Don't bother, the skirt is the only thing that rides along the cylinder wall. The crown is slightly smaller to allow combustion pressure to expand the ring and expansion. From what I've seen from other pistons the radius of the crown is about .015" less than the skirt. I'm seeing half the width of the ring there, at the bottom of the cylinder too. Sure I've seen engine's run with all kinds of defects run good without any issues, but sooner or later it catches up. So it's your choice you can fix it or just put the head back on and call it a day. If it was me seeing it's only has 13,000 miles and it just one cylinder, I would drop the oil pan and replace the one piston, a quick hone and you're done. This way it's a quality repair with out the drama of engine removal and expense of a rebuild.

 

If it ran fine do your head gasket and keep motoring
I had a 327 with way bigger gaps than that never smoked a bit.

 

Thanks for providing the logic on "Why" the crown is smaller than the skirt.

Isn't another issue that the skirt NEEDS to have less clearance to prevent piston rock and keep the piston stable? (1. It's exposed to less heat so it won't expand as much 2. It needs to have tighter clearances) ?


Adam

 

Pistons are interesting things indeed, designed by engineers that have a ton more knowledge of what's going on in a running engine than most of us mere mortals.
Yes the skirt thrust faces, (90 degrees of the pin). are designed to keep the piston from rocking, tilting in the bore. If the piston rocks or tilts in the bore not only will you get piston slap. Notable knocking. but the rings will not hold true to the bore. If the rings tilt on an angle. Well, think about it. if the rings are not exactly 90 degrees to the cylinder wall. then the bore becomes oval in relation to the piston and the rings can not seal properly. resulting in loss if compression, blow by and oil consumption. So yes, the thrust faces of the piston need to be fairly tight to avoid rocking.
The crown of the piston is round. unlike the bottom. and the ring lands need to give the rings the room to expand and move to seal properly. Yes it's true that the crown will expand more with heat that the skirt. but moreover, it is designed by those engineers to give the rings the ability to seal.
So, a piston is basically oval at the bottom. tight on the thrust faces and loose on the sides. (Where the wrist pin is) And round at the top but very tapered top to bottom and the amount of this tapper has a lot to do with the type of metal the piston is made of, expansion rates, ring design, etc.
As mechanics, if we are instructed by the mechanical engineers that designed a specific piston to run at .002 clearance. we set that clearance very near the bottom of the thrust surfaces of the piston. And any clearance at the top is not our concern. It was designed this way. Ring end gaps of course must always be checked and I have found in new engines with modern ring designs, the old "Rules of thumb". , .. (like .004 gap per inch of bore for instance,) Are totally out the window.
Bottom line. Your looking at a gap at the top of the piston and thinking it looks big to you. You are looking at it all wrong. The only way to measure piston to wall clearance is to mic the piston at the thrust surface and then using a bore mic. mic the bore. Clearance at the ring lands is by design.
If the engine did not have excessive blow by, and had good compression, no oil consumption. Replace your head gasket and drive on.
Now if you had piston slap, knocking especially when cold. excessive blow by. and poor compression. then tear it down.
All the photos you can take from above won't confirm piston clearance, although huge scratches or other damage might show. But then you wouldn't need to ask anyone would you.

 

Nice to know this. Thanks!


Steve

 

Steve -
It's entirely possible that your observed and noted condition about the piston-to-wall clearance, as observed across the top surface of the piston above the top ring groove, is completely normal. It depends entirely on the piston design.

As we know, piston diameter, and fit-to-bore, must be measured across the top of the piston skirt, perpendicular to the piston pin, just below the oil control ring groove. That's the only measured diameter than can be used to accurately determine piston-to-bore clearance. The diameter measured across the piston above the top ring will be smaller, and is not a representation of piston diameter or bore clearance.

Here's a good example:
I pulled this forged 4" bore piston out of my random parts stash. Diameter, as correctly measured across the top of the skirt, is 4.015"


But take a look at how the piston is machined: The very top diameter of the top surface is a necked-down reduced diameter from the rest of the piston. This is the only diameter that is visible from the top of the piston as it is installed in the bore:


This top diameter measures 3.961"


It is .054" smaller than the actual diameter of the piston. It would give the indication and appearance of a piston that is grossly sloppy in the bore.

