[73 TRANSAM]

I had the same style Scat crank from the past but sold them both. Which one will be the stronger crank? The one with the light gray and a smaller hole is a newer style. Thanks.

[77 TRASHCAN]

What length of rod do most folks use with 4.5" stroke cranks?
RPM list a 6.8 rod for the 3.25 main crank, and a 6.7 for the 3.00 crank.
I'm assuming this has to do with the overall diameter of the counter weights? (crank to piston skirt clearance...)

[Singleton]

Originally Posted by 77 TRASHCAN

What length of rod do most folks use with 4.5" stroke cranks?

I think more use the 6.8 rod vs the 6.7. I use a 6.7 rod, (Molnar), in my 4.5 3.25 stroker simply because it will work with an off the shelf autotec piston

[PontiacMatt72]

I’m using a 6.7 Scat rod on my 4.5” stroke/3.00” main/BBC rod journal Scat crank with Ross pistons. (Butler rotating assembly).

I thot the 6.7” rod was more common with 4.5” stroke, whereas 6.8” rod is more common with 4.25” stroke...

[Cruiser]

I saw this post & ordered a 4.5 stroke 3" main crank it arrived today. I'm going to build a 79 trans am nostalgia stock car next year. I want a nice torque engine & this 494 should work quite well. This is a club that does exhibitions type racing between rounds so there is no rules on engine size. Run what ya brung! There are ex-sprint cars, even a 1950s Indy car as well as 20s,30s,40s,50s,60s & 70s stock bodied cars. Once it's finished it will be the only Pontiac Powered car in the club. Here is an example of my buddies cars. I painted the Blue one. I will post a thread on the car once I start. The crank looks pretty good. Thanks Mark

[GTOGEORGE]

Originally Posted by Cruiser

I saw this post & ordered a 4.5 stroke 3" main crank it arrived today. I'm going to build a 79 trans am nostalgia stock car next year. I want a nice torque engine & this 494 should work quite well. This is a club that does exhibitions type racing between rounds so there is no rules on engine size. Run what ya brung! There are ex-sprint cars, even a 1950s Indy car as well as 20s,30s,40s,50s,60s & 70s stock bodied cars. Once it's finished it will be the only Pontiac Powered car in the club. Here is an example of my buddies cars. I painted the Blue one. I will post a thread on the car once I start. The crank looks pretty good. Thanks Mark

I have a nice set of GRP aluminum rods for sale 6.7" 2.20" rod end!



GTO George

[Jack Gifford]

Originally Posted by Cruiser

... This is a club that does exhibitions type racing between rounds so there is no rules on engine size. Run what ya brung! There are ex-sprint cars, even a 1950s Indy car as well as 20s,30s,40s,50s,60s & 70s stock bodied cars...

That's the coolest activity to come along in awhile! What track(s)? Have fun!

[lust4speed]

Think I would prefer the 6.8" rod. Not a lot of difference, but the 4.5" stroke is pulling the piston down farther into the cylinder and the wrist pin is getting close to the bottom edge of bore.

[BruceWilkie]

Originally Posted by Jack Gifford

That's the coolest activity to come along in awhile! What track(s)? Have fun!

Around 2004(2003?) a Rome car club did a show at Griffiss AFB.
They included a "reaction time" contest and setup a 1/10 mile run with sportsman tree. It cost @$10.00 per run iirc and whoever cut a perfect light won half the proceeds of the runs.

It was run heads up side by side and you could line up with whoever you wanted. Most of us weren't worrying about cutting a perfect light as long as we didn't red light ! LOL
Fun for sure and everything from old hot rods to stock old cars, and muscle cars and dedicated race cars.
I was among the earliest to start and cut a .002 light...which stood as best till late afternoon when a race only car with electronics finally won after lots of attempts.

Jack it reminded me of Spencer but almost no rules! The following year they used the county airport. Same format. No one cut a perfect light and I heard the club kept all the proceeds. Lots of grumbling and don't think the club did any more "races" or used either facility since.

[BruceWilkie]

Originally Posted by lust4speed

Think I would prefer the 6.8" rod. Not a lot of difference, but the 4.5" stroke is pulling the piston down farther into the cylinder and the wrist pin is getting close to the bottom edge of bore.

Agree!

I think that longer rod likely helps decrease bore/piston wear.
Personally think Pontiac stopped at 4.21 stroke for reasons other than making a 455 displacement.

[GTOGEORGE]

Originally Posted by BruceWilkie

Agree!

