Why is piston knurling all the rage on YouTube?

[Woody500]

back a long time ago , I bought a basket case street hemi, 66 belvedere , the block had been bored .030 over before he bought the pistons , was way too loose , I knurled the hell out of the pistons myself at a friends machine shop , way over did it (read heavy) .
Then did my best to sand the hi spots off from the tips of the knurl , ( to make it last) ,while trying measure them , measuring the knurl was time consuming and a ***** but I got it done .
It worked very good to great , ran good , lasted till I sold the car due to hard times , I beat on it every time I drove , and drove it to work every day till gas got too expensive for it ....

Have used a file to fit knurled pistons(much easier to clean) when rebuilding old Marine Engines, at my boat shop, and also Military engines, neither you could get new pistons for, when I worked at Ropkey Armor Museum. Only used a ball hone too.
They usually are tight when assembled, and some times had to use 12 volt to start a 6 volt system and 24 volts once for a 12 volt system. After a few starts normal voltage was all you needed.
Can't pull start a boat, and the Military one was onboard genset, M48 A1 tank.

 

[MOPARMAGA]

these days you have things like line2line coating to take up the piston to wall slop.

And it works very well because it holds oil

 

[Dart Frog]

We did a fresh up on a 413 back in the day the Piston skirts were knurled and new bearings rings and cylinder hones, rebuilt heads etc. ran great for about 10,000 MI pretty good for another 10 and was smoking noticeably at about 27.000 miles or so

 

[Woody500]

Apparently while talking piston UTG brought up in passing that he read Smokey use too knurl pistons and Smokey said he gained hp, now people are debating if it would, Brain said he gained like 8 hp but lasted only one 250 lap race.

Re-reading to remind myself:
And realized that Smokey also used the very heavy racing oil that was what they all ran back then. Valvoline had FR 50(IIRC). Just realized FR might mean Ford Racing(anyone know if true?), but Offy cars used the same.
Valvoline would give it to you in 5 gallon buckets. When you poured it, it seemed like gear oil thick! And we used oil heaters before cranking engines or it wouldn’t flow. Also longer engine warmups because of it.
Driven Oil had FR50 & it was disco'd. Vintage Racing guys might've tightened clearances for newer oils. I assume engine clearances were looser to use heavy oil, so the knurling & heavy oil might be the Smokey trick. Possibly special pattern knurling, Smokey prototyped a lot.

 

[Woody500]

Knurling, Read-CHEAP solution

And they did it years ago when they could get .010, and .020 pistons & occasionally .015s were avaliable.
Cheap to have knurl done or do it if had a lathe. Expensive to bore, although portable bolt on Boring machines were avaliable then, still too expensive for small shops then.
Nowdays Kwik-Way boring machines still available used.

 

[Woody500]

Have used a file to fit knurled pistons(much easier to clean) when rebuilding old Marine Engines, at my boat shop, and also Military engines, neither you could get new pistons for, when I worked at Ropkey Armor Museum. Only used a ball hone too.
They usually are tight when assembled, and some times had to use 12 volt to start a 6 volt system and 24 volts once for a 12 volt system. After a few starts normal voltage was all you needed.
Can't pull start a boat, and the Military one was onboard genset, M48 A1 tank.

My mind fooled me, the Pershing M26, was what that genset came from, a mini FH 4 cyl Waukesha( Maybe), and nothing was available for it, but rings & bearings and gaskets. Probably all old Military stock.

 

[MoparLeo]

Anything that looks like a cheap shortcut is popular....real precision.....
Better for a lawnmower or such today.