knurled...to restore the clearance

In Ms Peel 1st redoing we found her bored .40" over and oval barrel shaped worn in mid bore so to save money and a barrel an auto shop honed bores round but left pistons sloppy so they knurled pistons which worked a treat w/o notice of slap or nothing. Not longest lasting solution as new parts all around but gave me enough adventure to last a life time in a few seasons of glee before stuck throttle took the power out.



http://www.pbase.com/stealthfti/image/53355694

What you are seeing is one of the pistons after it has been knurled on a K-line knurler. We are looking at the major thrust side of the piston. The minor thrust side looks exactly the same. The knurling is done to extend just beyond any scuffing mark. That way the raised surface area will be all of the surface area that comes into contact with the wall.

The purpose of knurling is to increase the diameter of the piston skirt: at the surfaces that come into contact with the cylinder wall. That diamond pattern raises the surface of the skirt about four thou [0.004in] on each side; resulting in a diameter increase of about eight thou [0.008in]. Eight thou is about the max you can go; but eight is plenty for this motor. The cylinders had about 2.5 thou [0.0025in] of belling. Honing the bores to get things cleaned up nicely ended up at about three thou [0.003in] larger than STD size.

That means that I had about five thou [0.005in] of knurled metal to carefully remove, using fine emery cloth, so as to end up with the piston to cylinder wall clearance that I want: right at one and a half thou [0.0015in]. That does not happen in five minutes. It does take time and patience.

Pistons have to be carefully examined before you knurl them: they have to be usable. The ringlands have to be in excellent condition. The wristpin bores through the piston have to be unworn and undamaged.

The first step is to clean the pistons and rods before disassembly: to facilitate the wristpin removal without damage. And only when clean can the piston be examined and measured, and closely checked for any cracks or other evidence of damage.

If everything passes muster, the pistons are knurled to the eight thou diameter increase: called "blowing them up" eight thou.

Then I get to REALLY clean them; especially the ringlands. No carbon residue is allowable; and no ring scraper is allowable either. The pistons are soaked in carbon softening solvent, and the ringlands are scrubbed with toothbrush size brass bristle or stainless steel bristle brushes. Sometimes I actually use an old toothbrush: the plastic bristles won't damage the aluminum alloy, especially in and around the wristpin bore.

When thoroughly cleaned, the fit of the new rings in the ringlands are checked and verified.

Then I start the resizing to fit the bore procedure: sand a bit; clean; mike it; repeat. Once I get to about zero wall clearance, then it gets more intense, and the metal removal is done in smaller amounts...until I get to the right wall clearance.

Done carefully and done right, knurled pistons can perform as well as brand new pistons.

Another (bush) method of expanding the pistons to restore clearance someone told me about is to invert the pistons and blank off the wrist pin holes, then fill them with steel shot and seal off the skirt area with a close fitting cap with a hole to allow an air impact tool to be inserted then go to town with the impact tool, the resulting high frenquency impact will spread the skirts enough to restore clearance! when all things considered the shot peening effect could even have a stress releiving result as well.
I wouldnt do it though, new pistons arn't that expensive!

fredful said:

Another (bush) method of expanding the pistons to restore clearance someone told me about is to invert the pistons and blank off the wrist pin holes, then fill them with steel shot and seal off the skirt area with a close fitting cap with a hole to allow an air impact tool to be inserted then go to town with the impact tool, the resulting high frenquency impact will spread the skirts enough to restore clearance! when all things considered the shot peening effect could even have a stress releiving result as well.
I wouldnt do it though, new pistons arn't that expensive!

Click to expand...

I'd like to see a video of this before I tried it. Maybe you could use a small charge of C4?

Never did this on a bike but years ago, about 1972, did it with an Alfa Giulietta Veloce engine I rebuilt and I couldn't afford to get new pistons. Seemed to work OK but I didn't keep the car long enough to find out the longevity.

Knurling pistons & valve guides were a common practice back years ago---50's--60's when rebuilding auto engines. Less expensive than new parts for poor folks who couldn't barely afford a car in the first place let alone the upkeep. Also was used by quite a few used car lots to make a car easier to sale---no smoking for enough time and miles to pass the warranty limits---then the cars were pretty much turned into mosquito abatement vehicles----
I have heard a cheaper way yet some rebuilders used was to use a center punch on the inside of the skirts to make slight bumps on the od of the pistons to take up the clearance--lasting a bit longer than knurling. People were poor back then---had to do what they had to do to make the money stretch. Cars didn't put on the miles they do today---only used for necessity not pleasure so this procedure may have lasted for several years or more saving money for other essential items like food and clothing and a roof over their heads. And of course there were the "Jack Benny" type who hated to spend a cent on anything.