318 crank into 340? pitfalls

[j.d.duggan]

building a 340 that had very lightend pistons,rods,pins.crank was balanced to this.rods would have to go to .040 over which I don,t like and my mechine shop told me we could go to a 318 crank and rods to make balancing easier,taking weight off instead of adding a lot of it back in.located a 318 that's all there for $75.00.is there any problems with doing this.all the guy could tell me about the 318 is that it was a mid 70's and in really bad shape.what about pillet bushing,will it be drilled? or did some come with and some without?what about harmonic balancer?and the flywheel?any help would be appreciated.

 

[jazak5]

time to do the 340 nice with a crank kitand get new rods its seems you already have some good parts OR get a second opinion (make things easier?= less $$$) from another shop.

 

[MoparMike1974]

It boils down to is the 318 crank forged or cast? Only other difference will be the balance. The dimensions are the same on both cranks. If its a 4 speed car you will need to be sure the pilot bushing hole is drilled. Some were, some not. My 73 340 auto crank was drilled and had the bushing installed from the factory.

 

[j.d.duggan]

time to do the 340 nice with a stroker kitor get new rods

 

[j.d.duggan]

the rods that came out of motor may be reused,not sure yet,but they were realy lightend also.trying to build a mild 325-350 H.P. street cruiser that will run on pump gas (89) because we sometimes get in areas that don't have (91).trying to do this on a mild budget,mabey $2,000 total.

 

[belmateo]

If the 318 crank came from a truck motor, it should be forged.

 

[nm9stheham]

Since 318 cranks started in '68, then the pilot bushing ought to be drilled out. But check, of course. Forged versus cast shouldn't make a difference as, for the 318, both type were designed and balanced to the same bobweight.

Damper and flywheel will be the same. Make sure your damper does not have a cracked rubber ring; it probably will, and, if so, should be replaced. A cracked ring prevents it from doing its job of damping, and the outer ring might let loose and fly off with a lot of damage as the result.

What pistons are you using? Or, have those not been selected yet? And what is the 7 digit casting number on the side of the rods? Those are going to be critical for getting to the balance that you want without adding weight to the 318 crank. (If you can answer these questions, I can run some numbers for you.)

As noted, the 318 crank is balanced for a much lighter 'bobweight' (which is the sum of the weights around 2 of the big ends of the rod, added to 50% of the weight of the 2 pistons, pins, rings, locks, and small end of the rods). Your rods look to be well lightened at the small end on the beam but the big end is the more critical part of the rod weight. Stock 340 bobweight is around 2310 grams, and stock early 318 bobweight is around 2150 grams, + or -. We have used a 273 cast crank in a 340 with light rods and pistons; the 273 uses the same bobweight as the early 318's so it can be done IF the pistons and rods are right.

If you are using pistons like the L2316F forged replacement, it will be impossible to reach the bobweight needed to balance with the 318 crank without adding weight. Something like KB243's is needed. Even then, it will be close.

Other options to help:

• Drill out weight from the 318 crank's end journals like the 340 crank has
• Use lighter piston pins (not free)
• Imbalance the flywheel and use an imbalanced damper like on the 360 or later 340, and then you will be able to take weight off the crank. (But most folks prefer to stick with these parts being balanced.)
• Get a reconditioned crank from Crankshaft Supply in MN; their PN is 19060
• Go with the .040" under rod bearings...The crank strength will still be >95% of the original not it's a real worry IMHO.

 

[JBurch]

If the 318 crank came from a truck motor, it should be forged.

Food for thought:
crank coming from a truck engine does not instantly mean it is forged, I tore down a 318 that came out of a D600 truck, 1971, was a cast crank, not that I cared, just say'in....year (earlier) is more important than source

 

[j.d.duggan]

Since 318 cranks started in '68, then the pilot bushing ought to be drilled out. But check, of course. Forged versus cast shouldn't make a difference as, for the 318, both type were designed and balanced to the same bobweight.

Damper and flywheel will be the same. Make sure your damper does not have a cracked rubber ring; it probably will, and, if so, should be replaced. A cracked ring prevents it from doing its job of damping, and the outer ring might let loose and fly off with a lot of damage as the result.

What pistons are you using? Or, have those not been selected yet? And what is the 7 digit casting number on the side of the rods? Those are going to be critical for getting to the balance that you want without adding weight to the 318 crank. (If you can answer these questions, I can run some numbers for you.)

