Pistons to use with cobra jet rods.

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Anonymous

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I need to know what kind of pistons ot run with cobra jet rods in my caddy thanx in advance nathan.
 

dave brode

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Dale,
Non expert 2 cents: They are waaay too short, iirc. [6.5" or so?]. You could sell them to a ford guy and use olds rods if you are afraid of stock.
Dave
 
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SO with the 7 inch rods what is the number of the chevy piston i use.  I want to keep it at around 9 to 1.  The reason for these mods is, my motor developed a nasty knock over the weekend. And it aint the rockers blowing up
 
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Anonymous

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Depends on what heads your going to use
arent many dished chevy pistons out there most are flat or something with a big or BIGGER dome
There are alot of options for the open chamber heads
 

dave brode

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1978ScottsDale,

Most any flat 454 piston will do, but some are a tad shorter than others. Assuming stock CH is 1.910, deduct .250" for the 7" rod = 1.66". Problem is, most "Affordable" 454 pistons come only up to 4.310".
The block will have to be good shape to clean up at that. Beware, some flat top Sealed Power 454 pistons have a large chamfer around the edge. Adds to the combustion space.

The KB 454s come up to 4.350". Terry has flat top 454 KBs in his engine [I sold him those and the 7" rods, btw]. You can get to 9-1 with a flat top and proper quench with a 120cc. However, you can do it simpler by using the flat top KBs for the caddy engine, with stock rods. Comp will be a wee bit higher too, as all 454s have at least one valve notch.
Dave


link=topic=1062.msg10852#msg10852 date=1112207550]
SO with the 7 inch rods what is the number of the chevy piston i use.  I want to keep it at around 9 to 1.  The reason for these mods is, my motor developed a nasty knock over the weekend. And it aint the rockers blowing up
 
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Anonymous

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I have a couple questions...

So let's say you want to build a Forged Rod, Flat Top Piston, 120cc Head, 500.

You have a lead on a set of the hard to find 7" Olds Rods & of course the 455 Olds Rod.

What is more cost effective to build?

7" Rod & Chevy Piston combo ~vs~ 455 Rod & Caddy Piston combo

If all else where equal. Which of the two would run harder?

When I was buying parts I had heard the 7" Rod was scarce so I chose not to pursue it and went with the 455 & K.B. combo.

~JM~
who hasn't even finished building his first engine, yet is already planning the next one.
 
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Anonymous

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properly cammed....7" rod engine will beat the short rod engine
you dont cam them both the same
 
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That's great. :sarcastic:

Anybody like to contribute some useful information?

Thanks
~JM~
 

dave brode

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JM,

JW's info is probably the most useful you'll find. That is, if you buy into the long rod is better theory.
Most experts will say long[er] rods are best. Fwiw, look at the combo in Terry's. Low compression and a
tighter LSA than a caddy usually likes [according to at least some]. Seems to work well. But then, looks
the all caddy combo that Shawn ran in the yellow car. It kicked ass too.
Dave
p.s. - I forgot: the notches at the bottom of block must be enlarged just a little on a few holes to clear the rods nuts.


~ link=topic=1062.msg10891#msg10891 date=1112227955]
That's great. :sarcastic:

Anybody like to contribute some useful information?

Thanks
~JM~
 
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Anonymous

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Dave,

  Where does the block need to be notched? Do the original notches need to be enlarged or another location?

Do the style of rod nut used matter? 

How about the 455 Rod with ARP bolts?

Thanks
~JM~
 

Vern

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Cost wise both should be similar unless you get the chevy pistons for dirt cheap or you pay way too much for or for preping the 7" rods.  JW info was on the mark from what I have read in various places from knowledgable people in regards to longer than stock rods.  Any time you go to a longer rod its ideal to have a cam custom ground to take advantage or you will most likely forfit any power gain that you would have got out of it.  Naturally the log rod deal is more about wear & safty as their is normally less load on the piston rings, skirts, cyclinder wallls, and most importantly the rods themselves.  Also the cam ground for the longer than stock rod is not as commonly known by the general bolts some hot rod parts on my generic motor hotrodder so it was free info he added for your and other board readers benefit.  
 

dave brode

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JM,

With 7" rods: Original notches must be enlarged on some, but not all. The almost clear. Probably .030" or less needs to be removed. 12 point nuts *might* clear. Not sure.

