4.250 crank factory 340 block

-
72Dart360

72Dart360

Well-Known Member
Joined
Aug 7, 2004
Messages
131
Reaction score
4
Location
Mechanicsville, MD
I got a 71 340 block that's bored .040 over, the machine shop did a sonic test and said it has .275 in the thrust and that was good. I can't afford an R3 block yet, but I want to build a stroker using a 4.250 crank. So, my question is Can a factory 340 handle a big stroke using 2 bolt mains? Engine will see 6k rpm max.
 
72Dart360 said:
I got a 71 340 block that's bored .040 over, the machine shop did a sonic test and said it has .275 in the thrust and that was good. I can't afford an R3 block yet, but I want to build a stroker using a 4.250 crank. So, my question is Can a factory 340 handle a big stroke using 2 bolt mains? Engine will see 6k rpm max.
Click to expand...

6K? Bah! That's nothin'. I shifted my stock-stroke 340 at 7,200 with 2 bolt mains.
 
72Dart360 said:
I got a 71 340 block that's bored .040 over, the machine shop did a sonic test and said it has .275 in the thrust and that was good. I can't afford an R3 block yet, but I want to build a stroker using a 4.250 crank. So, my question is Can a factory 340 handle a big stroke using 2 bolt mains? Engine will see 6k rpm max.
Click to expand...

No!!!!!! I'm having to redo my short block because the original builder said I didn't need them a girdle or anything else. Do it right the first time and spring for some four bolt caps.

No need to spin it past 6k. Mine was faster short shifting at 5800 than 6 or above
 
why go 4.25" ? too much extra friction after the exhaust valves open, revs like a pig. when car craft built 6 similar engines, the 440,460,454, and the 3 455's, the 440 had the most torque and tied for most horsepower with the shortest stroke and least cubes.
 
6pkScamp said:
No!!!!!! I'm having to redo my short block because the original builder said I didn't need them a girdle or anything else. Do it right the first time and spring for some four bolt caps.

No need to spin it past 6k. Mine was faster short shifting at 5800 than 6 or above
Click to expand...

What happened to yours? Why do you 'need' 4 bolt caps?

Retrofitting 4 bolt caps to a 2 bolt block is probably not worth the time or money and will never be as strong as a block designed with them already. Think about all the machine work involved and you're only getting an extra angled bolt on either side. If you need that much strength in a block, just save your coin for an aftermarket block.

To the O.P., what are the performance goals? What type of application? Street? Drag race? Power adder?
 
4.25" crank is pretty cool....around 440 cubes with 4.080" bore diameter. I don´t know about the space to the oil pump pickup with such a big stroke - maybe must be moved to the cover of the pump?

If it´s done below 6000 rpm or not depends on the specific setup....with the right cam, intake and cylinder heads i see no problem such a motor making power past 6500 rpm...

Ramcharger is right, application is important to make suggestions.

Michael
 
The application is bracket racing while driving to and from the track. That is what I do now with a 360 with stock cast crank and flat top pistons. Best time on 100 octane is 11.10 at 115 mph. Last year I ran 87 octane with methanol & water 11.15 at 117 mph.
 
That's pretty thick in The thrust side, if he checked 12 or 16 spots per cyl you have a nice block. You may have to modify the oil pickup, I would get the billit 2 bolt main caps align home and use arp studs should be fine with no need for a girdle. You can always go 4.125 crank and only bore it .020 for a displacement of 426.
 
Marcohotrod said:
why go 4.25" ? too much extra friction after the exhaust valves open, revs like a pig. when car craft built 6 similar engines, the 440,460,454, and the 3 455's, the 440 had the most torque and tied for most horsepower with the shortest stroke and least cubes.
Click to expand...

I'm no expert but my 442 was quite snappy as far as throttle response and was an absolute blast to drive. It is a torque monster. 1500 rpm in 2nd gear, mat it and it blows the tires off.
10.79 @ 123 the first time to the track with minimal tuning. I was very happy. I'm sure there's more in it.
 
rmchrgr said:
What happened to yours? Why do you 'need' 4 bolt caps?

Retrofitting 4 bolt caps to a 2 bolt block is probably not worth the time or money and will never be as strong as a block designed with them already. Think about all the machine work involved and you're only getting an extra angled bolt on either side. If you need that much strength in a block, just save your coin for an aftermarket block.

To the O.P., what are the performance goals? What type of application? Street? Drag race? Power adder?
Click to expand...

The stock caps were not strong enough to keep the crank secure. I personally didn't know what was going on but when pulled the caps I knew there was something not right with the wear on the bearings. I took it to a machinist that's a Loves his mopars. The motor only had 1000 miles on it.
 
My street/strip 408 was close to the 600hp range, I ran it for six years before pulling it to freshen. It showed signs of block flex in the main and cam bearings, IMHO a girdle would not help. Also four bolt mains in a stock block will not help block flex either, cap walk, maybe. I'd save my pennies for a good block, my ¢02.
 
Build it and bend the floorboard with the gas pedal till it breaks. Be savin up for an R3 in the meantime.
 
