Use stock 1970 340 rods in rebuild?

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Richie

Richie

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Hi everybody:

When I get this 70 340 rebuilt to around 350 - 370 hp I was wondering if it would be a good idea to use the stock rods. They all look excellent as does the block and crank.
 
Yup, the only drawback will be the cost of reworking with good rod bolts. That cost will quickly approach and possibly eclipse new rods with good rod bolts ready to go.

But as to the question whether the stock rods can hold up. Hell yeah. And then some.
 
yeah, you can re-use the rods, but I would recommend installing new bolts and having the rods reconditioned. It will cost a little more than a new set of aftermarket rods, but I like that the stock rods have an oil squirt hole to help oil the bore on the opposite cylinder. The new aftermarket rods do not have oil squirt holes, so I would rather spend the extra $40 to buy new bolts and recondition the old rods... You don't know how many times the engine could have been taken apart and re-assembled before you bought it and the bolts could already be stretched...

You need to replace the bolts because if they have been taken apart and torqued more than 5 times, they can be stretched to much and break early... Are you willlng to blow up your engine to try to save $70 on new rod bolts????
 
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Or you can just replace the rods bolts and run them as is. I always got away with that.... never a rod problem yet, in many thousands of miles of hard racing. Mind you, this was with Opel and Mitsubishi and Ford small block rods and such. I do have a set of 340 rods that I will put in a 360 at some point, with just new bolts.

If you have the $$, I favor just going to SCAT rods for around $300 a set. Ready to go, and weight matched to each other, but beware they are lighter than stock rods by around 180 grams each so a crank rebalance is in order. So there is that cost.

But if you are going with lighter pistons like KB or SP hypers, you are already going to rebalance due to the lighter piston weight. In that case, it seems like a no-brainer to add the SCAT rods: the new pistons are balanced within 1-2 grams of each other, and so will be the SCAT rods. So you buy them, give the weights to the machinist ( the big end and small end weights for the SCAT rods comes marked on the box), and the machinist uses those numbers for the crank balance alone, and you match the rods/pistons yourself. That maneuver will save about $100-150 on the balance work.
 
nm9stheham said:
Or you can just replace the rods bolts and run them as is. I always got away with that.... never a rod problem yet, in many thousands of miles of hard racing. Mind you, this was with Opel and Mitsubishi and Ford small block rods and such. I do have a set of 340 rods that I will put in a 360 at some point, with just new bolts.
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The knurl on the bolt helps locate the cap to the rod. If the cap and rod are not properly aligned, it can create drag, in some cases can cause the crank not to turn or lock up with as little as .001" mis match...

When you replace the rod bolts, the new bolt may be in a slightly different position than the original one that it was machined with...

When you replace the bolts, it's best to have the bore re-machined so it's back to the proper size and alignment...
 
Maybe, just maybe..... you might want to just go with a new set of rods......stock 340 pistons weigh a ton....couple that with any serious rpm and there is a recipe for a spun rod bearing. NOT saying you will, just something to think about on your own......I'm not gonna tell you how to spend your money.......originally I was gonna say just have the big ends checked, if they are in spec, use them as is......the power level you are speculating is not that much of a stressor.

Good luck!!
 
I'll ask the engine builder at the machine shop and see what he thinks when I take it all in. I don't think this engine has ever been rebuilt
 
Richie said:
Something else...You should always have a windage tray right?
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If the engine has not been cracked open before, there should be a windage tray in there, it was one of the "perks" of a 340. If you got one great, if you don't, it's not the end of the world.

Another thought.........if you don't have a tray and you want to do something, go with a crank scraper.
 
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That 70 340 has advertised HP rating of 275 HP BUT the real HP output is more like 325 HP so your not shooting for an extreme HP hike. If I were you I would gamble on the old ones UNLESS you have plans to do some racing of any kind. Racing of any kind means high RPM and greater chance of parts failure.
 
if you are looking for light weight...go with the scat I beam...585 grams...make in china also...LOL

All Molnar Technologies connecting rods are finished here in the US at our factory near Grand Rapids Michigan.....kind of tells ya where they were made also...
 
There's argument for both sides, really.

The stock original rods are GOOD quality metal. You cannot argue that.

No tellin what type metallurgy the new rods are made up of. Especially coming from China.

It would be a tough call for me.......but I would rework the original rods.
 
In forty years of racing..I have had 3 connecting rods break...all stock rods...never had a chineese h beam rod break....two of current 360 in use have them..one assemble back in 2006...and 2008...

dont think the OP is going racing....so anything would work...just the H beams would probably save him money...but...WE all on here like to spend another peoples money..lol
 
Would be a tough call for me as well. Whatever you do , don't get the Eagle I beams. No personal experience but I've heard too many horror stories to consider using them even if they were free.
 
Stock rods are great and never break.....I have a set of seven I could sell you cheap!
 
Richie said:
Hi everybody:

When I get this 70 340 rebuilt to around 350 - 370 hp I was wondering if it would be a good idea to use the stock rods. They all look excellent as does the block and crank.
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the casting number on rods should be 496, last 3. theses are the good stock rods. I retired my set after 1800 runs in my 340, with stock weight pistons and trw lightened to match. these were ran up to 7000 rpm in the traps, 6500 shifted. only work done was new bolts every 5 years,( mopar bolts) ARP wasn't around yet. never broke a 496 rod. BTW, this took 18 years to amass 1800 runs, to resize rods cost $ 90, plus bolts, way less than new rods. nothing wrong with those rods!!!!
 
I have a whole box of stock rods like 27 I think are in there and a shitload of pistons they are all 6.123 rods most will need reconditioning you pay the shipping and give me 20.00 for the plastic container to ship them in and you get the whole box ,rods, pins, pistons the whole shooting match going to estimate 100.00 shipped if cheaper I will send extra cash back . I'm sure theres 8 good 1ns in there but not garoneeing any thing , you jude by pictures
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100_2201.JPG
 
they never break?...LOL
last set was 3 years ago...rods were shotpeened..magged..beams polished...and had ARP Wave lock bolts...5 passes ..had light piston on it too...so yeah..they never never never never break...LOL

40-+ year old rods with how many heat cycles...don't know if they were abused or what in previous life...with a heavy *** piston connected to it....yeah...I made thousands of passes with stock rods until one night going thru the traps...took out the whole side of the engine..and the one above 5 passes....Nope ...not any more..
 
the casting number on rods should be 496, last 3. theses are the good stock rods. I retired my set after 1800 runs in my 340, with stock weight pistons and trw lightened to match. these were ran up to 7000 rpm in the traps, 6500 shifted. only work done was new bolts every 5 years,( mopar bolts) ARP wasn't around yet. never broke a 496 rod. BTW, this took 18 years to amass 1800 runs, to resize rods cost $ 90, plus bolts, way less than new rods. nothing wrong with those rods!!!!
My%20340%20Rod_zpspkaxva5i.jpg



That's one of eight :)
 
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