I don't know about with 455 rods, but I suspect nothing needs done. The bolts are shorter in those.
Dave


~ link=topic=1062.msg10895#msg10895 date=1112231909]
Dave,

  Where does the block need to be notched? Do the original notches need to be enlarged or another location?

Do the style of rod nut used matter? 

How about the 455 Rod with ARP bolts?

Thanks
~JM~
 
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Anonymous

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Dave & Vern,

  Thank you for the informative replies to the original question. You state your thoughts & a reason why. I know that a long rod engine should have a custom cam ground for it as most any performance engine should. So what cam specs need to be different & why? That would be good info to know.

The original question was: "Cost effective?" and "If all else where equal. Which of the two would run harder?"

~JM~
 

dave brode

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JM,

<  Thank you for the informative replies to the original question. You state your thoughts & a reason why.

Glad to.

< I know that a long rod engine should have a custom cam ground for it as most any performance engine should. So what cam specs need to be different & why? That would be good info to know.

I'm not learned enough to say, except that from what I've read, there will not be a huge gain from the "proper" long rod cam. I once read a very detailed article on long vs short rods in [of course] a mild sbc. The long rods made more everywhere, but only a max difference on 6 or 7 horse and lbs-ft, iirc.&nbsp;

<The original question was: "Cost effective?" and "If all else where equal. Which of the two would run harder?"

JW answered that first. Cost should be similar: Small end must be opened up for caddy pistons on 6.725" rods, as well as bbc pistons on 7". Bolts cost same. BBC pistons might be a tad cheaper.

Fyi - KB makes a one notch flat 454 in 4.310" and 4.350", as well as an 18cc dished version . Speedpro makes cheaper [and not as good, imo] Hypers, and forged [chamfered, iirc] in 4.310". Ross, J-E, SRP , and other bigger buck versions are out there in various bores.

On the subject: I'v wanted to find time to see if the dome on a chev piston might clear into the chamber on a 120cc. The valves in the 120cc are apx 1/4" shorter than in 76cc. So, a small dome might not cause any valve to piston clearance issues with say, .550" lift. I have a set of new JE-700 [502bbc] pistons here [also for sale]. I'll try to look at it some time.
Dave

~JM~
 
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Anonymous

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&nbsp; &nbsp; &nbsp; JM

To clear a few things up
The power comes form the entire combination built around what your doing bottom end wise and what the engines intended useage is
If you are building a 7" rod engine you build it as a 7" rod engine.
That is the way we do things, not to just put a 7" rod for the hell of it .
If you want to build a 7" rod engine by all means build one ,and done properly there is additional power there.
My answer addresses the part of your questions that takes care of the rest of them in saying you dont build them the same.

The power doesnt come from the bottom it comes from the top,the bottom has more to do with reliability with most of the combinations thrown together anyhow.

BTW a 12 point bolt with the rod bolts we use has cleared and require no additional work.
The 6 point bolt and 455 bolts are a bit different and seem to have alot more clearance issues .
 
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Anonymous

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Sorry for question period here again. i have a few more to ask.&nbsp; So with the 7"rod if i use a 12 point bolt i do not need to shave the bolt holes"I hope i read that right".&nbsp; Next I use some sort of a flat top piston here. I want to keep it at 9-1 or so.&nbsp; It going to be a mud truck so its going to be off and on the throttle lots.&nbsp; And its going to have to spin 39s :yes: :yes:
 

dave brode

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1978ScottsDale,

Fyi - maybe some block are different, but on mine, even the end of the rod bolt hit the notches on two holes. A 12 point nut would not have helped. Imo, it'll be best to figure on an hour's work on the notches.
Dave
&nbsp;


link=topic=1062.msg10913#msg10913 date=1112278799]
Sorry for question period here again. i have a few more to ask.  So with the 7"rod if i use a 12 point bolt i do not need to shave the bolt holes"I hope i read that right".  Next I use some sort of a flat top piston here. I want to keep it at 9-1 or so.  It going to be a mud truck so its going to be off and on the throttle lots.  And its going to have to spin 39s :yes: :yes:
 
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Anonymous

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1978ScottsDale said:
I need to know what kind of pistons ot run with cobra jet rods in my caddy thanx in advance nathan.

Just wondering why you dont want to use the caddy rods? What kinda rpm do you think it will see?     :twocents:  I had good luck with the stock rods w/arp bolts   :twocents:  keep it simple if you have a choice.......... :thumbup:
 
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