318 build problems & new direction

-
There are plenty of cars out there that make decent power but are dogs out of the hole because they are lacking one thing : the proper converter.
 
Circling back to the 4” bore teen,


I have probably read or heard people say ‘sonic test’ about a thousand times now, but never once did I hear anyone give me a hard number in terms of min thickness. I suppose it’s also easy to repeat a min number heard previously, but again, how do we know that number is a true min.
 
OK, so my shortblock build uncertainty path is now certain. I bought a standard bore 340 block cast in 1969. Included is a steel crank and rods. No pistons. Also included is the windage tray. I will bore to 4.07", and not look back. I do have a question. The main caps had normal hex bolts. Caps 2/6 did not have extenders for tray. A quick look at Hughes, and I can get an ARP entire set for over $100. Why so freaking much? Any other options? Is finding just the 2 cap bolts with extenders difficult, and if i do, is it crazy to mix them with old?
 
cudaracer said:
OK, so my shortblock build uncertainty path is now certain. I bought a standard bore 340 block cast in 1969. Included is a steel crank and rods. No pistons. Also included is the windage tray. I will bore to 4.07", and not look back. I do have a question. The main caps had normal hex bolts. Caps 2/6 did not have extenders for tray. A quick look at Hughes, and I can get an ARP entire set for over $100. Why so freaking much? Any other options? Is finding just the 2 cap bolts with extenders difficult, and if i do, is it crazy to mix them with old?
Click to expand...


If all you need is the bolts to mount the windage tray I probably have some. I'll check tomorrow. If I have them, they are yours. You can PM me your info and I'll ship them to you.

I'll let you know in the AM if I have them, but I'm pretty sure I do.
 
Thanks Yellow Rose,
I have the main caps and all of them have regular hex head bolts. Did the factory include studs and nuts for the tray mounts? The kits I see have studs and nuts for all main caps. Is it OK to mix?

I will check out Mancini too.
 
I bought the Mopar Perf. tray kit. Came with the 4 special tall main cap bolts (like factory), 4 small bolts that thread into the tall main cap bolts, which hold the tray in place. (hope that makes sense!)
 
Damn I'm sorry, I only have one set of main cap bolts left, and they are for an engine on the stand. I used to have a bunch of them. Must have given them all away.

Sorry I couldn't help.
 
No worries yellow rose.

So I have my 318 short (72) that is running with 85K gentle miles, and I just purchased a 69 340 short from a 69 Cuda S. It included block, caps, steel crank and rods.

I am wondering what do I borrow from the 318. It has a cast crank that probably needs nothing, as the steel crank is rusted and needs to be reworked. Do I care that I am only building to 400hp if I just use the cast crank?

Also, should I bother with factory rods, or will Eagle I rods (~$250) be a much wiser choice to attach my KB hypers to?

Also, how do I know if crank is forged?

Any other build advice?

I am overboring to 4.08", using my comp 268 H cam, 308 bowl ported heads, DP intake with Fitech injection and Dakota logs feeding dual exhaust 2.5". Any guess as to my power?
 
I'd machine the steel 340 crank, if you use the cast 318 crank, it will require balancing to stay internal, and heavy metal ain't cheep.... food for thought.
 
The cast vs forged is seen in the casting line running down the 'spine' of the crank. That spine area will be all 'mooshed out' when forged; the cast spine will still be a sharp edge. Plenty of pix round showing the differences....

A '72 318 crank should be cast.

And I am repeating the advice from many on this point: With lighter rods and pistons, you ought to be fine at that HP level. (I have not done destructive failure analysis myself!) We have used a cast 273 in my son's 340 with SCAT I-beams and KB hyper pistons. The nice thing about those lighter parts is that it takes stress off of crank, AND, when you go to balance it, the shop will simply take off weight from the crank counterweights. And the 318 crank rotating mass is lighter too. Easy!

(And from what I have read the SCAT stuff seems to generally be better than Eagle.....)

BTW, is the block already over .020" or .030". IF the block is stock bore now, I'd just go over .020" to save some meat in the block for future rebuilds.
 
Good point on balance.

And block is std bore.

I calculated over 200 grams of savings per cylinder, if I use Eagle rods and KB pistons. So yeah, I think I would be removing weight from either crank, and the cast crank does weigh less and will in fact be stressed less and is ready to go and does not need other work.
 
Oh, and I will not be spinning this engine more than 5500. I have a tall gear and automatic, so over revving is a non issue.
 
Going .040" over versus .020" over to get a few more cubes won't do much. So I'd like to keep those rarer and rarer 340 blocks in circulation as long as possible.

It is a guarantee that either crank will have weight taken off. The shop took 81-82 grams off each of the largest 2 end counterweights for our 340 KB and SCAT rods and a 273 crank. (I have all the bobweight computations in a spreadsheet.)
 
Not a problemo... It is in Excel. You want it in an email? Send me a PM.

I have 7-8 columns for various SBM combinations. The weight data has been gathered from a variety of sources.

Or would type of stuff be useful to post 'out in the open'? I can do a screen capture and post it.
 
nm9stheham said:
Not a problemo... It is in Excel. You want it in an email? Send me a PM.

I have 7-8 columns for various SBM combinations. The weight data has been gathered from a variety of sources.

Or would type of stuff be useful to post 'out in the open'? I can do a screen capture and post it.
Click to expand...
Spreadsheet is perfect.
And I will give you an email, the minute I figure out how to send PM. Never done it here.
 
And here is the 340 bobweight stock, versus with KB243's and SCAT I-beam rods. This is to show the amount of operating weight taken off of each crank throw.
General disclaimer: These numbers are believed to be accurate but are not guaranteed.
Sorry for the poor quality....
Comparative Bobweights 340.gif
 
Turns out I need a crankshaft for my 340 short after all.
So, if I am adding lightweight rods and pistons and only spinning it to 5500 max.....do I really need steel? Certainly not forged. What about a Mancini cast crank for $250? It's ready to go and I bet better than production crank.

Thoughts?
 
Conventional wisdom and experience is that a cast crank is good for somewhere in the 400+ HP range. I personally would not bet on the Mancini cast crank to be any particular level of quality without knowing more about it.

If you start with a 340 crank and use SCAT or Eagle I-beam rods with KB pistons, you are going to be taking a lot more weight off of the crank for balancing than if you start with a 318 or 273 crank. I don't see a crank on Mancini for under $353...??? I personally would just put the 318 cast crank in there.
 
nm9stheham said:
Conventional wisdom and experience is that a cast crank is good for somewhere in the 400+ HP range. I personally would not bet on the Mancini cast crank to be any particular level of quality without knowing more about it.

If you start with a 340 crank and use SCAT or Eagle I-beam rods with KB pistons, you are going to be taking a lot more weight off of the crank for balancing than if you start with a 318 or 273 crank. I don't see a crank on Mancini for under $353...??? I personally would just put the 318 cast crank in there.
Click to expand...

Thanks for the reply and data above btw.
So I have a completely rusted crank as received, that is listed as a light duty cast part for 273 and heavy duty for 318. Part number is 2658393.
 
BTW, some 318 truck engine cranks are forged if you go to the j-yard... Start a Conversation and I'll send that spreadsheet over.

Another source of reconditioned stock crank is Crankshaft Supply; I can't speak for their quality but they have been around for a long time.
 
You must log in or register to reply here.
-

Back
Top