Pontiac Street Performance
Home | Profile | Register | Active Topics | Members | Search | FAQ
Username:
Password:
Save Password

 All Forums
 Pontiac Performance
 Blocks / Cranks/ Rods and Pistons
 Pontiac Rods On BBC Journals
 New Topic  Reply to Topic
 Printer Friendly
Author Previous Topic Topic Next Topic  

bigD
Indian Spirit

USA
643 Posts

Posted - 28 Apr 2014 :  7:08:10 PM  Show Profile  Reply with Quote
This is info for anyone interested in using a Pontiac spec 6.625 rod on a crank with BBC size rod journals. As I'm sure most of you know, the BBC rod journal is .050 smaller than the Pontiac journal. 2.25 for the Pontiac--2.20 for the BBC.

I suppose that's one reason why BBC rods are used for stroker assemblies. Another reason. of course, is because the BBC rods are available in lots of different lengths. And a 3rd reason is because most lengths of BBC rods are cheaper than Pontiac spec rods.

But we all know that most Pontiac motors, including 350, 400, 428 & 455, used the same size rod. Even if they were forged, they still had a 2.25 big end and a .980 small end. Now if you know of some of the rare engines that were different-great. And we know that there were at least 3 different strokes used--3.75, 4.00, & 4.21.

So how did they use the same rod with these 3 different stroke sizes ? Well, of course, they did it by using different pin heights (PH). Some call this compression height or compression distance. It's the distance form the center of the pin hole to the top of the flat part of the top of the piston. The 350 & 400 used aprox a 1.70 PH. The 428 had a 4.00 stroke, so the PH had to be decreased by 1/2 of that stroke difference in the 428 piston.

So if you subtract 1/2 of .250 from 1.70, you come up with aprox 1.575. I just looked up a Probe 428 piston. It has a PH of 1.59. it is said that Pontiac put the top of the piston about .020 in the hole (below the deck surface). So I've noticed that some of the forged pistons have slightly larger than stock PH to reduce the deck height without machining the deck.

A sealed power cast replacement 455 piston has a PH of 1.481. A forged L2359NF piston has a PH of 1.497. So the forged piston would reduce the deck height by .016 over the cast piston.

Anyhow, I said all that to say this. You can juggle stroke, pin height, and rod length to come up with lots of possible combinations that will work. Most now seem to favor the long 6.8 BBC rod for most stroker combos. In fact it is now possible to build a "Pontiac" motor without any GM Pontiac parts at all.

But it is possible to build a 350 or 400 block stroker, using the popular 4.25 stroke Eagle crank and and stock spec 6.625 rods. But why would anybody want to do that ? Why not ? It's just one more option that's available. If you are not interested, or you have nothing to add but negatives, please just ignore this thread and show me a little respect by keeping your negative comments to yourself ! THANKS in advance for that.

bigD
Indian Spirit

USA
643 Posts

Posted - 28 Apr 2014 :  7:54:54 PM  Show Profile  Reply with Quote
To match the Pontiac rod to the BBC rod journal will require a .050 undersize bearing. Most brands don't make that size. I heard that King brand offered this size but discontinued them. But I recently ran across both a .050 & a .060 undersize still listed in the Summit catalog. So I called Summit and got them to call King and find out about these bearings. They told him that these bearings had NOT been dropped and in fact a shipment including these sizes were expected sometimes in the middle of May.

Now, I have absolutely no way of knowing if this info is accurate or not. But if it is, then the .050 size will mate the Pontiac rod to the Stock BBC journal. And the .060 will allow for a .010 reduction of the journal later should it become necessary.

Now, should it turn out that the Summit guy was given false info, and these bearings do not become available, then this whole thread is useless. If however, on the other hand, these bearings do become available. Then it opens up several possibilites. And not just with the 4.25 stroke combos. This would allow offset grinding the Pontiac cranks for slightly more or less stroke. This could allow near zero deck heights without cutting the deck down. I assume that class racers have been offset grinding for years to max NHRA specs to get every last bit of power they possibly can legally.

So in some upcoming posts, I'll present some possibilities, for consideration. No, I'm definitely no expert on Pontiac strokers. But I did make A's in most of my math classes. So we'll just play with some numbers and see what possibilities arise. Now if you have plenty of $$$, just have one of the big name shops build whatever you want. but if you're on a low budget, you may find some of this info interesting--or not.

