The Stovebolt Site Forums The Shop Area Engine Shop Max overbore to re-use pistons

This is kind of a general question I have regarding vehicle engines in general, but for simplicity let’s just confine it to a 216/235.

Let’s say you have cylinders that won’t quite clean up with a hone-only. Is there a number at which you could bore/hone slightly and still use the stock or existing pistons? Like perhaps up to .003? Or 0.005? Where does piston slap become an issue?

Cylinder taper is the deciding factor. Cylinders do not wear evenly from top to bottom. The maximum wear happens in the top 1 1/2 inches or so of a cylinder. I consider a .003" TAPER, not total cylinder wear, to be a maximum. With a good quality rigid hone (not a spring loaded glaze breaker) I am willing to straighten out the taper and put a knurl on the piston skirts to re-establish the proper clearance. A race engine with forged pistons that do not have an Invar anti-expansion strut that must have a lot more piston clearance than one with cast pistons, a .005" clearance makes it sound like there's a bunch of little guys with sledgehammers trying to get out. That's particularly true when the engine is cold.
Jerry

I’ve heard of the piston knurling bing before to tighten up piston clearance. Is that kind of a lost art by now or still fairly common for machine shops to do?

The machinists who know how to do it like me are a dying breed. The procedure isn't difficult at all- - - -set up the piston in a 4-jaw chuck and roll the skirts past the knurling tool by hand until the diameter increases as much as you want. I run 3 or 4 knurl passes about 3/8" apart. I can usually expand a piston skirt up to around .005" maximum. .003" is more normal, as I can take out a lot of cylinder taper with that dimension. Honing a cylinder over .005" is getting into "rebore" territory. Rings in a straight cylinder will last several times as long as ones that must expand and contract 60 times per second at 2K RPM to stay in contact with the cylinder walls.
Jerry

Doesn't the knurl score the cylinder walls? But I guess with such a shallow knurl to gain the .003 dia.
The top of the knurl will be flat and not pointed like on hand tools tools. I have not heard of that trick being used on pistons before.
I learn something every time you type Jerry.

Don

There is even a machine to do it with.


Don

Check out this method.


Don

Shade tree method on how to knurl?


Don

A knurl on an aluminum piston, running against a cast iron cylinder wall, can't possibly score the cylinder. Actually the knurl pattern traps oil and lubes the skirt better. I also scuff the high spots down with emery cloth a bit before moving on to the next piston. After several thousand miles of running, the increased diameter can still be measured with a micrometer. Back when the average re-ring job only lasted 30K miles or so, knurling pistons was a very common practice. One of the pistons in John's 261 had a bit of excessive skirt clearance. I knurled it with the help of a lathe owned by one of his Amish neighbors.

For the "no-lathe" automotive machine shops, Perfect Circle sold the "nurlizer"- - - -a hand-cranked piston skirt knurling tool.

Just watched that last video, Don- - - - -my shade tree would die of embarrassment if I tried something like that under it!
Jerry

Interesting stuff.

Wonder how much a modern shop would charge if they’d even do it?

If they can't do it with a keyboard on a CNC machine- - - - - -"IT'S IMPOSSIBLE!"

Trapping oil is something I had not thought about and the light sanding would not scar the piston walls.

As for manual machinist, seems like there are less and less of us.
But, just do a search on manual machinist wanted, and some shops can't find any.
Ask John Lucas. He owns a job shop in Omaha.

Don

Once I've got the piston diameter up to where I want it, and before I loosen up the chuck, I give the skirts a quick shoeshine with 240 or 320 grit abrasive cloth. I keep rolls of the stuff nearby for polishing the parts I turn.
Jerry

JW51 I have seen where the pumper took a center punch and just covered a 52 fairbanks-morse oilfield engine piston with deep centerpunch marks same thing as a knurl. They would then file till it went in the cylinder,wash in coal oil put it together. Slow running engine,well worn from dry "field gas". In 1972 when I built a machine shop in the local parts store we got a new knurling machine. Some racers liked to knorl their pistons to hold oil,I just did what they paid for !!

2ton Piston knurling is related to guide knurling not a very good fix,if you notice lots of our type heads have the guides knurled and will have loose guides and worn stems !!

2ton That was a true shade-tree knurl,told you about the pumper maybe you'll see it in the cloud,used a center punch.

JW51 Don't waste your money on knurling !!!

Perfect Circle “knurler” put a PC on the shIrt.

Knurled piston and guides for an old truck in typical collector vehicle use will often last longer than the owner.

Many years ago I overhauled a V-4 Saab engine, which was a Ford industrial engine, such as used in forklifts etc. The owner was an artist without much money. Pistons were available only as an assembly with rods at $235 each. This was in 1975 or so. I had the local machine shop knurl the pistons; they just asked "how much." I straightened the bores with a hone, fit the pistons with a feeler gauge and a file, installed new rings, did some other stuff and buttoned it up. More than ten years later, after I was out of the business, another local mechanic called me on the phone and asked "What did you do to Libby's engine?" "OMG, what happened? Did it blow up?" I asked. "No, no," he said. "It's starting to use oil and I wondered if you remembered what you did." I told him and he put valve stem seals in it, lapped the valves and gave it back to her. This was around 100,000 miles after I'd worked on it. These were tough motors.

Hotrod Lincoln Did you ever see a tool to cut the top ring groove to install a steel spacer below the first ring?? Had one of those in72 it worked fine was Hastings I think. It liked to have it's cutter flat honed each cut because aluminum would build up on it. By chance still have the tool,if I could find the spacers.It would make the groove fit good with a steel sealing surface below,looked like an oil rail ,had to understand to stop at the groove bottom then hone it for the next cut.

Hotrod Lincoln When I ground crankshafts in 71 the grinder had an emery cloth buffer,well I had belts really rough ones to smooth-up a welded journal before grinding. Discovered you could put a mirror finish on your rod beams before balancing with it,still have a set running I done back then,50 years is pretty good service on those !!

JW51 Iv'e seen em where the ridge ream left the cylinder so sharp you could't gat the rings to go back in the bore,that's more than .003to .005 wear. Measured one worn so bad told em it was over,they insisted left out 1/2 a rod bearing on 3 killed it !!

jw51 left you some info