Inliners International Forums General Discussion Hi Performance Knurlized GMC Pistons

In California Bill Fisher's GMC Speed Manual there are references to "knurlized" pistons. The purpose is to make the piston fit tighter I suppose. He said that 1/2 the clearance could be removed. I remember my dad knurling the piston on my Cushman to make it fit. That was because we couldn't afford boring and a new piston. My question is does anyone use this any more? Do the newer piston metals change the expansion characteristics of pistons allowing a tighter fit to start with? If so can we use less clearance than shown in the old shop manuals?

With the higher silicone content of newer pistons,you can run a tighter clearance.
But there are still HP pistons that still require a loose fit.

You would need to talk w/the manufacture if there pistons have a high silicone content & of course ask what they recommend for clearance.

But if you are going to rev it really high ,you still to to set the clearance accordingly.

MBHD

Thanks, Hank. It's just one of the problems working with information that was printed over 50 years ago. I will definitely talk with the piston maker.

Knurling is typically a crutch to get you by, and generally doesn't last as long as the original skirt did. Mostly because you have less skirt surface area to bear the brunt of the piston load, as opposed to a non-knurled skirt.Just depends on the application,and what your expectations are.Cast pistons generally run at around .003 clearance on a stock street engine.Many non trained enthusiasts will try to tell you "the book" says this, or "the book" says that, and .003 is too loose. But you will(can)stick a piston if you load it heavy(tow a trailer), or run it hot if you run it tighter. I've built many Pro Stock Big Blocks that the piston clearance starts at .012, but they have a 4.600" bore or larger, and use a good quality JE forged piston. It also depends on how much taper the skirt has in it as well. An average 350 race motor for circle track with JE's will run .007 clearance on a 4.030" bore, and the skirt will have .003-.004 taper in it. With a good air cleaner and regular oil changes, those engines will run 1500-2000 laps at 7500-8500 RPM and only need about .001-.0015 of material honed out at freshen up, and ready to go again for 1500-2000 more laps. On the second freshen up, you usually need to go to .035" over and buy pistons. But by then you have raced almost 2 full seasons. Most races are 50-75 laps, 1500-2000 laps is a lot of racing.

The only times I have heard piston manufacturers speak of knurling pistons (in the context of someone else knurling theirs) it involved blue air and ususally ended with something akin to the phrase 'should be drug out in the street and shot'.

I'll be very careful who I tell!

Yeah, it's considered pretty much a "last rites" fix when there aren't any pistons.
Not only does the knurled surface not last as long as the original, the deformation of the skirt to do this weakens it quite a bit. With a really thick cast skirt (think 1955 Jahns) you can get away with it, but I wouldn't use it except to get a few more miles before biting the bullet.
The advantage was that the knurled surface did hold oil nicely, and I've seen scratches, saw marks, grooves etc. added to do this to pistons (that did not fail) by some pretty good shade-tree mechanics back in the day.

If we have a knurling tool in our shop it's with the babbit clearance tools for the old Chevy 216's and the valve hand lapping fixture.

Ron

One advantage that knurled pistons have is they will retain more oil in the knurling & lube the cylinder walls a bit better.

MBHD

How do you "knurl" a piston?

Well , you've seen all those cool looking billet aluminum dash knobs, right? Think of a piston as a big knob. You put a piston in a lathe and use a knurling tool to "rough" up the piston skirt surface to give it a slightly larger diameter. Knurling doesn't take away material. It moves it. It takes metal from the flat smooth sides of the skirt and stands it up to make the piston 'Larger". Back in the time when labor cost were less than material costs knurling, lapping, shims, and boots got lots of folks back on the road.
Thanks guys, This is interesting and useful to me! Beater

I recall having small groves around piston skirts for lubrication, whether they did good is the question--seems rational.

We did a lot of knurling '40s- '60s on many rebuilds. Often in chassis. It worked for budget jobs when you were honing a couple thou to freshen/ straighten bores. Haven't done that for decades.There were other ways to expand a piston which were a better method IMO. Still have the eqpt. Typical OE cast pistons could be knurled. Only do it on minor thrust side. Thorough knowledge of later alloys is important prior to any attempts.
Usually had to machine ring grooves and use GI inserts, too. Lots of labor, but at 65 cents per hr w/ 2 kids we worked lots of hrs. David