[takid455]

This has questioned me & I never got a solid answer?

Is it a manufacturing thing?

[tom s]

Cheaper rods.Tom

[LT4USMC]

Plus you can use longer rods and lighter pistons

Sent from my SM-G988U1 using Tapatalk

[FrankieT/A]

I have never seen the point unless you need an entire rotating assembly anyway. If I needed crank rods and pistons I would do it. To gain 4 cubic inches?

[firechicken]

I wonder why Pontiac chose a 4.21 stroke in the first place? Any thoughts?

[Dragncar]

Originally Posted by firechicken

I wonder why Pontiac chose a 4.21 stroke in the first place? Any thoughts?

Well, to get 455 ci.
Chevy had their 454. And Olds, Buick and Pontiac went with 455.
Numbers game.

[Gach]

This is how it started out, back in the 60’s we had no steel rods, well we kid of did, the early 62 steel rods, but the were called rubber rods. They weren’t very good rods. Maxey and Nunzi were the frist to do this. Chevy steel rods were allot cheaper. We desperately needed a good steel rod. Seeing companies made Chevy steel rods by the thousands. It was determined by them, turning our cranks down .040 which make rod Journal size of BBC. 2.200. And pin size .9270 which was SBC size it was a not brainer, to get companies like say Crower to make us a good steel rod, wasn’t going to be cheap.

Crank Shaft specialist was the main crank company in that era. They were the best, really really good crank grinders. They said why not off set grind the crank and make it 4.250 stroke. Now we had allot cheaper basically a Chevy rod. Allot cheaper then having to go with a whole new mold. So to speak for out Pontiac’s. Which would end up costing allot more money then a Chevy steel rod.

Besides that the early 62 steel rods were only available to the Pontiac NASCAR races. Very hard to get. Later when Butler came along they copyed and went to a 9900 pin. So guess what we were no longer limited to 5500 rpms with the stock cast rods. It open the door to make hp.

[Gach]

Once the companies saw their was a market for Pontiac you could get stock Pontiac rod Journal size. Steel rod. Thats kid of how the 4.250 stroke cam into play.

[grandam1979]

Originally Posted by Dragncar

Well, to get 455 ci.
Chevy had their 454. And Olds, Buick and Pontiac went with 455.
Numbers game.

All 3 of those engines have different stroke Chevrolet and Buick chose less stroke and bigger bore. I know it’s all we have but a 4.211 limit or so for a 455 block stinks a Buick stock bore is 4.3.

[Dragncar]

The 4.25 stroke thing started out at Butler. You can offset grind a stock 455 crank .040 and down to BBC rod size. Butler was the first to do this with off the self 6.8 BBC rods. At least he told me he was the first guy to do it.
Then you have Butler with their deal at ROSS pistons, they are the ones who got us our own forgings, not a other brand adapted to a Pontiac. They have more off the self Pontiac pistons than anybody. And they sell more Pontiac E heads. Have had a relationship with Vic Edelbrock(RIP) for years.
So the answer is, Jim Butler.
Also, there was the era when you could not get any steel Pontiac aftermarket forgings. Thats why H-O Racing went with the BME aluminum on the street deal.
So off the shelf Chevy rod combos were a fix for a need. AND, longer rods used to be a thing when all we had was iron heads. Once you fully port a RAIV or D port head stuffing a longer rod got a tiny bit more power. But once we got aftermarket heads the point was moot.

[Dragncar]

It was very hard to get a good Pontiac rod in the 80s, when my 455 was going together. I found 6 of the early SD 421 rods, but could not find the other 2.
So I paid 800$ for 8 new SD 455 rods and had them shot peened and re sized. Had 1200$ into them, the cost of Crower billets at the time.
It was kind of a mess. I mean, you had to try real hard to be a true Pontiac guy with hardly any decent rods to buy.

[steve25]

The reason Pontiac produced the 421 cid motor was to not only keep up with the Jones so to speak, but surpass them!

Chevy had the 409 and Chrysler had the 413 and Ford had the 406.

[firechicken]

Just always thought it strange they went from 3.75 to 4.00 but then 4.21. Why not 4.25 from the factory?

[Steve C.]

Not a specific answer, but two interesting related article....

https://www.drivingline.com/articles...ast-big-block/


https://www.liveabout.com/gm-455-cub...ig-bloc-726346


.

[grandam1979]

Originally Posted by firechicken

Just always thought it strange they went from 3.75 to 4.00 but then 4.21. Why not 4.25 from the factory?

I’m thinking it something to do with running one common rod.

[Stan Weiss]

Originally Posted by grandam1979

I’m thinking it something to do with running one common rod.

There should have been no problem running the same rod with either the 4.21" or 4.25" stroke. The 4.21" needed a 1.47" compression height piston the 4.25" needed a 1.45" compression height piston.

Stan

[johnta1]

Longer rods increase the side loading on the major thrust surface of the cylinder bore.

It's the R/S ratio that affects the side loading. It increases with SMALLER R/S ratio. (R/S - Rod length to Stroke length)
Higher R/S ratio makes for higher RPM with less loading.


The R/S of a 455 with stock stroke of 4.21 is 1.57, pretty bad.
The R/S of a 455 with same stroke and 6.8" rod length is R/S of 1.62, still probably bad but better than 1.57.
The R/S of a 4.151 bore and 4.25 stroke with 6.8 rod length is R/S of 1.60, which is better than stock 455 and bigger cid.


And, if the 4.25 stroke crank is used with the 6.625 rod - R/S of 1.55, really bad?

[PunchT37]

Originally Posted by johnta1

It's the R/S ratio that affects the side loading. It increases with SMALLER R/S ratio. (R/S - Rod length to Stroke length)
Higher R/S ratio makes for higher RPM with less loading.


The R/S of a 455 with stock stroke of 4.21 is 1.57, pretty bad.
The R/S of a 455 with same stroke and 6.8" rod length is R/S of 1.62, still probably bad but better than 1.57.
The R/S of a 4.151 bore and 4.25 stroke with 6.8 rod length is R/S of 1.60, which is better than stock 455 and bigger cid.


And, if the 4.25 stroke crank is used with the 6.625 rod - R/S of 1.55, really bad?

Chevy 454 is 1.53. How bad are those?

[grandam1979]

Originally Posted by Stan Weiss

There should have been no problem running the same rod with either the 4.21" or 4.25" stroke. The 4.21" needed a 1.47" compression height piston the 4.25" needed a 1.45" compression height piston.

Stan

So I looked in the shop for a stock 455 piston but I don’t have one I was hoping to mic it. So my option is they didn’t want to thin the area below the oil rings even .020 for service reasons. Also as I said before the more common they keep things the lower the cost for the company. It could have been a piston machining issue without being involved it’s hard to say for sure. I do service work in automotive plants and the more they can do in one cell “cassette” the better they like it. a lot of what we do if your off by .020 it might as well be 2” because bad things are going to happen.

[Stan Weiss]

My understand is there was a ci limit from GM. Pontiac could have gone 4.25" stroke and a 4.13" bore and came up with the same ci. But they wanted the extra bore.

Stan