[Cliff R]

I still remember being in the pits at Norwalk about 20 years ago and a guy pitted next to us "hit" his new 455 build with a shot of nitrous with disasterous results. He tore the engine down at the track since he was stuck there for the long weekend anyhow. Most of his new KB HT pistons were missing from the bores and piled up in the oil pan. They shattered like glass and left the rods and pins moving up and down in the bores void of any pistons on them.....NO THANKS!

I know that's an extreme scenario but it tells us that they have an elastic limit far below what you'd see from a decent forging, even the WAY too heavy TRW/Speed Pro forgings. Not a fan of those either because every set I've ran into ends up WAY to far down in the holes at TDC when I mocked them up required too much to be machined from the block to establish Zero Deck.

Anyhow, I've used Ross which are nice, and ICON, KB forgings. Since all my engines get forged rods and most will have the full floating option they fit the bill nicely. The modern alloys also allow for tight fitting compared to antiquated forgings from long ago. With a torque plate for final honing I set them at .002" here and have never had a single issue with that deal.

Even with all that said IF they made a nice flat top cast piston, even pressed pin I'd use them. Nearly all of my SBC builds get flat top cast pistons and they are fit really tight for minimal "rocking" in the bores, good ring seal and long life expectancy. The cast piston options for the 400's are absolutely pure bovine excrement and always have been. I still remember mocking up an engine I was hired to assemble decades ago using those pistons. They were WAY down in the holes at TDC and already taking over 15cc out of the squish area like using a dished piston. Team that up with a 4X or 6X-8 head in a 400 build and you'll have about as much compression as a Briggs and Stratton lawn mower engine, and left wondering why it doesn't idle or run for chit once placed in service......

[tooski]

Originally Posted by Cliff R

Most of his new KB HT pistons were missing from the bores and piled up in the oil pan. They shattered like glass and left the rods and pins moving up and down in the bores void of any pistons on them.....NO THANKS!
.

Wow!

[Kenth]

Originally Posted by tooski

Wow!

That's why KB does NOT recommend nitrous shot for the hypers.
But racers are racers, they will ALWAYS run their stuff until they break, one way or another.
(This is the "Street" section BTW).

[ponchjoe]

Originally Posted by Kenth

That's why KB does NOT recommend nitrous shot for the hypers.
But racers are racers, they will ALWAYS run their stuff until they break, one way or another.
(This is the "Street" section BTW).

Thanks Kenth. Seems like every post goes to drag racing/max effort somehow somewhere, that’s why I posted the build of a 9ish:1 street cruiser. Sounds like KBs might be an option I’ve never considered. My builder is well versed on Pontiacs so I’d just have to reminds him about the clearance.
Do you know how much weight difference there is between the KBs and the Speed Pros?

[Joe-Touring]

Originally Posted by ponchjoe

Do you know how much weight difference there is between the KBs and the Speed Pros?

Per the Speed Pro website: 589g/783g with pin

KB’s website does not list weights, from CNC motorsports website: 541g/704g with pin

https://www.drivparts.com/part-detai...rand_code=BHVW

https://uempistons.com/product/647/K...TM.030?sflag=0

My personal recommendation is to go with the speed pros. They were the go to for many years, I haven’t seen many examples on here of them failing. Multiple local machine shops have said the KB Hypers have issues even when you open the rings up to their specs. The shops said you wanna go REALLY big on the gaps

Of note: KB’s website says they sit 0.025” under the deck. Quite a bit of deck to trim off

[ID67goat]

Originally Posted by Joe-Touring

Per the Speed Pro website: 589g/783g with pin

KB’s website does not list weights, from CNC motorsports website: 541g/704g with pin

https://www.drivparts.com/part-detai...rand_code=BHVW

https://uempistons.com/product/647/K...TM.030?sflag=0

My personal recommendation is to go with the speed pros. They were the go to for many years, I haven’t seen many examples on here of them failing. Multiple local machine shops have said the KB Hypers have issues even when you open the rings up to their specs. The shops said you wanna go REALLY big on the gaps

Of note: KB’s website says they sit 0.025” under the deck. Quite a bit of deck to trim off

In my opinion, it is hard to compare a hyper piston with a forged. But just for info here, I have a set of the KB 2618 forged pistons (+.030 428 piston with a dish) and they weigh 714.4 grams with pin and 567 grams without the pin.

[tooski]

Originally Posted by Cliff R

The cast piston options for the 400's are absolutely pure bovine excrement and always have been. I still remember mocking up an engine I was hired to assemble decades ago using those pistons. They were WAY down in the holes at TDC and already taking over 15cc out of the squish area like using a dished piston. Team that up with a 4X or 6X-8 head in a 400 build and you'll have about as much compression as a Briggs and Stratton lawn mower engine, and left wondering why it doesn't idle or run for chit once placed in service......

I had them in a build that was done in '82. I asked the machinist to do a zero deck. I found out he did not and was glad for that as the pistons were .040 down. Swapped heads to a 6X-4 to alleviate detonation. Resulted in a CR of 7.6. Engine still ran ok . Swapped pistons to the heavy SpeedPro which brought the CR up to 8.2. Still not great but the engine runs much better.