Difference between Valve stem leak and blow by

[supraguy31]

Hey all,

Now I know that this isnt something that you get asked everyday but I just Picked up a 90% Complete engine from a guy here local. The head was already off the block and the Pistons were already out. Now after looking at the Pistons, there was lots and lots of burnt oil on top of them. These are .040 overbore Pistons just to let you know. Now I did take a look at the cylinder walls and they are pretty clean but it I put one of the Pistons back into the cylinder without the rings on it then they are Pretty loose but not that much. Question is what is too much clearance to possibly get blowby and can you tell the Difference between blowby and Valve stem seals leakage. Everyone of them had alot of oil buildup.

I took them and scan blasted all the crap off them and they came out looking brand new. Not a Scratch or very very little scratches on the Side Skirts of everyone.

Thank for your time all.

Robert

 

[supraguy31]

No one can give me anything on this. Im just trying to make usre that this block is still good as Im not for sure what the max Clearnace on .040 pistons should be.

 

[gaboonviper85]

you didn't sandblast the whole piston did you?

The oil could be from the pcv system....and pistons are made smallen than the bore for a reason (gives the piston room to expand under heat) so the slip fit is normal...they would be much tighter if the had the rings....blow by is when exhaust gas/fuel leak past the piston rings into the crankcase...valve seals will leak and the motor will smoke on startup or when you let the motor engine brake to slow you down.

If you plan on rebuilding this motor I highly recomend you take it to someone who knows what they are doing

 

[Poodles]

shouldn't have touched the piston with a sandblaster....

 

[Kckazdude]

^^At least now it makes a clean paperweight.

 

[supraguy31]

I didnt hit the sides of the Pistons and it was a very very cheap sand blaster. Basically just to get the stuff off the tops, it took about 2 or 3 minutes per Piston. I can see the size and everything on them stuff. Washed them off when I got home and all so I dont think I hurt them in any way.

Ive built these engine plenty of times before but Ive never had the chance to check cylinder clearance and this is something that Ive always wanted to learn. I usually take it down to the machine shop and have them bore and/or hone the cylinder to fit. Now this is something that I want to learn and I basically need to know where to start. if you have a 84mm piston then what clearance would be the best for the walls and such? I want to check these and make sure Im not wasting my money at the machine shop on a block thats bored out too much.

I have a Set of Stockers, .020 overbore Pistons and a set of .040 overbore Pistons here. This block was .040 over when I picked it up and I figured that this one would be something worth learning on.

 

[HommerSimpson]

you need to get a NEW ring and stick down the cly and check the end gap....

How much ridge is at top of each clyinder wall ?


the diferance between blowby and valve stem seals leaking is....

blowby is usally all the time... its the compression that gets past the rings into the crankcase...

valvestem seals leak at 2 diferant stages... while car sits the oil tends to get into the cly and smoke on start up... the 2nd time is under hard load were the pistons acually suck the oil off the top of the head past the seals

as far as sandblasting the pistons... put up some pics.. but i have seen many people sandblast pistons and never have a problemm .. main thing is to make sure all the sand is out of the ring groves and wristpin area...

 

[adampecush]

if you sandblasted the pistons, chances are they are toast.

 

[supraguy31]

I will get some Pics of them up this afternoon when I get home. I did take some Pics, just didnt get them loaded up on Photobucket last night.

 

[Koenigturbo]

Piston's are a precision part, they have a very slight tapper to them,they are a little larger at the botton. Chance's of doing a precision sand blasting is very slim to none. Forge piston need more piston to wall clearance than cast, because of expansion. So you need to measure the piston skirt and you need to measure the piston body, the thrust side of the piston. then you need to measure the cylinder walls top , middle, bottom of the stoke, also on the thrust side. You take your largest reading and compare that to your measurement from your piston's if there's more than .005 (a half a thousands) to .01 differance between piston to cylinder wall, then I would bore it out, me personly I'd bore it with that many miles, and especially if you sand blasted them. Honestly If there's any variation in the cylinder walls I'd bore it out. I think it's 2 to 3 thousands for cast pistons and 3 to 5 thousands for forged piston, but look it up in the repair manual. Different rings reqire different cross hatch pattern and different coursenes of the stones.Some rings reqire a courser finish than other rings Once you do the pistons and new rings, you better do the heads, if you don't then the cylinder presure is just gonna sneak past the valves or vice versea. Turbo engines are very picky as far as clearance goes.
You'll need piston's from sand blasting them.

P.S. I know your gonna post pictures soon, I'll say this in my opinion all the pictures in the world won't help you, your honestly gonna tell me that you can tell by looking at pictures that those pistons are in spec's just by looking at a picture?? Wow! then the person looking at them must have very presise eyes. Pictures will not help you, a micrometer will, just my two cents.

 

[supraguy31]

What type of Micrometer do I need. I have one but its for shims only and its really small, I asked a couple machine shops but they couldnt tell me where I could Purchase one as I think they want me to just bring it in to them and let them charge me to do it.

Any recommendation on the name of the Micrometer that I need to Pick up.

Thanks All.

 

[Koenigturbo]

What type of Micrometer do I need. I have one but its for shims only and its really small, I asked a couple machine shops but they couldnt tell me where I could Purchase one as I think they want me to just bring it in to them and let them charge me to do it.

Any recommendation on the name of the Micrometer that I need to Pick up.

Thanks All.

Well, If your piston are over three inch, which in your case think they are over three inch, You'll need a three inch Micrometer if you give a little bit of time I'll go check what size your pistons are (i'm have lunch right now) I think it is three inch and change. You can pick up a mic at "Harbor Freight",( don't know what it cost) "Starret" is a good brand name. next you need a "Dial bore indicator" bore mic made by I believe "Sunnen" is one brand (very costly, very costly) and you need to know how to set them up. This is a specialty mic, "Harbor frieght" will not have this one. (or they shouldn't) because it's not for the average every day person to use, this needs training to use and it is not cost effective. by the time you bye one , you may have just as well brought it over to the machine shop and have them do it. yes, you are correct, they want you to bring it to them, to be honest unless you have an engine machine shop, it's not worth your money to get an a "Dial bore indicator" In my opinion. I'm to sure where to get a "Dial bore indicator'.
Do your reseach, I'm willing to bet it's cheaper for you to get it measured for you than it is to buy these gauges
P.S. Mic's come in different size's one inch, two inch three inch, e.t.c.

O.K. luch was good. Yep, your (stock piston) are 3 1/4 inches roughly, I just used a tape measure, but you'll need a mic for the exact measurement, so, yes get a three inch mic, and you'll be good.

 

[supraguy31]

thank you sir. I think Im going to take your Advice and just take it over to my usual machine shop. They said since I do alot of business there and all I need it some measurements then they will do it for free. Im hoping that this comes out like it should but we will see.

Thanks Again