Marcohotrod said:
why go 4.25" ? too much extra friction after the exhaust valves open, revs like a pig. when car craft built 6 similar engines, the 440,460,454, and the 3 455's, the 440 had the most torque and tied for most horsepower with the shortest stroke and least cubes.
Click to expand...
LOL, great joke. I take it when you built your 4.25 stroker you took measures to fix the issues?
72Dart360 said:
Is there any extra stress on the block using a longer stroke?
Click to expand...
Yes, but it is the amount your introducing in the form of extra torque and HP of the build. You could build a very mild (AKA Turd) 350HP 4.25 stroker with minimal stress.
6pkScamp said:
The stock caps were not strong enough to keep the crank secure. I personally didn't know what was going on but when pulled the caps I knew there was something not right with the wear on the bearings. I took it to a machinist that's a Loves his mopars. The motor only had 1000 miles on it.
Click to expand...
Doesn't Program make caps for small block MoPars?
 
rumblefish360 said:
LOL, great joke. I take it when you built your 4.25 stroker you took measures to fix the issues?

Yes, but it is the amount your introducing in the form of extra torque and HP of the build. You could build a very mild (AKA Turd) 350HP 4.25 stroker with minimal stress.

Doesn't Program make caps for small block MoPars?
Click to expand...

You bet, that's what's being installed
 
justinp61 said:
My street/strip 408 was close to the 600hp range, I ran it for six years before pulling it to freshen. It showed signs of block flex in the main and cam bearings, IMHO a girdle would not help. Also four bolt mains in a stock block will not help block flex either, cap walk, maybe. I'd save my pennies for a good block, my ¢02.
Click to expand...

About 550 to 600HP is what I have always heard is the limit on what a SB mopar can handle. Over 600 chances of it living long drop off fast if you use all that HP they way it should be.
 
6pkScamp said:
The stock caps were not strong enough to keep the crank secure. I personally didn't know what was going on but when pulled the caps I knew there was something not right with the wear on the bearings. I took it to a machinist that's a Loves his mopars. The motor only had 1000 miles on it.
Click to expand...

Does not sound like a block/cap/strength issue, more like machining/line hone/balance issue. Could even be an oiling issue or something totally unrelated like a misfire or detonation. "Something not right with the wear on the bearings" seems pretty vague and doesn't make you think "oh, this thing needs 4 bolt caps". I could be wrong though, it's happened before.

But, you really don't read about guys using factory blocks with lower HP (under 600 let's say) with those types of issues unless their junk was put together poorly or worn out.

Everyone wants to know the limits of the factory parts because they 'can't afford' an R block. But when you factor in all he extra cost to 'beef up'/band-aid said factory parts, you could have just bought the good stuff to begin with. Again, to the O.P., if you are making enough steam, step up to an aftermarket block and build everything around that.

Or like RRR said, build it, run it and see if it blows up. Then we can all read the story and ooh and ahhh at the carnage pics.
 
I'm more impressed he caught it. Carnage is only nice when it's not yours...
 
moper said:
I'm more impressed he caught it. Carnage is only nice when it's not yours...
Click to expand...

I wonder what the symptoms were and why it was pulled apart? I'd like to hear the explanation of how installing 4 bolt main caps will solve uneven bearing wear. Sorry O.P. to sidetrack here, just curious. Maybe it will be pertinent.

And yeah, carnage sucks if it's its yours but this is the internet, we want blood.
 
Fixed:
"And yeah, carnage sucks if it's its yours but this is [NASCAR], we want blood."
 
rmchrgr said:
I wonder what the symptoms were and why it was pulled apart? I'd like to hear the explanation of how installing 4 bolt main caps will solve uneven bearing wear. Sorry O.P. to sidetrack here, just curious. Maybe it will be pertinent.

And yeah, carnage sucks if it's its yours but this is the internet, we want blood.
Click to expand...

I apologize, I had explained the whole sorted story to moper in a pm. The build has seemed to be cursed from the start. Long story short I pulled the pan to repair a rear main seal leak the the original builder failed to fix in two previous attempts. After pulling it apart I see why. There was no crunch at the ends of the seal. The seals didn't extend past the surface of the block or the cap. A gob of silicone was his fix. Anyway while the pan was off I decided to pull a few caps and see how things were. That's when I saw in my words striped bearing surfaces. Like a candy cane in a way. That's when I loaded it up and took it to a different shop. I'm basically starting from scatch. Cylinders are out of square and out of round. The registers on the caps were loose had the wrong style bearing. The list goes on. Thanks to the previous shop I now have wasted a lot of coin and I can't afford $2500 for another block plus the new machine work needed. Just tryin to get what I have back together.
 
6pkScamp said:
I apologize, I had explained the whole sorted story to moper in a pm. The build has seemed to be cursed from the start. Long story short I pulled the pan to repair a rear main seal leak the the original builder failed to fix in two previous attempts. After pulling it apart I see why. There was no crunch at the ends of the seal. The seals didn't extend past the surface of the block or the cap. A gob of silicone was his fix. Anyway while the pan was off I decided to pull a few caps and see how things were. That's when I saw in my words striped bearing surfaces. Like a candy cane in a way. That's when I loaded it up and took it to a different shop. I'm basically starting from scatch. Cylinders are out of square and out of round. The registers on the caps were loose had the wrong style bearing. The list goes on. Thanks to the previous shop I now have wasted a lot of coin and I can't afford $2500 for another block plus the new machine work needed. Just tryin to get what I have back together.
Click to expand...

Yep sounds like things were poorly assembled. I agree that what you found was alarming and it's good you took it to another shop. I still don't see how 4 bolt caps will solve your issues though.

Good luck with it, let us know how it turns out.
 
You must log in or register to reply here.
-

Back
Top