Piston cylinder wall clearance-Don't hate me for asking

Posted By: VITC_GTX

Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 12:28 AM

The 383 out of the '71 SSP is standard bore with stock pistons. I want to rebuild the engine back to stockish (daily driver with decent fuel mileage). Thinking 300-325 horse.

The loosest cylinder is .007" over standard (most fall in the .004"-.006" range). The couple of pistons I measured seem to be .001" over at the skirt.

I know there is the right way to do things and the wrong way to do things but there is also a space in the middle that allows folks to do things "good enough" sometimes (that space is large for some folks, narrow for others).

Here is the question that you probably already see coming. Can I hone and re-ring my original pistons for a "low" horsepower, low compression build? I think I am out of most, if not all, new pistons specs but would piston slap, etc really be an issue? What would the consequences be?

Posted By: dOrk !

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 12:38 AM

7 is waaaay tooo much ... it is new piston time.

You will have a ton of piston slap and rock ... and that will not benefit ring-seal.

Posted By: buildanother

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 12:44 AM

If the worst bore is just .007" over stock bore size, you should be able to get some more time-miles out of it. Just don't get too crazy with the glaze breaking and if the ring grooves and piston tops and skirts look to be in decent shape, go for it. If you do put new rings in it, make sure you have a gap of 016" measuring at bottom of bore where the least amount of wear is. IIRC, the old shop manuals had around .007" for wear limit too. (440 4.320") i personnally drove a cava w/2.2 for 25k miles with one piston with .018" clearance, and I heard it rocking in there hot or cold, and did not use oil, ran strong.

Posted By: John_Kunkel

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 01:36 AM

All of the "loosies" I've overhauled did use a little oil but oil's cheaper than the alternative.

I'd rather hear them than smell them.

Posted By: GTXX

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 01:38 AM

It won't be perfect or ever make all the HP it could, but .007 will live a long time and run well when you consider the anual mileage the motor may see. The question I have though, was the taper honed out of it before you measured it? Not with a dingleberry, but a real ridgid hone? The problem with running a cylinder in your situation if it has taper is the rings don't as seal well at RPM. Having said that, I ran a 440 std. bore with .007 on stock cast six pack pistons well into the 10's, still had good leakdown after 300+ passes and 10k+ street miles.

Posted By: RapidRobert

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 01:59 AM

dingleberry hone for the right finish & get a set of cast rings (they're dirt cheap) as the taper is prob substantial. Break one of your old rings in two & grind a "hook" on a broken end & straighten the edge & use it to decarbon the grooves. CLEAN the cyl walls good.

Posted By: 69Chrgr

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 03:27 AM

.007 on the worst piston is definitly loose for cast pistons ( I run almost 007 on my race motor with forged pistons) I would get a set of .005 over rings and file fit them, at least you wont have a ton of end gap on the rings. you also could go 030 with a set of hypereutectic pistons for not a ton of money if you wanted to just do it 100 %

Duane

Posted By: 383man

Re: Piston cylinder wall clearance-Don't hate me for asking - 07/12/11 09:24 AM

I did a 383 that was like that and I had the pistons knurled and then fit them to the cylinders. Worked great. Ron

Posted By: crlush

Re: Piston cylinder wall clearance-Don't hate me for as - 07/12/11 12:05 PM

If it was me id run it, i got a buddy that put together some real turds that ran fine, no smoke.

Posted By: BB Dart 69

Re: Piston cylinder wall clearance-Don't hate me for as - 07/12/11 06:32 PM

Run it forever

Posted By: dogdays

Re: Piston cylinder wall clearance-Don't hate me for as - 07/12/11 09:27 PM

The old "knurling the pistons" trick, right, Chief?
It actually works. But try to find someone who knows how to do it!

About rings, the bore wear is probably greatest close to the top. If you use file-fit rings and set them to the correct gap at the top of the cylinder the odds are they'll be really close or butting at the bottom of the stroke.

I know my first reaction was "yuk" but I've run worse than this.

R.

Posted By: 383man

Re: Piston cylinder wall clearance-Don't hate me for as - 07/13/11 03:28 AM

Quote:

The old "knurling the pistons" trick, right, Chief?
It actually works. But try to find someone who knows how to do it!