Edited by - bigD on 29 Apr 2014 05:10:07 AM
Go to Top of Page

Merrik66
Cochise

USA
370 Posts

Posted - 28 Apr 2014 :  10:01:33 PM  Show Profile  Reply with Quote
Interesting topic D.
Go to Top of Page

bigD
Indian Spirit

USA
643 Posts

Posted - 29 Apr 2014 :  04:57:49 AM  Show Profile  Reply with Quote
OK, here are links to the King rod bearings I mentioned.

http://www.summitracing.com/parts/kgb-cr815cp050/overview/make/pontiac

http://www.summitracing.com/parts/kgb-cr815cp060/overview/make/pontiac

Now, as most of ya'll know, for a while, the cheap Chinese cast cranks were available which would transform a 400 block into a stock stroke(4.21") 455. They had the 3" mains and 2.25" rod journals, so that you could use 6.625 Pontiac rods and 455 pistons. But they are no longer available, as far as I can tell. The forged cranks are still sold for around $700.

http://www.summitracing.com/parts/esp-440042106625/overview/make/pontiac

Now back in the day, some would turn down big main cranks, to work in small main blocks. I've read that Mr.P Body used to do this as a common practice. And PDude used to make some sort of adapter for the thrust bearings, for use with these cranks. And I've read that this is how Smokey got the 421 motors to last for a 500 mile NASCAR race. he would turn down a 421 crank to run in a 389 block. The smaller mains would give the motors a better chance to survive the sustained rpm of a long race.

But nowadays, if you don't wanna spend $700 + on a crank, or use a 428/455 block, then you are stuck with the 4.25 stroke crank, which has BBC size rod journals.

http://www.summitracing.com/parts/esp-104004250/overview/make/pontiac

So that's what I wanna talk about, using a Pontiac spec 6.625 rod with the 4.25 stroke crank. Most guys want at least an Eagle 5140 BBC rod, instead of some of the cheaper Chinese rods on the market. The best price for these rods that I have found is about $336 shipped from CNC.

http://www.cnc-motorsports.com/catalogsearch/result/?order=relevance&dir=desc&q=sir6800b

The best price I've found for Pontiac rods are the 5140 rods sold by CP. I think RPM Machine is the dist for these rods--not sure.

http://www.competitionproducts.com/5140-I-Beam-Rods-Pontiac-6625-Press-Pin/productinfo/4556/#.U19oUVVdVZ4

http://www.racingpartsmaximum.com/sae5140stockrod-2.html

But my choice for a decent Pontiac rod is the Tomahawk rod.

http://forums.performanceyears.com/forums/showpost.php?p=5050633&postcount=1

http://forums.performanceyears.com/forums/showpost.php?p=5137912&postcount=18

Since the 350P blocks have a smaller bore size, I've decided to discuss them in the "350P Parts ?" thread in the Restoration and Performance forum.

Edited by - bigD on 29 Apr 2014 05:30:06 AM
Go to Top of Page

bigD
Indian Spirit

USA
643 Posts

Posted - 29 Apr 2014 :  7:12:53 PM  Show Profile  Reply with Quote
The 1st option that comes to mind is to use the cheap, strong and popular, but heavy SP L2359NF 455 pistons. But the 4.25 stroke crank will raise the piston top .020 higher in the cylinder. This should put the piston top slightly above the deck. But I've read that the top of these pistons are plenty thick enuff to take some off the top. So, a good shop should be able to take enuff off to create zero deck height.

Or if you want a lighter 455 piston, there are several available in the $500 range. Or you can go all the way and buy the Ross custom pistons with the exact pin height you want, for about $800.

I'll look up the best prices on some 455 pistons when I get time.

http://www.summitracing.com/parts/slp-l2359nf30/overview/make/pontiac

http://www.rpmmachine.com/shop/index.php?l=cart_view

http://www.cnc-motorsports.com/pistons-1/probe-pistons/srs-series/pontiac-400-428-455/probe-srs-10-06-1-flat-top-pontiac-455-4-181-bore-4-210-stroke-for-6-625-rods.html

http://www.flatlanderracing.com/probe-sportpontiac01.html

The following site always has a MUCH lower price on everything I've priced on their site. For example, these pistons, shipped, are priced over $30 cheaper than Summit. This site seems bogus to me. I know a guy who ordered some 5140 forged rods from them because they were cheaper. They took his payment, then called him later saying they did not have the rods and wouldn't have any for several weeks. Another thing that makes them look bogus is that the site shows that they have a HUGE number of that particular part in stock, which you know is not even close to the truth. So, have any of you guys ever bought anything from this place, or heard anything either good or bad about them ???

http://www.buydart.com/pontiac-big-block-455-455-1485-flat-top-55-2618t6-p-15439.html

Icon has a 455 piston # ic888-030. RPM sells them for $493 + freight and says they'll meet any competitors price.

https://rpmmachine.com/kbpistons-premiunforged-menu.shtml

http://www.rpmmachine.com/shop/index.php?l=product_detail&p=6226

All the other 455 pistons I can find right now are well over $500.

