Author Topic: Need info on piston deck (Read 4898 times)

jimcarrier · « **on:** July 19, 2015, 10:36:32 AM »

Does anyone have the stock piston deck / piston vs top of cylinder height ?

would help me a lot

thanks

Anthony0327 · « **Reply #1 on:** July 20, 2015, 11:40:57 PM »

They all may differ a little. I don't know what the spec is suppose to be but the only way to know with your setup is to have your cylinder deck height checked. I was told by my builder that a lot of the oem cylinders that he has worked on the deck heights where off and the piston sits a little lower in the cylinder than it should. To get compression In spec, you would have to order a custom gasket from Cometic but like I said the only way to know would be to have the deck height checked. My deck was off and I had to get a custom copper gasket made to bring my setup into spec. One of the builders would know what the height is suppose to be.

jimcarrier · « **Reply #2 on:** July 21, 2015, 05:37:11 AM »

i know that.... im just asking if someone has a spec range, since the bike sit at 150 mile from where i get everything done. im doing a 0 deck with oring set up and would like to save a trip by knowing how much is the piston deep from factory.

i mean with all the engine builder over here, someone must have took the deck height.....

udontknowme · « **Reply #3 on:** July 22, 2015, 01:28:49 AM »

i never bothered much with the top deck. what everything looks like at bdc is what i payed more attention to

wilkin250r · « **Reply #4 on:** July 22, 2015, 02:31:07 AM »

All different avenues to arriving at the same answer. I'll be honest with you, I don't know deck height on any of my machines, but I CAN tell you the squish on each and every one. They're obviously related, but the squish is much more important to me. The ONLY reason I would adjust the deck height is to use it as a calculation tool, knowing that a .010" change in deck height results in a .010" change in squish (as long as I don't change the gasket thickness).

I wish I had an answer for you, but I don't. Only a suggestion. Squish. It's all about the squish. Forget the deck height and focus on the squish. The deck height is just a calculation tool to arrive at the proper squish, so just start with the squish.

jimcarrier · « **Reply #5 on:** July 22, 2015, 05:50:01 AM »

God... my head and porting are done according that the piston will be at 0 deck.... how are you suposed to get your porting right if the piston isnt at the proper deck set for the porting

?? if you port for 0 deck and make the piston lower by 0.040.... yeah squish is going to be big, but the porting is also going to be off. and 196 on the exaust instead of 190 is enought to make a dragster vs a mx motor...

seriously i just needed the stock deck so i can have the bottom of the cylinder decked, That would save me a second trip to the milling shop wich is 150 miles away !

Got the awnser from carlos,

0.040 -0.050 negative for those who want to know.

thanks

mx250r91 · « **Reply #6 on:** July 22, 2015, 07:14:28 AM »

Gee thanks

:carlos:

udontknowme · « **Reply #7 on:** July 22, 2015, 11:35:32 AM »

Theres nothing special about having a 0 deck at tdc as far as I know but alot can be said about the transfloors above or below the piston edge at bdc. Piston to head clearance is easily adjusted via the cylinder top deck or head.

jimcarrier · « **Reply #8 on:** July 22, 2015, 01:56:34 PM »

Cmon cant you ****ing read. I made the porting on a 0 deck base. If i make the deck to 0'040 and ajust by the head. I will have to weld all the transfert and exaust in order to keep the curren duration. anyway.

udontknowme · « **Reply #9 on:** July 22, 2015, 02:14:57 PM »

You must have misunderstood my post. I didnt say anything about changing your cylinder height . just mentioned theres nothing special about having a zero height. Hopefully you didnt ignore all the other important stuff just to simply put the piston at 0 deck. Did you chop the base or take it off the top ?

jimcarrier · « **Reply #10 on:** July 22, 2015, 05:29:44 PM »

ho sorry , i dont get all the english thing lol.

im going to mill the spacer plate to lower the port so i get zero deck and match the porting we did,,, its just that i had no piston with the stock cylinder when we decided to mod it, so i coulnd take mesurement....

i like to put cylinder at 0 deck, less trouble when it come to making a new dome, or in this case modded stock head.

i also agree that 0 deck give nothing xd. i think that the easy set up is flat top piston with the corect squish beeing the negative piston deck... make a lot easyer to build the dome as you only have to build a bowl and dont worry about the squish and squish angle...

anyway, gonna post some dyno result when we end up runing. thanks guys

Jerry Hall · « **Reply #11 on:** July 22, 2015, 06:59:08 PM »

Quote from: jimcarrier;56352

ho sorry , i dont get all the english thing lol.

im going to mill the spacer plate to lower the port so i get zero deck and match the porting we did,,, its just that i had no piston with the stock cylinder when we decided to mod it, so i coulnd take mesurement....

i like to put cylinder at 0 deck, less trouble when it come to making a new dome, or in this case modded stock head.

i also agree that 0 deck give nothing xd. i think that the easy set up is flat top piston with the corect squish beeing the negative piston deck... make a lot easyer to build the dome as you only have to build a bowl and dont worry about the squish and squish angle...

anyway, gonna post some dyno result when we end up runing. thanks guys

I would be doing my porting based upon crank angle and not deck height when developing ports especially if you are using some of the online software . Remember the software and calculations are used to give you a ballpark starting place and seldom are optimum off the drawing board.

After port optimization and development has been finalized through a lot of dyno and field testing then I use the head gasket surface as a reference on my port dimensions on my blueprints. My blueprints also contain the base and head gasket thickness, spacer plate thickness if used and the length of the cylinder from the base gasket surface to the head gasket surface.

I would measure your cylinder length to determine if any has machined. The standard cylinder length on the ATC/TRX 250 R is 4.916"

udontknowme · « **Reply #12 on:** July 22, 2015, 07:04:00 PM »

i see now your reason behind the zero deck thing. no messing around with figuring how much above or below the deck your piston top is and you didnt have a piston anyways so you just set your window heights based on the cylinder top

normally i dont recomend dropping the cylinder because i see no good reason to do so but i wont preach to you about it and im sure you dont want to hear it so ill just shut up about that. this is just my opinion but in the future if you do another engine like this and insist on keeping the piston edge flush with the cylinder top, i would be more inclined to leave the cylinder up alittle higher and shave down the cylinder top.

you never said how much you need to drop the cylinder but if its only .020-.030" you might find it easier to just use thinner gaskets ( if you have to go .040" or more then you may have to cut down the spacer plate). most any gasket shop will likely give you some scrap pieces for free then cut the gasket yourself. i been making my own base gaskets for several years and it only takes about 15min. then again if you have a mill in your garage that may be just as easy as driving across town to find gasket material

jimcarrier · « **Reply #13 on:** July 22, 2015, 07:40:39 PM »

Yeah i get the whole thing about not beeing able to face the cylinder again, but it has been engraved for oring . so i dont really plan to face it again in the next 10 bore avaible ^^.

the only reason i do this is because of the way we made porting, xd