About rings, the bore wear is probably greatest close to the top. If you use file-fit rings and set them to the correct gap at the top of the cylinder the odds are they'll be really close or butting at the bottom of the stroke.

I know my first reaction was "yuk" but I've run worse than this.

R.

The 2 machine shops in my area knew how to knurl a piston real easy. They only run a few lines of knurls on each side of the piston. Then they will usually be to big to fit in the cyl. Then you can file fit them and the knurls hold oil in them which is good for keeping oil on the cyl walls. Ron

Posted By: MoparforLife

Re: Piston cylinder wall clearance-Don't hate me for as - 07/13/11 03:35 AM

Quote:

Quote:

The old "knurling the pistons" trick, right, Chief?
It actually works. But try to find someone who knows how to do it!

About rings, the bore wear is probably greatest close to the top. If you use file-fit rings and set them to the correct gap at the top of the cylinder the odds are they'll be really close or butting at the bottom of the stroke.

I know my first reaction was "yuk" but I've run worse than this.

R.

The 2 machine shops in my area knew how to knurl a piston real easy. They only run a few lines of knurls on each side of the piston. Then they will usually be to big to fit in the cyl. Then you can file fit them and the knurls hold oil in them which is good for keeping oil on the cyl walls. Ron

Sure glad someone else mentioned knurling. Last time I did you would have thought that the world was coming to an end from all the perfectionists here. Knurling if done right will last many thousands of miles. ore than likely many more miles than any of use will put on these cars. Knurled valve guides will last many miles too if done right.

Posted By: dodgeboy11

Re: Piston cylinder wall clearance-Don't hate me for as - 07/13/11 03:56 AM

And while you're at it, throw some c4 in the oil pan and make it a good explosion. Ok, so that may be a little bit dramatic, but you have the engine apart, build it right. You can't afford to do it right the first time, but you can afford to do it twice right? With taper in the bore, the rings are constantly expanding and contracting as they go up and down the bores. IF there's a groove at the top, you could possibly break rings when the pistons come to TDC. Do yourself a favor and do it right the first time. Piston slap is likely the least of the problems as you have a tall piston, but ring seal will be compromised and on top of that, if you get some new pistons, then you can get the compression up where it belongs and make some real power. Just a suggestion, but it's what I would do.

Posted By: 383man

Re: Piston cylinder wall clearance-Don't hate me for as - 07/13/11 06:35 AM

Quote:

And while you're at it, throw some c4 in the oil pan and make it a good explosion. Ok, so that may be a little bit dramatic, but you have the engine apart, build it right. You can't afford to do it right the first time, but you can afford to do it twice right? With taper in the bore, the rings are constantly expanding and contracting as they go up and down the bores. IF there's a groove at the top, you could possibly break rings when the pistons come to TDC. Do yourself a favor and do it right the first time. Piston slap is likely the least of the problems as you have a tall piston, but ring seal will be compromised and on top of that, if you get some new pistons, then you can get the compression up where it belongs and make some real power. Just a suggestion, but it's what I would do.

Knurling is good to take up the extra clearence in a bore for wear but you are right that if it has much cyl taper that has to be fixed. I was under the impression that it just had to much piston to wall clearence from cyl or piston wear but not alot of taper.
Knurling is good to close up the piston to cyl wall room and works very good if done right. In fact it is very good for holding small amounts of oil in the knurls and helps keep the piston and cyl wall wear down with the oil in the knurls that keeps extra oil between the cyl wall and pistons. It is not a half a$$ job as it has been done for many years and holds up for a very long time. My old 383 still has the knurled pistons in it after 10 years on the road and the knurls still look good.

Many people are on a tight budget as I was and just cant afford knew pistons and all. I honed my cylinders alot to get it as close to round and as little taper as I could get. Then I knurled the pistons and file fit them for .0015 to .002 piston to wall clearence. It saved me alot of money on a budget build. I am not trying to be smart as you are correct in boring and replacing pistons to be the best way to repair it but done right knurling is very effective and is a good repair that will hold up for many years and miles. Ron

Posted By: nomore65BelvJim

Re: Piston cylinder wall clearance-Don't hate me for as - 07/13/11 07:19 AM

I ran knurled valve guides and ran them hard for several years with no issues