[63gpman]

I am interested to learn a little bit more about rods available and to check understanding. I am not an engine guy but am trying to learn so here are a few questions.

I see a lot of the new stuff having a 2.200 big end or the BBC size, can a stock crank be cut down to that size?

I see that the RaceTech pistons can be adjusted for your pin height for no charge, so can I use these rods with the racetech pistons and just adjust them to the deck height? http://www.ebay.com/itm/H-Beam-6-660...dWltre&vxp=mtr

Are there limitations on pistons or cranks as to the length of rods that can be used?

[Steve C.]

You may already be aware of this... but, when you cut the stock crank down it typically increases the stroke. Example from the stock 4.210" to 4.250". Keep this in mind when evaluating the pin height / rod length / block deck height.

http://www.hotrod.com/features/1507-...kshaft-stroke/


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[Schurkey]

Originally Posted by 63gpman

I see a lot of the new stuff having a 2.200 big end or the BBC size, can a stock crank be cut down to that size?

A crank can be cut to the BBC rod journal size. This can be done without affecting the stroke, or, it can be done to increase or decrease stroke a little. The crank grinder needs to not be an idiot.

One has to be careful to get the rod side-clearance correct, either by altering the journal width or the rod width.

Originally Posted by 63gpman

I see that the RaceTech pistons can be adjusted for your pin height for no charge, so can I use these rods with the racetech pistons and just adjust them to the deck height? [url]http://www.ebay.com/itm/H-Beam-6-660-2-200-990-Bronze-Bush-4340-Connecting-Rods-Suits-Chevy-BBC-454-/181990904527?hash=item2a5f80d2cf:g:wnwAAOSwoydWltr e&vxp=mtr[/url

eBay connecting rods are almost certainly generic Communist Chinese junk. "H"-beam rods tend to be very heavy.

Originally Posted by 63gpman

Are there limitations on pistons or cranks as to the length of rods that can be used?

The rod needs to be long enough to keep the bottom of the piston from hitting the counterweights. It needs to be short enough to keep the rings in the hole at TDC. It needs to be the right width to appropriately fill the space on the journal. It needs to have corrrect-size big- and small-end holes where the crank journal and the pin go through. The big-end of the rod needs to be small enough to not hit the block, especially in a "stroker" application--which often means clearancing the block rather than making the rod big-end smaller.

The whole works needs to be "balanceable".

[blykins]

Yes, a stock crank can be cut down to that size. My last 400 had BBC rod journals. Was not offset ground, so factory stroke stayed at 3.750".

You open yourself up to a variety of rod lengths when you do that, as well as less expensive rods.

You would just order your pistons and specify which rod you use.

I will say that a 7.100" rod with a 3.750" crank will fit just fine. I betcha Cliff R. hasn't tried that one....

[63gpman]

Originally Posted by Schurkey

eBay connecting rods are almost certainly generic Communist Chinese junk. "H"-beam rods tend to be very heavy.

When you say heavy, are they heavier than stock rods? This build is not likely to see over 5500 rpms but am trying to build something that is stronger than stock. I don't know what fasteners they use, but at the price, if they work, it would allow for upgrade of ARP 2000 bolts, at least that was my thought anyway.

[Tom Vaught]

Originally Posted by blykins

Yes, a stock crank can be cut down to that size. My last 400 had BBC rod journals. Was not offset ground, so factory stroke stayed at 3.750".

You open yourself up to a variety of rod lengths when you do that, as well as less expensive rods.

You would just order your pistons and specify which rod you use.

I will say that a 7.100" rod with a 3.750" crank will fit just fine. I betcha Cliff R. hasn't tried that one....

Stock 366 rod was 7.080" with the 3.375" stroke. Maybe not the best combination but it allowed using a normal ring package in the block. I would not worry too much about a 7.1" long rod with a 3.75 stroke or even a 3.5" stroke. A 2.1 R/S might not be the best for some applications (stock 366 R/S ratio).

Tom V.

[tom s]

R/S ratio is way over rated.Tom

[slowbird]

Originally Posted by tom s

R/S ratio is way over rated.Tom

X1000!

[Tom Vaught]

In what manner? Most imply a shorter rod length is the way to go.

Some old timers say Just hook the two parts together and forget about it.

Overrated in what way, bragging about being longer??

I always paid more attention to where the rings, pin and the bottom of the bore was.

Tom V.

[tom s]

Tom,from many top engine builders.Pick your piston- ring pack,pick your stroke and what ever connects them is what you use.Tom

[blykins]

That's generally how I see it too.

If I have a smaller engine with a big fat low velocity intake port, I may change to a shorter rod to help out.

[tom s]

I used a touch shorter set with a 10-1 street engine to maybe make it a little less detonating prone.Less hang time.Tom

[Tom Vaught]

Originally Posted by tom s

Less hang time.Tom

Less hang time (also known as Dwell time) and what is right is mostly engine specific and generic rules typically do not mean much.

Tom V.

[hurryinhoosier62]

Originally Posted by blykins

Yes, a stock crank can be cut down to that size. My last 400 had BBC rod journals. Was not offset ground, so factory stroke stayed at 3.750".

You open yourself up to a variety of rod lengths when you do that, as well as less expensive rods.

You would just order your pistons and specify which rod you use.

I will say that a 7.100" rod with a 3.750" crank will fit just fine. I betcha Cliff R. hasn't tried that one....

