Thread: Piston clearance??

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So, I started with a .030 over 509 block 400 with a good bit of wear, and I really didn't want to go .060. So I did what everyone says not to do and bored it without having the pistons just to see if it would clean up before I bought the pistons. I bored it 4.163 which would be .038 over and it wasnt clean on three holes. So I ended up having to go dead on .040 over at 4.165 and two holes still had a small place at the very bottom where the bit didn't touch. I measured it and I could see where it didn't clean up but couldn't really measure it. So I got my neighbor (who owns the machine shop...he is in his 80's and retired) to measure it and he got the same results which was less than half a thousandth. So I bought a set of .040 keith black kb147's and thats as far as I got. That was in 2009. I just got back on it this week and as you can tell I havn't exactly been in a hurry. I measured the pistons last night and found that they measure 4.161, so with my block being 4.165 I have .004 clearance. Keith Black recommends .002-.0025 clearance for street applications so should I go ahead with the .004 clearance I have or do something different? Here is what gripes me....if I was going to design a piston that was meant to be installed in a .040 over 400 block, I would make them 4.063 which would give .002 clearance in a 4.065 block. Isn't that too simple?? My block wouldn't clean up on the .038 over that these pistons require, so what do I do now?

what are you talking about sbc 350?or400 you go from 4.063 then you say 4.065 then say 4.161 i bore engines and hone them all the time first off you never finsh a bore till you have the pistons and check real size.... i all ways leave 005 to 002 less for honing this has alot to do with your boring bar and how good a finsh cut it makes my F2b rottler puts a very nice finsh cut so if i am not QT honing i leave .002 to 003 to hone to finsh size with deck plate i will bore with it on the engine or leave .005 under size and hone to finsh size . with cast pistons and hypers you can not go + over bore much . some forged pistons made from 2618 you can go + over size lets say you have a bore of 4.125 +030 =4.155 and you want more piston to wall for hard use . if you have lets say .003 at 4.155 then you could go to 4.157 if you wanted .005 piston to wall but you would finsh the bore with boring bar .005 under size to hone to 4.157 so you stop boring at 4.152 if honing with a deck plate and all 400s chevys should be Tq plate honed all pistons have built in size they are made to. so they have the right piston to wall size when hole is bored and hone to size of over bore

Sorry Pat, I didn't realize I posted those crazy #'s....it's a 400. I went back and edited my post and fixed that. I understand that it's not a good idea to finish a block without the pistons, but the point is that this block wouldn't clean up any less than 4.165, and even then it still had faint signs of wear at the very bottom of two cylinders. I couldn't have went any less. I just took a gamble on this one cleaning up without having to take it to .060 over. I took it the bare minimum to clean it up and then ordered the piston after it cleaned up at 4.165. I should have measured the pistons when I got them to make sure they would work but I didn't so now I have to come up with a fix. Should I run them with the .004 clearance I have or try to find different pistons? It just seems to me that the keith black pistons I have are too small. If they require .002 clearance and they measure 4.161 then the block would need to be fiished at 4.163........that's not .040 over LOL.

were are you measuring the pistons at. how far down the skirts if hypers they are not going to like 004+ they will hammer .i know how to fix it

I'm measuring at the largest point on the skirts.........Is another set of pistons the fix? As you can tell.....I'm a technician, I'm only posing as a machinist playing with my own junk. My neighbor is a retired machinist. He's in his 80's so I try not to bother him too much. His shop is closed but I have access to it. I don't have all the knowledge I need but I'm learning.

there is 3 ways to fix this 1# is to knurl the piston with a pistons knurler.2# try another set of pistons set of forged pistons can take more piston to wall3#bore block out to the next size up. there is others ways as well custom pistons were any size can be made i have done this were we just bump up the pistons size by .002or more put a sleeves in the block or new block.send pistons out for skrit coatings

I called keith black and a tech told me the pistons should measure 4.163 so I'm going to take more measurements this afternoon. Hopefully I was a little off on my measurement. I might not have been measuring in the right place. Hopefully its just a micrometer operators malfunction.

Originally Posted by 35chevy

I called keith black and a tech told me the pistons should measure 4.163 so I'm going to take more measurements this afternoon. Hopefully I was a little off on my measurement. I might not have been measuring in the right place. Hopefully its just a micrometer operators malfunction.

did you check your mic with a standard

Your getting awful techenical there Pat

Yep that was it............just my mistake. Took the pistons to my neighbor's and used his mic and they measure 4.163, exactly what the Keith Black tech told me they should. Rechecked my measurements on the block and it's 4.165 so I'm good with .002 clearance. If my first measurements had been correct I was going to look into forged pistons. Thanks for the help. I learned something today LOL.