Edited by - bigD on 30 Apr 2014 05:26:13 AM
Go to Top of Page

beertracker
Cochise

350 Posts

Posted - 29 Apr 2014 :  10:53:03 PM  Show Profile  Reply with Quote
Good info!

I want a LS engine in my Pontiac
Go to Top of Page

Mr. P-Body
Running Bear

USA
2258 Posts

Posted - 30 Apr 2014 :  7:58:55 PM  Show Profile  Visit Mr. P-Body's Homepage  Reply with Quote


In olden times, Federal Mogul made a "1555-AP-060" for the expressed purpose of reducing bearing "speed". Tha "AP" series were popular for the "softer" rod bearing applications and worked very well with the nodular cranks. Gone for 30 years now...

The kit Ken has is a "shimmed" thrust bearing, using an Olds 455 thrust bearing with shims "behind" it to support it. It's the correct length bearing for the large journal (.060" longer than the smaller journals). For cast cranks, this is really the only good choice. Welding a cast crank is asking for disaster. Forgings, however, can be welded up on the thrust face and have it ground to the correct size. This is how Smokey modified them. When properly done, welding a forging won't hurt it at all.

The Icon FHR (Forged Head Relief) are "replacement" pistons. For 455, no more desirable than Speed Pro L2359F-xxx. Maybe a LITTLE lighter, but not much. Biggest advantage is they're readily available in "standard", which the Speed Pro are not. A .030" over 428 block with a 455 crank IS a a 455...(:- If a "dish" is needed to reduce compression, the FHR piston won't "do". The head isn't as thick as that in the Speed Pro, which WILL allow for an extensive "dish". The "good" (light weight "racing") Icon pistons are $550-up for most Pontiac numbers. Don't confuse the two.

Jim
Go to Top of Page

bigD
Indian Spirit

USA
643 Posts

Posted - 30 Apr 2014 :  10:56:03 PM  Show Profile  Reply with Quote
quote:
Originally posted by Mr. P-Body



The Icon FHR (Forged Head Relief) are "replacement" pistons. For 455, no more desirable than Speed Pro L2359F-xxx. Maybe a LITTLE lighter, but not much. Biggest advantage is they're readily available in "standard", which the Speed Pro are not. A .030" over 428 block with a 455 crank IS a a 455...(:- If a "dish" is needed to reduce compression, the FHR piston won't "do". The head isn't as thick as that in the Speed Pro, which WILL allow for an extensive "dish". The "good" (light weight "racing") Icon pistons are $550-up for most Pontiac numbers. Don't confuse the two.

Jim



The Icon ic888-30 pistons I mentioned are the good ones. And RPM sells them for about $493 + freight. And they advertise that they will meet or beat any competitors price on 'em.

http://www.rpmmachine.com/shop/index.php?l=product_detail&p=6226

https://rpmmachine.com/kbpistons-premiunforged-menu.shtml

Here's the ugly FHR pistons:

http://www.cnc-motorsports.com/pistons-1/icon-pistons/icon-ic9960-030-fhr-forged-flat-top-pistons-4-181-bore.html




Edited by - bigD on 30 Apr 2014 11:01:06 PM
Go to Top of Page

pmd400
Tribal Scout

3 Posts

Posted - 08 Jun 2014 :  03:28:11 AM  Show Profile  Reply with Quote
From what Iv read the bbc rod is .990 wide on the big end while the pontiac is 0.996.
Could this cause a problem?
Also is the 6.625 Pontiac rod and stock piston sure to clear the counter weight of a crank designed for a longer rod?
Go to Top of Page

bigD
Indian Spirit

USA
643 Posts

Posted - 08 Jun 2014 :  06:39:54 AM  Show Profile  Reply with Quote
quote:
Originally posted by pmd400

From what Iv read the bbc rod is .990 wide on the big end while the pontiac is 0.996.
Could this cause a problem?
Also is the 6.625 Pontiac rod and stock piston sure to clear the counter weight of a crank designed for a longer rod?



I have not heard of any problems with these, but if you find some info that points out problems, please post a link to that info. Thanks!

http://www.hotrod.com/pitstop/1311_pit_stop_pontiac_chevy_rods_421_sd/

Edited by - bigD on 08 Jun 2014 06:47:07 AM
Go to Top of Page

Mr. P-Body
Running Bear

USA
2258 Posts

Posted - 09 Jun 2014 :  7:50:54 PM  Show Profile  Visit Mr. P-Body's Homepage  Reply with Quote


PMD440,

No, and No. Have had a serious issue in the past using a rod only .060" "short", much less .175... Don't scrimp here! Get good rods.