As noted, the 318 crank is balanced for a much lighter 'bobweight' (which is the sum of the weights around 2 of the big ends of the rod, added to 50% of the weight of the 2 pistons, pins, rings, locks, and small end of the rods). Your rods look to be well lightened at the small end on the beam but the big end is the more critical part of the rod weight. Stock 340 bobweight is around 2310 grams, and stock early 318 bobweight is around 2150 grams, + or -. We have used a 273 cast crank in a 340 with light rods and pistons; the 273 uses the same bobweight as the early 318's so it can be done IF the pistons and rods are right.

If you are using pistons like the L2316F forged replacement, it will be impossible to reach the bobweight needed to balance with the 318 crank without adding weight. Something like KB243's is needed. Even then, it will be close.

Other options to help:

• Drill out weight from the 318 crank's end journals like the 340 crank has
• Use lighter piston pins (not free)
• Imbalance the flywheel and use an imbalanced damper like on the 360 or later 340, and then you will be able to take weight off the crank. (But most folks prefer to stick with these parts being balanced.)
• Get a reconditioned crank from Crankshaft Supply in MN; their PN is 19060
• Go with the .040" under rod bearings...The crank strength will still be >95% of the original not it's a real worry IMHO.

 

[j.d.duggan]

speed pro l-2316f40 is the one we are considering at this point.not sure yet.machinist has another option he will check on after cristmas.so you would not be concerned about the crank having to be turned to .040 on rods? what is imbalance? machinest said he can ad wight to my 340 crank,just thought it might save me some $$$ to go with the 318 crank.but if he has to add weight to that crank I may not come out ahead.

 

[toolmanmike]

speed pro l-2316f40 is the one we are considering at this point.not sure yet.machinist has another option he will check on after cristmas.so you would not be concerned about the crank having to be turned to .040 on rods? what is imbalance? machinest said he can ad wight to my 340 crank,just thought it might save me some $$$ to go with the 318 crank.but if he has to add weight to that crank I may not come out ahead.

I would agree with the machinist if a .040 turn is needed. Probably tough to find bearings that size too. .010 or may be .020 is standard. .030 or more is a bit too much in my opinion. There's lots of cranks around. New and used.

 

[nm9stheham]

What year is your 340? The '68-'71 and early '72 340's had forged cranks, so you could be going from a stronger forged crank to a less strong cast crank from the 318. And it this is a later 340, then the comments on the damper and flywheel change.

I personally would not be concerned with turning the crank down .040".... these cranks, forged and cast, have a reputation of being quite strong and durable. And your build as described is not at a 'gnarly' level of torque and HP, so that makes it even safer to do. The crank journals' cross-section area will stay at 96-97% of original with .040" undersize. And folks turn these cranks down .030" regularly and even .125" if using a 2.000" journal Chevy rod.

Adding weight to the crank tends to be a more-$$ thing to do to a crank when balancing; I've read reports of $75 to $200 to add weight. With the L2316F's, the 318 crank is not going to balance without adding weight. The L2316F's are stock weight, so if you used them and the 340 crank, then any re-balance would be simpler.

 

[nm9stheham]

I would agree with the machinist if a .040 turn is needed. Probably tough to find bearings that size too. .010 or may be .020 is standard. .030 or more is a bit too much in my opinion. There's lots of cranks around. New and used.

FWIW.... even RockAuto lists .040" under size. But yes, lots of crank around to be had. I'd be more worried about the shop undercutting the shoulders' radius on the journals than turning down .040".

 

[crackedback]

Plenty of .040 bearing available. CB481P40 summit has them

That's the last time that crank is going to be used, unless it's ground to SBC size journals.

 

[toolmanmike]

Don't forget heat treat. "Heat Treating
In addition to materials, and casting or forging techniques, heat treating can greatly impact the strength of a crankshaft. Nitriding is the most prevalent method of heat treating used in aftermarket cranks, where ionized nitrogen is vacuum deposited onto the crank surface in an oven. By penetrating .010 to .012 inch into the metal surface and changing the micro-structure of the steel, surface hardness is doubled from 30 to 60 on the Rockwell scale, and fatigue life is increased by 25 percent. The OEs usually favor induction hardening over nitriding, which results in deeper penetration into the metal surface (.050 to .060 inch). This process uses a magnetic field to heat the surface. “There are pros and cons of both methods, but nitriding is most common in the aftermarket,” explains Humphries. “Induction hardening is more localized, whereas nitriding treats the entire crank at once. However, induction hardening penetrates more deeply, which enables turning down the journals once or twice during rebuilds before having to heat treat the crank again.”

