maxpower105 Posted January 28, 2017 Share Posted January 28, 2017 I am in the middle of replacing my OEM pistons with CP forged pistons due to a cracked ring land. I caught the issue immediately and drove less than 5 miles with the issue. The pistons were completely intact upon removal, just had excessive blow by. My question is with the piston to wall clearance. I have done a significant amount of research and I feel it should be OK to proceed but it never hurts to ask. I used a flex hone with proper grit to de-glaze the cylinders and then used soap and water to clean the cylinder walls. I have all cavity B cylinders. Below are my measurements for my cylinders, I did the best i could with a micrometer and dial gauge. I am sure there is some measurement error. My PTW clearance will be between 0.0026" and 0.0033". Do these measurements alarm anyone. If you need more info please ask. Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted January 28, 2017 I Donated Too Share Posted January 28, 2017 You made all the measurements but ultimately if you didn't have a torque plate then it wasn't going to be worth a damn. I am guessing they just supplied you a set of standard 99.5 pistons, there isn't anything special about set SC7420 so again the point of measuring was for fun maybe? Were the rings installed correctly? Did you use your figures to set the ring gaps? It sounds like you took the heads back off after the 5 mile drive... pictures? Link to comment Share on other sites More sharing options...
maxpower105 Posted January 29, 2017 Author Share Posted January 29, 2017 I took the heads off immediately, there was no damage to them. I had them machined flat. They supplied me the standard SC7420. The measuring was to make sure the piston to wall clearance would be sufficient. I did not use a torque plate when I measured, was hoping there would be a correlation between measuring with it and without it. Some info is better than none was my thought. I did not install the rings yet, I measured the top ring in the cylinder and had a 0.022" gap near the top of the cylinders. The engine has about 30k on it and I took the engine out immediately seeing smoke on a start up. I have not driven since. I can take as many pictures as needed, what would you like? Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted January 29, 2017 I Donated Too Share Posted January 29, 2017 I did not install the rings yet, I measured the top ring in the cylinder and had a 0.022" gap near the top of the cylinders. wait. What? You put the pistons in, but you haven't installed the rings yet and you ran the engine for 5 miles? Am I reading that right?!? I am so confused... Never mind - I am confused and I am frankly just an idiot - I understand what you're talking about. OKAY! Hang on - let me answer better... Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted January 29, 2017 I Donated Too Share Posted January 29, 2017 I apologize because I didn't read your first post properly and somehow thought you installed the CP's and went for a drive for 5 miles discovered you had a ton of blow by... you were talking about the original pistons! Okay your numbers seem okay. Is FB one type of measurement device and SS the other because any number below 3.9173 would be a B and we can assume Subaru measured your block better than you did. Two of your SS numbers are slightly over 99.5 For reference the outer diameter of factory pistons: A pistons are 3.6214 to 3.6218 inches B pistons are 3.6211 to 3.6214 inches Ideally you still need a torque plate to measure the ring gap but there are 100's+ of engines built without them. There should be a chart in your instructions about how to calculate the ring gap for turbo charged applications. Bore x 0055" for the top ring etc. I would follow that and measure with a feeler gauge and see if that measure provides some further sanity your ring gap is going to come out correctly - I think you can't have less than 0.16" You can also put that bit of paper aside and start again with measuring and if the numbers don't match very closely you might start to be more concerned by the accuracy of your measuring but otherwise use those numbers and put this thing together. If you are at all unsure and you want this engine to be built for max, with lowest possible oil consumption and reduce blow by as much as you can, then you might just want to let a machine shop with a torque plate and better experience measuring do this step for you. Where are you based? Link to comment Share on other sites More sharing options...
maxpower105 Posted January 29, 2017 Author Share Posted January 29, 2017 I took your advice and remeasured the block, being in WI the temp here is well below the recommended 70 degrees F. So I warmed the block to about 90 degrees then let it cool down to about 65 - 70 degrees then remeasured everything. Below are my results. They look a lot better than the original, meaning they are a bit larger in general which is what I wanted to see. My ring gap is way to small (0.017"), it needs to be 0.022" according to the sheet. So I will need to grind the rings a bit, but my PTW clearance is now between 0.027" - 0.0034" a much more acceptable tolerance. Do you see any issue with that PTW clearance? I only run about 23 lbs of boost on E85, not insane HP numbers ~ 400whp on the dyno. Not a race engine by any means I just don't want it to blow up. Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted January 29, 2017 I Donated Too Share Posted January 29, 2017 What is FB and SS? and are you using bore x 0.0055 number? http://www.cp-carrillo.com/file/6852-Piston+%26+Ring+Installation.pdf Your CP pistons should have come with a Spec Sheet that will have the recommended piston to cylinder wall clearance written on it, as well as where the piston should be measured - is that the only sheet you got that says customer 0008 on it? Manley pistons are a spec of 0.0035" but I don't know what CP says for theirs and that's what is meant to be on the spec sheet? Link to comment Share on other sites More sharing options...
maxpower105 Posted January 30, 2017 Author Share Posted January 30, 2017 The FB stand for "Front" and "Back" the SS stands for "Side" to "Side." That way I can do roundness of the cylinder. The spec sheet asks for 0.0030". In some places of the cylinder it is over and others it is 3 tenths smaller. For the ring gap I am using 0.0055" per inch. Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted January 30, 2017 I Donated Too Share Posted January 30, 2017 I get what you have done... but. Anyway... My only thought is those B piston markings mean that cylinder was less than 99.5 which is less than 3.91732283464567" - to be pointlessly exact. Now your numbers are all in the A range. So unless you honed the living hell out of them I don't know why they got bigger. I think it's not worth worrying about. Stick the rings in the cylinder, measure the gap and set it to what the sheet calls for and your PTW is fine - those are 99.5 pistons. It will all be fine. Don't over think it because you don't have the rest of the tools to overthink it anyway and you are on a road many others have traveled and been just fine. I have installed many Wiseco pistons and just put the rings in that come in the packet, don't measure anything, no trimming as long as the factory hatching was clean in they went and no problems have occurred. I wish you the best of luck. Link to comment Share on other sites More sharing options...
maxpower105 Posted January 30, 2017 Author Share Posted January 30, 2017 Thanks for your help, you have literally told me the exact same thing everyone else has. I need to start listening to that advice. I will move forward with the build, hopefully I will be telling good news in a week or so. Link to comment Share on other sites More sharing options...
dga Posted January 30, 2017 Share Posted January 30, 2017 Any engine I've built, I've gone off the piston manufacturer's recommended specs, unless it was a stock piston replacement, then the FSM. It's hard to go wrong this way. Good luck and let us know how things pan out. Link to comment Share on other sites More sharing options...
maxpower105 Posted February 4, 2017 Author Share Posted February 4, 2017 Reading through the CP instructions it is difficult to understand which is the top ring and which is the second ring. Does anyone know which is which? Also it says they should already be gapped correctly but as I am checking them they are all to small and need grinding. So at 3.9175" bore x 0.0055" I need a gap of 0.021" - 0.022". I am reading gaps around 0.017". Should I grind them or just install and hope its ok? Link to comment Share on other sites More sharing options...
I Donated Too sigmafour Posted February 4, 2017 I Donated Too Share Posted February 4, 2017 0.017" will be too small. Do you have a piston ring filer tool? Second ring will have a taper on one side and an under hook on the other for pushing oil back towards the crankcase and is cast iron. Top ring is steel and won't have those features... look at the edge. Link to comment Share on other sites More sharing options...
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