Originally Posted by 35chevy

Yep that was it............just my mistake. Took the pistons to my neighbor's and used his mic and they measure 4.163, exactly what the Keith Black tech told me they should. Rechecked my measurements on the block and it's 4.165 so I'm good with .002 clearance. If my first measurements had been correct I was going to look into forged pistons. Thanks for the help. I learned something today LOL.

well your not good if you bored to 4.165 you should be at 4.163 or less you need to hone the finale size to 4.165 . you could use a good chewing out but i will not do that only to say you should allways verify that setup mics for boring bar interface with all mics used threw out the job before you start the job .this machine stuff is not like baking cookies

Originally Posted by cffisher

Your getting awful techenical there Pat

this is the way it Must be i need to stay at or under .0002 at all times or i can not live with myself

I understand. Where I used to work we made the metal (aluminum) for Ford hoods and trunk lids. If it wasn't exact back it came. we'd melt it down and try again.. Not much profit there.It had to be right gauge, right tencile strenght, right width and length. There wasn't much scrap coming back unless we made it

just go ahead and chew me out...LOL. I definately learned from this.

Originally Posted by Louey

Now I'm going to tease ya...Take that sign out of there...

Why should I take that sign out of there? You must have noticed where I edited my signature earlier? Yes it did say earlier today that I was a high shool automotive technology instructor. They closed my program this year due to funding and more programs are on the chopping block next year. I didn't have to be a machinist to teach the class and as far as that goes engine repair isn't even required by the National Automotive Technicians Foundation to be taught on the high school level. Actually it is often discouraged due to the fact that we are required to teach brakes, suspension and steering, electrical and electronics, and engine performance and a student could only take the class six semesters. I have worked a long time as a tech but I have generally left the machine work to someone else except for running a brake lathe. My precision measuring is generally not so precise and limited to measuring brake rotors and drums to make sure they aren't warped or too thin and measuring spark plug gap and a/c compressor clutch air gap and that sort of thing. I am not usually looking for and exact number but for a number that falls within a set of specifications. Definately not stuff that is to the nearest ten thousandth and not as precise as engine machining. I use a vernier caliper, dial indicator, feeler gauge, torque wrench, and a measuring tape quite often, but a micrometer very seldom. I have access to some machine shop equipment and I'm trying to learn more about it on my own stuff. I might be messing it up but oh well, it's mine and i have to learn some how or I can just give up all together and never learn it. I learned this time that blocks aren't always bored dead on 20, 30, 40, or 60 over, and its a good idea to measure the piston in the right spot to get the correct reading. I only measured on place and I didn't realize they had as much taper as they do so I was off a tad. I'm reading up on the benifits of torque plate honing as well as many other things. My neighber is a retired machinist in his 80's and I guess he did a lot of stuff the old school way. The equipment is pretty old but it has been used to build countless engines. I had him bore a 300 six for me with his old bar and honed with no torque plate and it had over 300,000 on it the last time I saw it. He machined one for us in the 90's for our mini-stock car that won the track championship six years in a row and the engine turned 9800 rpm every lap. Of course we broke it a couple of times but the crap worked. I didn't know his techniques weren't correct.....the crap just worked and very well. We were blowing the doors off the stuff being machined by the other shops doing it the "correct" way. I didn't know there was anything "wrong" with the engine that's been in my '35 for close to 15 years now until recently. You sure can't tell by the way it runs. It makes good power and doesn't knock, smoke, or use oil, but according to some it is a piece of junk because it wasn't honed with a torque plate. It was bored with a antique quick way boring bar to size and just lightly honed before assembly. I didn't know until I started digging deeper into this stuff that it was unacceptable to do that. The crap he did worked on a lot of engines for a lot of people that's all I know. Now I know it's not the right thing to do and his grandson and myself are looking into trying to update a few things. The old timer told me he was looking into getting torque plates around the time he closed the shop but decided to retire. I'm actually enjoying learning more about this stuff. It's quite a challenge. We just learned how to set up the surfacing machine Saturday and we replaced the rock and figured out how to dress it. We played with an old 302 block and milled the decks. We are going to try to set up an old junk head next and mill it. As far as the ASE master tech thing goes...it is what it is. I passed all the tests the first time with flying colors. I'm proud of that. I know what I know and what I don't I can learn. I guess that means I don't know everything.