DITTOS. As long as the machinist grinding the crank has set the correct stroke into the heads on the grinder, the stroke SHOULD remain same. ONLY you choose to offset grind the rod journal would the stroke change. A 7.100" rod with a 3.750" stroke is a 1.89:1 rod ratio (PERFECT) but would give you a 1.275" compression height with a 10.250 deck.

[hurryinhoosier62]

Originally Posted by tom s

R/S ratio is way over rated.Tom

For most street engines, I would agree. In race engines(especially road race/ oval track racing), it can be critical.

[Cliff R]

We've assembled and dyno's a number of 455 builds with the 4.25" stroke cranks and BBC 6.8" rods. Can't see any improvements on the dyno vs Pontiac 4.21" and the Pontiac 6.625" rods based on the heads and cams being used, in other words they all make the power expected for the CID, compression ratio, head flow and camshaft chosen.

This doesn't mean that one may not have a slight advantage once placed in service, or run a few hundredths quicker at the track in the same vehicle.

I saw this played out a few years ago when I swapped from a flat cam to a hydraulic roller cam. The HR cam didn't make but 3hp/4ft lbs more power on the dyno, but the car ran faster at the track, and we didn't make any other changes.

I suspect the HR design allows the engine to rev a little quicker with less frictional losses than the flat cam. With the flat cam the car ran very consistent 11.70-11.80's for most race days. After the cam swap it ran a solid tenth quicker in the same weather/track conditions and nearly 2mph faster.

Best runs ever for the flat cam was 11.64 @ 116mph. The roller cam went 11.52 @ 118mph, showing a clear advantage over the flat cam beyond the 3hp improvement that we saw on the dyno.

I haven't done a 6.8" BBC 4.25" engine for my car to date, so don't have any feedback on whether or not it would show any improvements. All of my 455's have been 4.21" stroke with 6.625" Pontiac rods.

If it matters we have done some offset stroke crank grinding on a few builds to increase CID slightly and at the same time get the pistons closer to TDC without having to machine a lot off the deck surface for the parts being used. I usually reserve moves like that for "max-effort" engines subject to class rules where you need to make a little more power in every area to be competitive......FWIW......Cliff

[Steve C.]

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 Pontiac 455 with the 4.210-stroke and the stock 6.625-inch rod is producing a rod-to-stroke ratio of 1.57:1. This is not considered ideal, but is much better than a 454 Chevy, which has a 4-inch stroke and a rod length of 6.135 inches with the result of a 1.53:1 ratio, which is much worse.

I mentioned offset grinding the crank here at the beginning of the thread since I would assume most engine builders would probably use the popular longer 4.250" stroke.

Jim Butler mentions that stroke length within his Q&A answer, and he also notes in conjunction with a 6.8" rod length. With that in mind Jim went on and notes it would increase the displacement. But at the same time, the better rod angle, the rod-to-stroke ratio of 1.6, and the raised pin location in his custom pistons relieve the stress on the block. The higher pin pushes up on the piston instead of out on the shirt and results in less fiction.

Please don't shoot the messenger here, these are Jim Butlers thoughts on the subject. And as more often not presented only for conversation around the coffee table.


.

[63gpman]

Interesting stuff and thanks for all the info. So the build is going to be a 73 455. I am trying to do as much as I can on a budget with the lower end because I would like to buy some aluminum heads.

Stock 455 Ratio would be 1.573 but with the 6.660 @ 4.25 stroke 1.567 so not that far off of stock, just not ideal. Not sure if I am concerned considering the low RPM's that I will be running.

So I guess the next question is to go with an offset stock crank or to purchase a 4.25 cast crank? I see RPM has them for $256. I would guess the cost to offset grind a crank is going to be close to that?

[b-man]

Originally Posted by 63gpman

Interesting stuff and thanks for all the info. So the build is going to be a 73 455. I am trying to do as much as I can on a budget with the lower end because I would like to buy some aluminum heads.

Stock 455 Ratio would be 1.573 but with the 6.660 @ 4.25 stroke 1.567 so not that far off of stock, just not ideal. Not sure if I am concerned considering the low RPM's that I will be running.

So I guess the next question is to go with an offset stock crank or to purchase a 4.25 cast crank? I see RPM has them for $256. I would guess the cost to offset grind a crank is going to be close to that?

I can remember about 20 years ago or so reading about rod to stroke ratios of around 1.4 being used in Pro Stock engines in an article written by one of the top engine builders. Simply put, you connect the crank to the piston using whatever rod length fits the block deck height and piston being used.

I used to bracket race with a group pf Pontiac guys, two of the fastest cars had 455 combos that retained the stock cast cranks and 6.625" rod length. One was in a '64 GTO that ran in the nines, the other was in an Oakland roaster than ran in the 8s. The roadster regularly shifted at 7500 and ran a stock oil pan along with ported 1970 #614 RAIV heads.

No need to split hairs on rod length here, using the factory dimensions would be more than fine. However less expensive good forged rods with the BBC 2.200" journal size are readily available in both 6.7000" and 6.800" lengths along with pistons to match, both can be bought right off the shelf. Either way is good.

On crankshaft selection, I'd trust a good Pontiac cast crank properly prepped long before I'd ever use a questionable offshore casting.

Good luck.

[63gpman]

Maybe I am way off here, but my thoughts were that the pistons would come last since I they can set the pin height anywhere I want (mostly) as well as bore size, would that not be the last thing to order?

If I am going to have the stock crank cut down, I would assume I would want to have the rods there at that time to confirm side clearance and sizing?

I need to confirm a few things with my machine shop on Monday, but unless I am missing something, I will be purchasing the rods above. Thanks everyone for their input here, I have a lot to learn!!