 

[Mopar Sam]

Cut the crank .040, and put the money you would spend on a core towards buying light pistons.

 

[j.d.duggan]

keep in mind what I,m trying to do.build a 325-350 H.P. 340 that will run on (89) pump gas.bolted up in front of a 4-speed tranny.$2,000.00-$2500.00 budget.I have the 340 block that has been squared and decked-will bore to 0.40-$200 - ,pistons come out of top .020,thinking that new pistons may need to be milled -turned to lower cr.$80- have a set of 587-j heads that have 2.02 valves, ported ,polished will need to be fresh,nd up and surficed-$350 turn crank-$120,true up my already lightened rods-$80 cam bearings and freeze plugs install $50,balance-$,300 vat and mag heads and block-$100,press new pistons-$80.that puts me at $1360.00 at the machine shop.thats if my valves are all good.then we move on to parts,pistons-speedpro L2316f40-$313,rings $60,bearings & freeze plug set $160, felpro bebuild gaskets-$100,valve springs $50,timing chain cover and water pump $100,considering reusing my timing chain and high val. oil pump?cam and lifters $300 ? I haven't really started checking prices very close because not sure what I need in this area.misc $150 paint,trowout bearing,filters,ect.ect. that puts me at about $1150.00 in parts.$2500.00 or I could sell all this stuff I have and buy a engine already built.any idea what the parts I have might bring??? I see some really high prices out there but I don't know what this stuff will actually sell for in a timely manor.whats you,re thoughts????

 

[Max1196]

What is the casting date of the 340 block?

 

[j.d.duggan]

I think it was 5/26/72

 

[Max1196]

Thanks, personally still looking for a mid July 67 block. For you, does it have to be a 340 or does a build on a 360 thrill you?

 

[j.d.duggan]

I don't know.I'm really a chevy guy,,oh,oh,said the c word.it seems that mopar guys really love these 340's and I thought if I put $$$$ in the 340 if I ever decided to part with this car I might come closer to getting my $$$$ out.I really like the look of these cars and have been looking for one for a long time.but I don't know if I'll even like the thing when I start driving it.think its as much to buy parts for 360,318,so why not build the 340?

 

[Jadaharabi]

Use the crank and rods with new pistons that you have. Just have it balanced. Your going to have more money in a bunch of parts that you don't need when you can use what you already have.
I have .060 over 2316s and they are heavy. Did not have to add any weight with them.

Use a Mopar oriented shop.

 

[nm9stheham]

Just some notes on your costs.... what does 'true up the rods' mean? And the 340 uses pistons with floating pin & retainers, not pressed-in pins, so no need for the press cost, if you assemble them yourself.

With your build date, it is hard say if it is an early or late 340. Does you damper have an obvious imbalance on the front surface, and have the words 'for 340 only' cast into it? That would be the later 340, with the cast crank.

From the factory, the early 340 will be close to 9.75 static CR. For the 89 octane, you probably want to drop the CR to 9.0 or so, IMHO. There are H405CP pistons made for lower compression. RockAuto shows a set for $150-160. And Silvolite has their 1267 cast pistons for lower compression; Northern Auto lists sets in .020", .030" and .040" oversize for $184. Both of these pistons will drop the CR to between 8.7 to 9.0, assuming your block has been decked around .005"

I seriously doubt you will get as good an engine built for the same $$, with the parts right and the machining done reasonably well. I don't see where building a 360 will be any different in price; it is just if you can realize a profit on the 340 parts vs a decent 360 core. And the value of your parts depends a bit on if it is an early or late 340. A 340 in an A body does have a certain added value for the non-engine knowledgeable types who buy these cars so I agree on that point.... you ought to see the slobbering and hear the 'oooohs' when my son mentions a 340 in his '65 'cuda LOL.... even though the 340 did not exist in '65 and the only 340 part left is the block, there is just a mystique around this engine.

Did you ever look up the 7 digit casting number on the sides of the rods? Just curious to see if they are the heavy or light rods.

Sent you a PM.

 

[j.d.duggan]

my crank has been very lightend already,old pistons were ross that had been worked and weighed 430 grams I believe.rods had been worked also.super light pins.so, will have to ad lots of weight back in, but I think it,s the way to go after what most people here are saying.

 

[j.d.duggan]

damper does not say for 340 only.are this h405 cp pistons forged,seems I need to stay with a forged,I will get out to my shop in alittle and ck. but I think the # have been removed with wight removal.