Engine: Piston Knurling

[JoeHall]

I am now in the third month since pulling the 56J motor for rebuild/replacement. Between chasing parts, waiting on the machinist, limited personal time, and occasional "oops", the project is dragging out.

One oops was yesterday when I attempted to hone a spare block to fit a set of NOS, Ertles pistons. I'd had the block sleeved back to standard, about 10 years ago, and the Ertles were a little snug, at .0005" to .0015". Long story short, I overshot the honing and now have .004" to .006" clearance. I slept on it last night, did some reading on the internet today, and have decided to knurl the Ertles.

Today, some view knurling as an old school band aid for a worn out motor, in taking up excess clearance. But yester-decade piston refurbishing was common practice (ring grooves, pin holes, and knurling) and many considered refurbishment of OEM pistons preferable, since the aftermarket pistons then were not as good as they are today. Arguably, knurling can be an improvement since it traps oil in the piston skirt. I am comfortable with knurling, since the Ertles pistons in the original motor were knurled about 160,000 miles ago, and the skirts are currently the least worn part of those pistons. So now I have plan B, just gotta find a place to do the job.

Here is a pic of one of the 160,000 mile, knurled pistons, it is the one in the forefront:

[Mike Van Veghten]

Think about it for a minute...........................................
You "used" to have a whole section of piston to bear upon the outside (thrust side) of the cylinder wall when you have a proper piston to wall contact.

So now, you have a little extra gap to fill, so you upset (or knurl) the side of the piston to make it larger in dia. This is sort of like rolling a thread on a good quality fastener. You force some material into the parent metal so that material in unsupported areas is squeezed out and is now above the original surface...just like making threads. Note that knurling cannot be performed into the lower skirt area because the lower skirt will break off from the pressure. So by nature of the process, you have less piston material to support it in the cylinder. (Note: if you have older full skirted pistons, then you're ok to knurl both sides of the piston).
PLUS...you now only have these tiny little extruded shapes that will bear upon the cylinder wall. I don't know the true percentage, but knurling a piston probably leave in the neighborhood of about 25% (or less ?) or so of original piston surface to take the beating on the thrust side of the piston.

How long do you think this "upset" or extruded material will last...being that the surface area has been removed/cut by 75% or more before it wears away and is now back to the original piston surface.

MANY, MANY people have done this over the years. But I'd bet only a few have looked inside to see what happened after only a few miles.
On the other hand, knurling the diameter of a piston, only a little more than half will wear away within 6 or 8,000 miles, so a "little" of that extra gap filler will remain in place.

If you don't plan on putting that many miles on the engine (few thousand a year), then go for it.
BUT if you plan on actually driving it, get another block, or spring for new pistons.

Mike

[JoeHall]

Mike,
Did you not see the pic above, of a piston I pulled out of the original motor, with about 160,000 miles on the knurling? In order to re-use those pistons, I'd only need to have the top ring grooves widened for a .030" spacer, to bring side clearance back to .0015", and fit .003" OS pins. As is, the skirts on those pistons are still in excellent shape, ditto for the skirt wear area in that motor.

Here is a pic of a Volvo piston, I found on the internet, that has been knurled (to the lower skirt edge); it was also common practice for BMW boxer motorcycles, and they wrote the book on longevity:

[bezhawk]

How much material is added if you get them coated? There are many places that will put a permanent DFL [dry film lubricant] . Just thinking out loud.

[Lamar]

Not going to get into the yay or nay on the knurling but would like to point out the extra clearance in the bores will also cause a considerable increase in ring gap if you use standard fitting rings. This can lead to excess oil consumption and increased blow by. Just another piece of the puzzle. Lamar

[Lamar]

We do not increase the bore size to use coated pistons, the coating is thin enough to not cause a noticeable decrease in skirt clearance on a correctly honed bore. Lamar

[swvalcon]

To bad they didn't make a forged piston for this you would be right about where you need to be.

[PackardV8]

To bad they didn't make a forged piston for this you would be right about where you need to be.

