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**Dukes69** · 02-25-04, 08:09 PM

What about a stock 258 Crank and rods? I *think* thats what I was going to do. If I remember, its the same crank, just longer stroke. It might save a little.

**rick** · 03-09-04, 03:19 PM

The crank Chuck is using is a 258 crank. The 258 crank is 3.9" stroke, and the 258 rods are 5.875" long. Stock 4.0L rods are 6.125" long, which makes the piston top higher than the deck at TDC. Thus, you need a custom piston with a higher pin location. Using stock 258 5.875" rods increases the quench hieght to 0.081" (I think 0.051" is stock on a 4.0), resulting in higher emissions, not as much power, higher fuel consumption, and prone to detonation. If the block is milled to restore the quench height to closer to stock, the combustion volume is decreased even further, resulting in close to 10:1 compression, requiring high octane fuel and ECU remapping. Hence, the custom piston which allows use of the longer 4.0 rods. It is possible to increase the combustion chamber volume to help reduce the CR, but this also costs money unless you are into cc-ing heads yourself and using a die grinder very very carefully. Thus, the real cost of the custom pistons is the difference in cost between purchasing stock 4.0 pistons and 258 rods, assuming you have good 4.0 rods to mount the custom pistons on:

Short 258 rods + stock 4.0 piston = $350

Custom pistons + reuse 4.0 rods = $550
Difference = $200

The longer 4.0 rod also results in a higher, more desirable rod ratio. This is not really a major concern in an engine with a redline of ~5,000 rpm, but what works on a race engine just makes a lower rpm engine last longer. Custom JE pistons are $550 a set, possibly lower if I buy more than one set. The machine shop I use would probably not feel good about ordering you pistons though Chuck unless they look at the block. They don't like doing things twice

rick

**cbremer** · 03-11-04, 05:05 PM

what rick said

i'm using a 258 crank, 242 rods, and custom pistons with a higher pin height. i haven't decked the block at all; assuming it was flat when I took it to the machine shop. if it's not, then it'll likely have .010 taken off. I'll likely shave the head (which is also getting rebuilt) a little, then use a thinner than stock head gasket to bump up the CR a bit. I'm looking at ~9.5:1 right now (actually more like 9.45:1

) throw in a mild cam with a little more lift & duration, and viola! that's my new engine.

also getting all new/reconditioned goodies- high volume water & oil pump, distributor, harmonic dampener, H.O. alternator, etc.

good stuff!

**sarah** · 03-12-04, 04:40 PM

rick says don't get a high volume oil pump.
use a stock one!

**Dukes69** · 03-12-04, 04:53 PM

Originally posted by sarah
rick says don't get a high volume oil pump.
use a stock one!

Why?:confused:

**cbremer** · 03-15-04, 01:48 AM

and we're under way...

the color is AMC blue

since I took the pics, i got the crank and cam installed, and discovered that all but one of the mains are slightly bigger than spec, so now i'm waiting on the next undersize bearings

the crank was 'reconditioned' and all journals were turned .010" smaller than factory. that, combined with the hone job at the machine shop, made the journals too big. the service manual says I can mix bearing sizes, so i'm going to use the .010" undersize on top, and .020" in the caps, tightening up clearance by .005", putting them all into spec.... hopefully

pistons are due to arrive on Tuesday, and the head's being rebuilt as we speak! yay

:hat:

**Dukes69** · 03-15-04, 11:59 AM

Originally posted by cbremer
the service manual says I can mix bearing sizes, so i'm going to use the .010" undersize on top, and .020" in the caps, tightening up clearance by .005", putting them all into spec.... hopefully

So the hole is out of round? That doesnt sound good. are you using plasti-gauge to check the clearence?

**cbremer** · 03-15-04, 12:22 PM

Originally posted by Dukes69
So the hole is out of round? That doesnt sound good. are you using plasti-gauge to check the clearence?

yup... used plastigage. i know what you mean about that method leaving the mains out of round... but the manual says it's OK :confused:

i'll try using all .020" bearings- maybe it'll be a little tight, but within spec. at least that way they'll all be round

**Dukes69** · 03-15-04, 12:32 PM

Yeah, I'd try all the same bearings, just to see if they are in spec. I've never heard of using different size bearings top and bottom. Sounds eff'd up to me

:confused:

**rick** · 03-17-04, 09:01 PM

Sounds bad to me also... I always thought bearings were matched to one another.

There was much debate and many posts in several forums a couple years ago when I built my 258 about hi-vol oil pumps and problems with everything from priming to life of the pump. I had a hi-vol pump the first time I rebuilt the 258 and I had priming problems for the first two oil changes. I put a stock pump in it after the second rebuild and then I had priming problems because I had the wrong oil pickup tube, but the pump was fine after correcting that problem.

Nice looking engine Chuck.

**rick** · 03-25-04, 09:53 PM

What bearing clearance are you shooting for in the swapping/mixing of oversizes? If they are all 0.002 - 0.0025, you are sittin in tall cotton... I wouldn't make them tighter. I think factory specs are a ridiculously tight 0.001-0.0015 or something. This is too tight in my opinion. The only difference you will see is in the oil pressure at idle. My mains are 0.0025" on the nose and I run 15-20 psi at idle, then it jumps up immediately off idle to 30-50 depending on rpm and temperature. If the lower oil pressure bothers you, just run 10W-50 oil. I get 20-25 psi at idle with the thick stuff. The larger bearing clearances help protect the bearings under high loads, particularly at low-medium rpm where harmonics and high torque can deform the crank slightly. This is particularly true of a high torque, low rpm six cylinder with inherent vibration issues like a stroker. These engines have a high rotating inertia, so once they are spun up to higher rpms the damper and flywheel actually do very little to smooth things out enough to where the bearing loads are more even, as is the case in most 4 and 8 cylinder engines where the rotating masses are in opposition during most of the crank cycle. This is where tight bearings can eat your lunch. One mishap at higher rpms and that 0.0015" will disappear very quickly and send metal flying. The more oil that flows between the bearing and the journal, the more heat transfer occurs between the metal surfaces. Of course, this is only true up to a point (I'm sure 0.05" clearances are not good), but if 0.0025 is good enough for a big block chevy on nitrous putting down 800 hp, it's good enough for me. Just my $0.02.

**blkTJ** · 03-26-04, 02:12 AM

just one question... what the heck are you guys talking about???:confused: j/k

man, I know so little about building engines.:yay:

**codfish** · 03-26-04, 04:33 PM

that is a nice looking block you have there.. how is progress? did you resolve the bearing issues?:oink:

**cbremer** · 03-26-04, 10:56 PM

.020 undersize main bearings were delivered this week, and i'll be (re) installing the crank over the weekend.

got the head back from the machine shop too

i suppose i'll give the ports a once-over with a Dremel too.

the FSM recommends .002" clearance, but .001-.0025" is acceptable. i was getting >.003" on all the mains (some as high as .004"). this is really all that I was waiting on... now I can really get crackin' on assembly

:hat:

p.s.: I just ordered this beauty:

it's on backorder, but I'm pretty patient :yawn:

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