I think that longer rod likely helps decrease bore/piston wear.
Personally think Pontiac stopped at 4.21 stroke for reasons other than making a 455 displacement.

Seems to work for me, but everything I do according to the PY experts is wrong! LOL!


GTO George

[Steve C.]

This might be of interest, and somewhat related.......

In part from Jim Butler in a related Q&A topic- March/April 1997 PE magazine....

The Pontiac block has thin cylinders, and there not supported or reinforced in the lower part of the water jacket. Most friction is produced as the piston starts up the cylinder from BDC. The longer the stroke and shorter the rod, the more angle the rod has relative to the direction of piston movement. The pressure or load on the outside of the cylinder increases with the longer stroke. The cylinder wall starts to distort from the outward force caused by the rod angle as the piston stops and begins the upward movement. The force on the piston and cylinder is drastically increased at this point over any other part of the stroke or cycle. The rod angle and pin location in the piston can effect the amount of pressure the piston has on the cylinder.

The force caused by the rod-to-stroke angle can cause the cylinder to distort, which produces friction, since it does not match the shape of the piston moving against it. The problem has been solved for quite some time by filling the lower part of the water jacket. Jim Butler built two engines for his son's street car. Both engines had 455 blocks. One was filled and the other wasn't. The filled block ran 10 to 15 degrees cooler than the unfilled block. Upon tear-down and inspection, the cylinder walls on the unfilled block were seen to be scratched and scored from excessive friction. When measured with a dial bore gauge they were found to be out of round. The filled block was smooth and the bore round. You do lose significant coolant capacity by filling the block. However, the difference in the amount of friction produced is much improved. I'd certainly consider it knowing this.

Can we presume the longer 4.500" stroke contributes to this situation ?


.

[Dragncar]

Whittmore was a fan of long rods. I like the idea of the pin staying as far up the cylinder at BDC as possible. Side loading gets bad with short rods, long strokes.
400 sbc have that, and the middle of the bores get very worn. Pontiac 455s have some of the same issues.

[GTOGEORGE]

If I had it to do all over again id go at least 4 5/8" stroke and smaller bore.


GTO George

[Hooter]

I did not have good luck with a 4.75 stroke and 6.7 rods. Lots of piston scuffing in my street car. Now has a 4.375 Molnar crank and 6.7 Molnar rods. Pin is out of the ring land, so no ring supports needed. This is a street/strip car.

[77 TRASHCAN]

Originally Posted by Hooter

I did not have good luck with a 4.75 stroke and 6.7 rods. Lots of piston scuffing in my street car. Now has a 4.375 Molnar crank and 6.7 Molnar rods. Pin is out of the ring land, so no ring supports needed. This is a street/strip car.

I think your new combo is a good deal. Getting the pin out of the oil ring groove for a street engine would be best, also freeing up more room for ring pack. My engine builder was a player in the alky dragster world, years ago. I saw his "buttons that went in between the wrist pin and cyl wall, and I think also the oil ring groove. I asked about that. LOL he said it's a goofy racing only deal.......

Reason for my question is... a member here a few years ago had a 4.5 " crank in his engine (6.8 rod). Was built by a reputable builder, but developed problems. Was getting piston rock, rings came unsealed... He didn't publicize it too much. I think he got help from a different builder, used 6.7 rods, everything went much better.

[greggutknecht]

My 542 has 6.565 rods in it and yes the pistons are scuffed,I got a good deal on a Moldex crank and had the pistons so i put in what would fit.The crank is cross drilled so thats my 7000 rpm rev limiter.

[lust4speed]

Originally Posted by 77 TRASHCAN

...a member here a few years ago had a 4.5 " crank in his engine (6.8 rod). Was built by a reputable builder, but developed problems. Was getting piston rock, rings came unsealed... He didn't publicize it too much. I think he got help from a different builder, used 6.7 rods, everything went much better.

Except that's backwards since a longer rod should have less piston rock. Could have been a build issue not related to rod length at all. I'm running the 6.8 rods and 4.5" stroke in the drag engine, and believe Frank at AP was the one suggesting the length.

[PAUL K]

Originally Posted by lust4speed

Except that's backwards since a longer rod should have less piston rock. Could have been a build issue not related to rod length at all. I'm running the 6.8 rods and 4.5" stroke in the drag engine, and believe Frank at AP was the one suggesting the length.

I agree.....Mick, are you using a factory block with the 4.5" arm?

[lust4speed]

Mine is an AllPontiac aluminum block. We do have a club member with a 4.5" stroke in a 428 with billet main caps and 6.8" rods that only sees racing duty, and it has held up well.