to bring the piston back flush at TDC.

Again, I know nothing about the operation of 2-strokes so am I thinking about this the wrong way or is this indeed a problem? What is the proper relationship between the piston top and ports/top of cylinder?

THANKS!

[flotek]

the only logical explanation i think of is your jug is ported for a 4mill stroker crank ....and you dont have a stroker crank currently in your bike

[INTMD8]

Yeah, for some reason it seems the ports are too low.

I added an extra base gasket to put the ports up a bit higher. I then tested compression (cold before running it) and it was only 90psi.

I then replaced the 23cc cool head with a stock head and the compression went up to 145psi.

After a few heat cycles and seating the rings compression is now 195psi hot.


I found it interesting that before freshening it it had an FMF pipe and a 192 main jet. Now that I've freshened it and installed an esr trx5 center mount and open element air filter I'm down to a 170 main jet and it still seems a bit rich. (plug chop shows a dark ring about .100 tall from the bottom of the porcelian).

Going to do some riding on a 168 jet tomorrow and see how it is.

[flotek]

i dotn see how you could gain an additional 50 pounds by breaking the rings in a typcial heat cycle it must run like total shit and rich with such a dramatic loss of veloicty fro m the ports being way too low..you gotta fix this situation

[INTMD8]

Well pm me a price on a mid-top end ported cylinder with a 72.95mm bore.

I was wrong initially as the piston was slightly above deck. The extra base gasket brought the piston even with the deck and about .065 above the bottom of the side ports. At this point I could buy a spacer plate, have it milled until the ports aligned correctly, then deck the top of the cylinder to bring the piston back even, but I'm probably better off replacing it rather than going through that much trouble.

What is considered 'correct' for port alignment. Should the piston stop even with the bottom of the ports at BDC or actually go below them entirely?

[INTMD8]

-also, not sure what to say about the compression, but it was definitely 145psi cold, before it was ever ran verified with 2 gauges. After a couple heat cycles I pulled it into the garage and did a compression check and it was 195psi.

Seems to run rather well to me in the low/mid range but I don't really have anything to compare it to.

[flotek]

the honda is not a zero deck motor that is to say the piston top edge will not come dead even with top of the deck instead it sits about .035 below it,now the bottom of the stroke bdc the edge of the piston should be even with bottom of transfer ports and a few degrees from bottom of exhaust port ,be sure your not talking about the ring edge here ... im talking edge of the piston where light could be seen as port opens

[INTMD8]

Gotcha, and yes I am talking about the very top edge of the piston and not the ring edge.

I've got bigger problems than that though as looking down the intake port I could see the piston was scored/gouged so I removed the cylinder and piston. I did not check piston to wall clearance before assembly (though I wish I would have) but it seems as if it's zero.

Dropping the piston in the bottom of the cylinder (with no rings) results in the piston only making it in half it's own length. I then have to force it into the cylinder and the piston skirts are very tight against the bore.

[all250r]

Originally Posted by INTMD8

Gotcha, and yes I am talking about the very top edge of the piston and not the ring edge.

I've got bigger problems than that though as looking down the intake port I could see the piston was scored/gouged so I removed the cylinder and piston. I did not check piston to wall clearance before assembly (though I wish I would have) but it seems as if it's zero.

Dropping the piston in the bottom of the cylinder (with no rings) results in the piston only making it in half it's own length. I then have to force it into the cylinder and the piston skirts are very tight against the bore.

Wow, this all really sux. You should try to post some pictures of the deck height and transfer ports. It does sound like you have a stroker port though, in which case your timings might make it more worth it to pick up a used oem cylinder of some not too crazy degree of modification and start over.

Have your builder hone to a particular piston is the best route, though wiseco seems pretty consistent with their production tolerances.

[flotek]

id save your self a ton of greif and just get a newer unported fresh cylinder off ebay