Today, custom forged pistons are as near as your credit limit. A set of forgings for the Packard to your specified diameter would be about $850 and a month's delivery time. I've got a set of .060" JEs on the shelf just waiting for the right build.

Not going to get into the yay or nay on the knurling but would like to point out the extra clearance in the bores will also cause a considerable increase in ring gap if you use standard fitting rings. This can lead to excess oil consumption and increased blow by. Just another piece of the puzzle. Lamar

The good quality moly file-to-fit rings come .005" over. The CASO way would be to buy .010" over rings and file them to fit.

jack vines

[Dan Timberlake]

Are the NOS pistons the same style as the used, knurled piston in the OP?

Some pistons expand a few thou when the skirts are shot-peened with coarse glass shot. I'd consider that a bit longer lived than knurling.

[jnormanh]

First off, I'm not sure that .004-.006" clearance is anything to worry about. The usual rule of thumb is .001" per inch of bore, and It's hard to argue against .0015 per inch. You'll want to check ring gaps. Most folks think .004" per inch of bore is about right, (.016" for 4" bore) and I wouldn't be concerned about a few more thou. If necessary, you can buy oversize rings and file the ends to the gap you want.

Knurling has been around for probably a hundred years by now, and I know of engines with knurled pistons which have run just fine for a hundred thousand miles. I knurled a set for my Sprite because, although it ran perfectly, the piston slap when cold irritated me. That's been probably fifteen years ago.

Get them coated if you like. The only set of coated pistons I've ever seen after use had the coating worn off after 10K miles.

Personally, I'd use them as is, possibly with OS rings filed to fit. If the present rings don't gap more than, say, .020" I'd use them too.

If you really want them knurled, any older auto machine shop should be able to do the job.

[JoeHall]

Are the NOS pistons the same style as the used, knurled piston in the OP?

Some pistons expand a few thou when the skirts are shot-peened with coarse glass shot. I'd consider that a bit longer lived than knurling.

Dan,
Yes, they are made by same company, "Ertle" and are 1960s to 70s vintage. Except these are standard size, and the old ones were .030" OS.

[PackardV8]

Dan,
Yes, they are made by same company, "Ertle" and are 1960s to 70s vintage. Except these are standard size, and the old ones were .030" OS. [ATTACH=CONFIG]41782[/ATTACH]

According to this, Ertel Manufacturing was independent up until 1998. Since Ohio Pistons came out of Ertel, OH, wonder if they were the same with different boxes?

jack vines

[JoeHall]

Jack,
Looks like your info squelches the rumor I heard, that EGGE had bought Ertle at some point. However, the latest EGGE Packard pistons are a dead ringer for the old Ertles, since EGGE finally began installing the metal expansion plates. Both Ertle and now EGGE are similar to the OEM Bona-lites, but still a little different, mainly in how the metal plate is set.

I guess a key question would be what kind of aluminum is EGGE using now days. If its as good as OEM or Ertle, I'd run them without any qualms.

[JoeHall]

Jnormanh,
I agree .004" to .006" will not hurt anything, but they will slap when cold. I ran a 352 with loose pistons once, for about 50,000 miles. The loosest one was .007"; it ran nice and cool, but slapped a bit even when warmed up.

I like your idea of OS rings, and will go that route, since they are currently around .025" to .030". But I read recently, far more compression is lost past loose ring side clearance than through excess end gap.

Thanks,
Joe H

[jnormanh]

But I read recently, far more compression is lost past loose ring side clearance than through excess end gap.

Thanks,
Joe H

I'm sure that is correct . We all know it is a "fact" that piston ring gaps have to be clocked, 120 degrees apart if three and 90 degrees apart if four. However on many engines, when dissassembled, the rings will have moved around until the gaps are almost lined up. And it makes no discernible difference.

Bores need to be round, and piston rings have to be damned near round too, although they will wear-in if not too bad. Lots of engines burn oil and smoke when first fired up, but settle in as the rings wear to the same shape as the bore.

Put it together, knurled or not (my experience, knurling can eliminate piston slap. OTOH piston slap is more a cosmetic annoyance than a real problem), give it a hundred miles of moderate use, then drive it like you stole it.