91banshee Posted January 25, 2007 Report Posted January 25, 2007 i am almost done with my la sleve 409 ported big bore kit , i have a stock shaved head, will it need to be cut or can i use it like it is . i have heard it can and cant , whats the deal Quote
dajogejr Posted January 25, 2007 Report Posted January 25, 2007 Are you running a stock stroke crank? Quote
Forcefed Posted January 25, 2007 Report Posted January 25, 2007 You can run it like that, but you really should not... here`s why. It is VERY important to maintain the proper squish between the piton and the head. If the inside diameter of the dome is smaller than the outside diameter of the piston, then the result is that the difference between the 2 will net a MUCH tighter squish than the rest of the area in the squish band. This could be bad. Lets say your 68mm pistons come to "zero deck" at TDC, which means perfectly flush with the top of the cylinder. And if the i.d. of the dome is say 65mm ... well, you get the picture .. ZERO squish. Detonations problems will plague you from day 1. You would have to run very rich with like no timing to keep things from getting messy. What you want to do is verify the piston to deck height at TDC. Yes, thousandths of an inch are critical. They may be in the hole .010, or out of the cylinder .005 ... whatever it may be ... BE SURE to be accurate. Once you have this number, you now know how deep to cut the "step" in the head, to achieve the proper squish clearance. Typically you want to be right around .035. No less than .030 and no more than .040. Imagine if your 68mm pistons come out of the hole by .005 while running a dome with ANY LESS than 68mm dome i.d. ! BANG! BANG! BANG! BANG! Set your tolerances up properly and you can assure happy, healthy, high performance fun much longer than leaving these items to fate. :thumbsup: Quote
dajogejr Posted January 25, 2007 Report Posted January 25, 2007 You can run it like that, but you really should not... here`s why. It is VERY important to maintain the proper squish between the piton and the head. If the inside diameter of the dome is smaller than the outside diameter of the piston, then the result is that the difference between the 2 will net a MUCH tighter squish than the rest of the area in the squish band. This could be bad. Lets say your 68mm pistons come to "zero deck" at TDC, which means perfectly flush with the top of the cylinder. And if the i.d. of the dome is say 65mm ... well, you get the picture .. ZERO squish. Detonations problems will plague you from day 1. You would have to run very rich with like no timing to keep things from getting messy. What you want to do is verify the piston to deck height at TDC. Yes, thousandths of an inch are critical. They may be in the hole .010, or out of the cylinder .005 ... whatever it may be ... BE SURE to be accurate. Once you have this number, you now know how deep to cut the "step" in the head, to achieve the proper squish clearance. Typically you want to be right around .035. No less than .030 and no more than .040. Imagine if your 68mm pistons come out of the hole by .005 while running a dome with ANY LESS than 68mm dome i.d. ! BANG! BANG! BANG! BANG! Set your tolerances up properly and you can assure happy, healthy, high performance fun much longer than leaving these items to fate. :thumbsup: damned good explanation.... I personally like a little more squish gap, low to mid .040's....but, each builder/tuner has their own preferences... Nice post, forefed! Quote
Forcefed Posted January 25, 2007 Report Posted January 25, 2007 Yeah thats true, I shouldn`t have been so arbitrary ... you can be more or less than .030-.040 - but that is a general rule of thumb starting point. Altering your squish more or less WILL require timing and jetting changes to achieve desired results. That is one of the reasons i would never tell someone to run their timing high unless I know ALL the variables within the confines of the head & jug. :wink: 91Banshee - something else to look into .. what series are the pistons you are using? 513 - 795 ? Reason I ask is because the angle that the crown of the piston is cut at and the angle of the squish band should be near parallel, and since both series of piston utilize different crown angles, you may need even more machine work. Did you shave your head allready knowing your piston to deck height ? Quote
91banshee Posted January 26, 2007 Author Report Posted January 26, 2007 Yeah thats true, I shouldn`t have been so arbitrary ... you can be more or less than .030-.040 - but that is a general rule of thumb starting point. Altering your squish more or less WILL require timing and jetting changes to achieve desired results. That is one of the reasons i would never tell someone to run their timing high unless I know ALL the variables within the confines of the head & jug. :wink: 91Banshee - something else to look into .. what series are the pistons you are using? 513 - 795 ? Reason I ask is because the angle that the crown of the piston is cut at and the angle of the squish band should be near parallel, and since both series of piston utilize different crown angles, you may need even more machine work. Did you shave your head allready knowing your piston to deck height ? i sold the stock head i had and bought a cool head with 19cc big bore domes in it , i got freaked out thinking about the bang bang . thanks for the input Quote
Forcefed Posted January 26, 2007 Report Posted January 26, 2007 No problem ... glad to help. For peace of mind, when you get your new domes, check the i.d. and the step. Nothing like assembly with confidence. Quote
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