I have almost finished my 340 bored .040 with 4 inch stroke scat cast steel crank . It has the scat ibeam rods, and diamond forged dish pistons. it is internally balanced . the actual compression ratio is 9.8 to 1 , with sidewinder aluminum heads .This is going in my 67 barracuda convertible street car with automatic transmission . The car is for street use , and I do not plan on reving the engine over 6,000 RPM , also it is a relatively light rotating assembly . The question is, will it hold up with stock main cap bolts . I had this professionally done by a machinist that does primarily race engines . Originally I had him assemble it with ARP main studs . He used their lube , and their higher torque spec .He measured the saddles and bearings , and the mains moved with the studs out of round too tight . It measured perfect with the stock bolts, so I told him to assemble it with the bolts , as I did not want to start over and Line bore for the studs . I felt that the motor built mild for my purpose would be fine. Now I am second guessing before i finish, looking for opinions . Thank You , Ron

Plenty good,in my opinion.

I made something like 550 hp with the stock cap bolts(378" W2) so not as much stroke, but 12:1 SCR and a four speed. Never had a bottom end issue. I used a cut stock pan for 2 add'l qts of oil, but it regularly saw 7000 RPM for hundreds of passes and a few thousand street miles. If the machine work and assembly are good I can't foresee you having a problem unless you try and road race it in the future.

they're not little tiny chevy bolts holding the mains in on there, don't worry about em

I've helped many NHRA division 7 stocker racers with SB Mopar's set world records and win NHRA races with stock main bolts in 318, 340 and 360 making from 380 HP up to 435 HP in NHRA blueprint race trim years ago

Thank You Guys for your input , experience , and opinions !! This helps me to feel more confident in my own opinion that the stock main bolts will be fine in this application ! I did have concerns going back to my own previous experiences back in the eighties when I raced a 340 in a Duster . I was running heavy pop up pistons with a 12:5 to 1 compression ratio , huge cam , shifting at 7200 RPM , and crossing the line at 7200. with 4:88 gears . I had a problem with main cap walk on the center main until I used main studs . I was only worried with this street 418 because of the long stroke , but since the rotating assembly is lighter, the compression lower, and rpm much lower, I don't believe it will be under the load that 340 was !

Long rods and light weight pistons as well as a good tune up are your friend to avoid cap walk and detonation

Under high torsonal loads the crank tries to twist itself into an 'S' so you will tend to see the highest wear on the #3 because #2 and #4 main are constantly trying to see-saw between eachother.

You said you're running a cast steel crank so yours safest bet for longevity might be to cam it where the motor doesn't want to rev that high, if the heads can't pump up there the bottom end friction will start to act like a rev limiter.

The other thing is on the street your tires are the safety fuse, you're not really dead loading the motor because you'll break traction before you'll ever twist anything to failure.

I have a 427 ford windsor (4.04" bore and 4.17" stroke with 340 rods) in my cobra replica and that is a torque monster, rarely sees over 5800-6000 but in a 2400 pound car it doesn't need to. I pretty much build all my SB strokers (except my Rocky that spins 6800-7000) that way.

340 block bored .040?

Good thing it's mild because those cylinder walls might be a little thin.

As opposed to the others replying here, mine was a different experience.

These pics show cap-walk in a 360 SB (non-stroker) street engine. Nothing crazy here - stock crank with HP116CP pistons - other than all using ARP fasteners and the block having been machined correctly to true everything up. That was a flat tappet hydraulic cam with high-rev lifters, I regularly spun to 6500 RPM.

So did this cause any damage?

I do not think so, the crank came out looking very nice, no scoring on the journals, although the bearings did show a little wear and those locations coincided with where the cap-walk witness marks showed up.

Because of this I went to main studs on my W2 stroker build. The assembly difference was a 'day & night' kind of a thing: for the lack of better terms, the bottom end - as I was torquing down the studs - literally felt like a SOLID piece of iron! lol

...and here are the matching bearing pics for both the mains and connecting rods


EDIT
====
OH, all this is after about 10 seasons (summary only) mostly street crusing and a few trips to the track! In total I put on about 10K KMs, so about 6K hard miles?

I only use studs when I line hone the block, that is what I prefer but if it's not line honed I just just use high quality ARP bolts.

How much clearances did those rod bearing have originally? What brand, weight and type of oil?
Some of those shiny ones worry me
Same thing on the #1 and 2 main bearings

Ref Sreetwize post . The cam is relatively mild for the 418 . It is a comp Hydraulic Nostalgia Series . 239/246 duration at .050 . Lift 484 . It is rated for 2,500 - 6400 powerband . I figure a little lower with the big motor . Should be just right . Car will always be run on street tires > I was thinking along the same lines you mention .

oh?...they are not ideal for sure, but really that bad you think?

1) MAINS (B - means Block, C - means Cap)
1 - 0.0037
2 - 0.0037
3 - 0.0035
4 - 0.0034
5 - 0.0034

2) RODS (R - means Rod, C - means Cap)
1 - 0.0018
2 - 0.0023
3 - 0.0018
4 - 0.0024
5 - 0.0018
6 - 0.0025
7 - 0.0018
8 - 0.0023

I used a high volume oil pump (not high pressure), and ran 10w30 oil (VR1) the last three seasons and 10w40 Delvac earlier on.

Oil pressure was always good.

The mains were SealedPower 3/4 grove 120M (which had that flat non-zinc finish, so it looks a bit green/brown in the pics, you can see it clearly in #3), and rods were SealedPower 8-2130CP10 (with the zinc plating that still remained on some of them), with the crank being 10/10.

If anything I would say that the rod bearings show more wear on the ROD side of the bearing as opposed to the CAP side, therefore in my amateour way of thinking that'd be a result of not enough oil film during the power stroke??? maybe detonation??? (i'm fishin' here...lol)

I like and use the old rule of .0010+ oil clearances for every 1 inch of crank journal diameter, IE, 2.375 rod journal diameter I like to have .0025+ clearances, main bearing bore size at 2.750 at .0030,+ on all of my engine builds, street and strip or race only
On most rotating parts that have pressure oil feeding them I like to see them a little looser than needed instead of a tiny bit too tight

Intersestingly, Nicks Garage on youtube just tested a 390" 318 with ported factory Iron heads, 10.2:1 with a Performer RPM and a really mild 231/237 @.050 hydraulic flat tappet with factory 340 Manifolds and it made 405 hp at 5400 and 440+ LB/Ft through the middel and I would think his dyno is very conservative (or at least real).

I would think headers alone would put it at least at 460 lb/ft and +430 hp at about 5600. It any event it's a sweet hard pulling street combination, probably enough to put an A body with decent traction in the 11's and a mid 70's D-100 in the 12's.

it's funny, I commented on one of his more recent videos that most people (that actually watch dyno videos on youtube) would really like to know more specifics about the head and cam specs...and (Thank you Nick!) it looks like he took that advice to heart,

In that pic of the cap walk block is that a Heli-coil on the main cap threads? There really should be at least a solid insert like a Time-sert.
I could be wrong but the one on the left looks like it's been repaired.

Gus

sure looks like a heli-coil to me.
i have only installed thousands upon thousands in my working life, and i agree, a time-sert or keen-sert would have been the better repair.

Many say its ok but id question that on a performance build. Sort tells on itself since that the worst of the main caps. However he is pretty well informed and at the end of the day his problem if something goes amiss. Sorta like the stranger who says "Im gonna drive my car into the brick wall there"... Im like ok, I will watch.

What do the mains on the Al. blocks have?