piston crown at bdc and squish...

ian van opijnen /

helloo everyone,

my piston crown at bdc has about a mm to go before it will completely open the exhaust port on my kit, i already have two thin base gaskets and want to add a thick 1.5 or 2mm but my squish clearance is already wayy too much, between 2.5 and 3mm! (i dont have digital calipers yet just mechanical)

should i just leave the piston crown showing a little bit to get better squish? or space out the cylinder, maybe shave some of my piston skirt and deck 2mms off the top of my cylinder?

is LOWERING the ex. port something people do to allow it to open all the way?

i need to check all my port timing which i guess would help me out, but i havnt had much free time to tinker lately, running ...

e50

piston port 70cc kstar

hihi comp head with modified squish/head gasket

biturbo (bleh)

phbg20

17btdc

very tall gearing

Re: piston crown at bdc and squish...

Space the cylinder up then either shave down the top of the cylinder or have the head machined on a lathe to sink it into the cylinder 3mm.

Re: piston crown at bdc and squish...

ian van opijnen /

thats what i figured! thanks POWERRR!

Re: piston crown at bdc and squish...

Either that or shave down the piston crown as long as the ring fully seats. If I recall the k-star transfers fully open even though the exhaust doesn't. A milled flat k-star head, a head gasket and 1 thin base gasket dial me in at ~1.6mm for squish.

Re: piston crown at bdc and squish...

If it is only 1mm difference then just mill a short ramp in the piston crown so you get rid of the shelf.

Re: piston crown at bdc and squish...

the amount of time the piston spends at bdc is minimal, so "not clearing" is less of an issue then it appears.

more about time/area then fully opening.

small side issue is it will block some of the pipe hit, again, minimal.

Re: piston crown at bdc and squish...

Same setup same issue. Why would Kstar make a kit with a huge squish like that?

Re: piston crown at bdc and squish...

Overpriced Parts /

These kits and many others nowadays are like this

Stupid huh

We're stupid for not complaining and still buying these kits ,

Junk cdis, junk pipes that fall apart, stuffed cranks that the stuffing comes out and much more aftermarket "racing" parts. That is how they get away from warranting this junk saying "for race only no guarantee"

Re: piston crown at bdc and squish...

Yup its trash or tuners material to make blast.

Re: piston crown at bdc and squish...

If you put a shelf at the exhause when the super heated charge exits the combustion chamber it creates eddy currents which will eat away at the periphery of the piston and can also scorch the ring and draw the temper from the ring.

Also the shelf will disrupt the reversion wave and not allow overstuffing the combustion chamber.

You are contradicting yourself mentioning time the piston spends at BDC and time/area, the piston spends more time at BDC and TDC then any other point in its travel. Do the math.

Glenn Kuehn Wrote:

-------------------------------------------------------

> the amount of time the piston spends at bdc is

> minimal, so "not clearing" is less of an issue

> then it appears.

> more about time/area then fully opening.

> small side issue is it will block some of the pipe

> hit, again, minimal.

Re: piston crown at bdc and squish...

Kits are being produces with looser tolerances and less quality controls to keep the prices down. Just makes the consumer have to bear the task of ensuring that the product they buy suits their intentions or they have to rework it to meet that means.

It is better then back in the day when there was not much you could get aftermarket and would have to start from scratch, no internet forums back then to guide you, everything was ordered over the phone or in writing. A build could take months back then just waiting for parts.

Re: piston crown at bdc and squish...

IAN!!!

1. Try a different piston? Or

2. Use a thicker base gasket to make the crown flush with the bottom of the exhaust port, then measure your deck height and head gasket thickness. Send all that info and the head to Graham Motzing and he'll fix you right up :)

Ps let's ride! (edited)

Re: piston crown at bdc and squish...

Or bring em to mars in Boston he did all my head work. And the piston is the right one. As long as the ring reseats its OK. Metal base spacers are the only real option, paper doesn't cut it.

Re: piston crown at bdc and squish...

RM, you are correct, time, yes, area, no.

draw a bell curve representing time/area

draw a horizontal line clipping the top off, representing the piston stopping before the bottom of the port.

most of the area, vertical is still there, but horizontal, time, it's a fraction.

my choice of wording was poor.

"the amount of time the piston spends at bdc is minimal"

should have read:

"the amount of area the piston blocks at bdc is a fraction of total time/area"

i experienced a similar problem on my grande but modified it like graham M. did

Re: piston crown at bdc and squish...

ian van opijnen /

Pay mars if he's down or deck the cylinder on a piece of marble I have hmmmm

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