Even if you measure across the non-necked area of the piston just below this minor diameter, and just above the top ring groove, the piston is significantly smaller than at the skirt measure-point. This piston, just above the top ring groove, but below the necked-down diameter, measures 3.981", which is .034" smaller than the actual diameter of the piston. Even without consideration to the necked-down top diameter of this piston (which not all pistons have), this actual top diameter would allow the piston to be firmly pushed to one side of the bore and tilted, taking up all clearance on one side, and you would be able to get a .030" feeler gauge between the piston and the bore:


So based on piston design, it is absolutely possible to have a very large piston-to-bore clearance at the top visible surface/diameter of the piston (allowing a .030" feeler gauge to be inserted). This is no indication of the actual piston-to-wall clearance.

If you were not experiencing piston knock/piston slap, and the engine did not sound like a diesel engine starting up on a cold day, slap it back together and run the hell out of it!

Lars

 

You are The Man Lars.
Not that the previous posts haven't made me aware of new things, you answered my original question, with photos, to a T. Thanks for your time and effort in doing so.


Thanks again my friend, and to you and yours, ....
MERRY CHRISTMAS!

 

By your math that would leave .017 of a inch around between the crown and cylinder wall. I agree that's the correct amount of difference between the skirt and the crown to have. I think the confusion here lies with the OP stating he was able to get .030 feeler gauge in there. From the picture I bet he could have gotten a .100 feeler gauge in there. Whether it requires attention or it will last 100,000 miles it debatable. But that piston looks to have a serious case of erosion on the crown.

 

Yes, if the piston is centered in the bore, that would be the gap all the way around. If you look at his photo, all "slop" is taken up on one side, and he has produced the full .034 clearance on one side of the piston only. The piston is not centered in the bore, and he has cocked it and pushed it fully to one side only.

We used to take advantage if the "slop" in the bore with the ability to "****" a piston in the bore during NHRA teardown inspections when you win your class. Knowing what side of the bore the inspectors would check the compression height at, we would "smack" the piston with a rubber hammer to "****" it in the bore and lower its compression height for a lower indicated compression ratio... When we did this, the piston would appear to have a huge gap on the "high" side of the piston, exactly as Steve's photo shows.

 

Thank you for clearing that up Lars, Marc

 

That piston's stain looks like a fluid level stain, No? Water or oil-water mix causes that. I wonder how long that head gasket's been leakin. This car sat for 6+ months while I messed around swapping out the TH400 with the M20. Damn! Never hydro-locked though.


This was it's Leak-Down results just before teardown.
Interesting.

Steve

 

Assuming that leakdown test was done on a cold, non-running engine, your actual "hot" leakdown numbers are probably around 15%. That would be in the "acceptable" range, and I'll bet it would seal up even better after running for a while to get the water stains and slight surface oxidation off the cylinder walls... I'm still voting to clean it up, wipe up the bores, fix the head gaskets, and run it.

Lars

 

I feel like the pistons are gonna be alright. #6 cleaned up well anyway. Since the Cylinder Head is at the shop and it's the holidays, I'll spend the time waiting by bringing all four Pistons to their TDC and clean em up.




The Piston's ID. .030 overs. Ah, it's all coming back to me now.

Steve

 

Looks exactly as it should. As noted, I don't think you have a problem at all. Keep doing the nice work, slap it together with the refurb'ed heads, and it's gonna' run well!

Lars

 

Now to get the deck as clean as the pistons.

Machinist called yesterday and has the Head and Intake Manifold done. Happy today.

Steve

 

Hell, Steve, that looks better than many full-up "rebuild" jobs I've seen..! Some nice, **** work there.
Lars

 

Thanks Lars.

Now when I picked the Head up at the Machinists, I asked him about the Block's surface quality. He wasn't as concerned about a polished surface as much as the surface just being clean, making sure all previous crap cleaned off.


Cylinder #8 had a busted Inner Valve Spring. He just replaced them all.

I forgot to ask Greg what the torque should be on these Rocker Arm Studs. They also hold down the Push Rod Brackets. Were assembled when bought.
Also forgot to get his thoughts on running a dry engine for a few seconds before adding coolant in order to help the Cylinder Head Gasket seal better.
Any comments appreciated. Thanks.

Steve