We grind many Pontiac cranks for BBC rod journals. A little more side clearance only means the oil gets through faster, a GOOD thing until TOO much gets through. For a street engine, no more than .025" side clearance (and that's "loose") and "race", .035". Nothing UNDER .010".

Jim
Go to Top of Page

bigD
Indian Spirit

USA
643 Posts

Posted - 11 Jun 2014 :  05:49:36 AM  Show Profile  Reply with Quote
quote:
Originally posted by pmd400


Also is the 6.625 Pontiac rod and stock piston sure to clear the counter weight of a crank designed for a longer rod?



I've been thinkin about this question some. And since I have never measured any of these stroker cranks, I can't say for sure. But here's just a few thoughts I have about it. Now, keep in mind, this is only my opinions and thoughts on the subject. Anyone who differs has a right to their opinion.

OK the 4.25 stroke crank has a stroke which is only .040 longer than a stock 455 stroke. 4.25 - 4.21 = .040. So, that means that the stroker crank will raise the piston .020 higher in the cylinder on the upstroke and lower the piston .020 lower in the cylinder on the down stroke than a stock 4.21 stroke crank would. .020 + .020 = .040 increased stroke.

So that means that the 4.25 stroke crank will lower the bottom of the piston skirt by a distance of .020 further than a 4.21 crank would at the bottom of the down stroke. So, if the counterweights of a 4.25 stroker crank are the same size a those of a 4.21 stroke crank, then the piston skirt will come .020 closer to the counterweights at the bottom of the stroke.

So that brings 2 questions to my mind. #1 Did the Pontiac engineers design the 455 engine so that the skirts of the piston would only clear the counter weights by .020 or less ?

#2 Although there is only .040 difference in the stroke, do the 4.25 stroker cranks have larger counterweights than the stock 455 cranks ? And if so, why ? Actually, since the 4.25 stroke cranks are designed to use rod/piston combos which are much lighter than the stock 455 rod/piston combo, it would make more sense that the counterweights would actually be a bit smaller, since less weigh would be needed to balance the crank with the lighter rod/piston combo.

Here's some more thoughts on the subject. Nobody would recommend stock cast pistons for a performance 455 motor. The min piston anybody would use is the SP L2359NF forged piston. The pin height or comp distance on these is higher than the stock 455 piston, which will raise the piston higher in the cylinder than a stock piston. So, if the SP piston is exactly the same size as a factory 455 piston, then the skirt of the SP piston will have more skirt to counterweight clearance than the factory piston.

I have not found any specs on the L2359NF pistons which lists the distance from the top to the lowest edge of the skirt. So, I can't say for sure if that distance is exactly the same as a factory 455 piston or not. But, just looking at pics of both, the skirt on the SP pistons looks shorter. If so, then that's even more skirt to counterweight clearance.



If anybody has a link to this info, please post it here.

Now, that should answer that question. But, most guys would opt for some of the lighter 455 pistons, to reduce the reciprocating weight of the rotating assembly. My guess is that most of these have even shorter skirts than the SP pistons ? If so, that's even more skirt to counterweight clearance.




If you buy custom pistons you can get any pin height that is possible. Here is a set of Ross pistons that are said to be on 6.625 Oliver rods for a 4.21 stroke build. Looks like the pin is too low for that app, but that's what the caption to the pic said. Money won't buy everything, but it will buy any Pontiac parts that can be made.



One last thought before I leave this post. Even if you use the longer 6.7 or 6.8 rods, you must reduce the comp distance of the piston to keep the top of it from going above the deck. So moving the pin will lower the bottom of the piston skirt in the cylinder, and it will be in just about the same place as it was with the 6.625 rod and SP piston. So the only way I can see to increase skirt to counterweight clearance is to use pistons with a shorter skirt, or reduce the size of the counter weight. But hey, I could be wrong. If so, it won't be the 1st time !

Edited by - bigD on 11 Jun 2014 07:38:56 AM
Go to Top of Page

bnorris_74
Crazy Horse

USA
1442 Posts

Posted - 06 Jul 2014 :  07:12:26 AM  Show Profile  Reply with Quote
TTT
Go to Top of Page
  Previous Topic Topic Next Topic  
 New Topic  Reply to Topic
 Printer Friendly
Jump To:
Pontiac Street Performance © 2006-2017 Go To Top Of Page
This page was generated in 0.22 seconds. This Site Sponsored By:
Powered By: Snitz Forums 2000 Version 3.4.05