4 inch stroke crank

[dwg86]

I remember Alex22 on this board was talking about offset grinding a crank to a 4 inch stroke and using chevy 250 6 cyliner H beam rods, 6.000 long used on a 2.000 crank pin. I have been thinking about my next stroker project, and I am going to look into this. The chevy 250 rods are 1.050 wide at big end, so they are .010 narrower than the 4.0 rods. The rod side gap will be a little wide, but I don't think it will be a big problem. The rods can be purchased at cnc motorsports for $375.00. I have a 12 wt crank that was run at .010. so if my figures are right I should be able to offset grind the crank to a 4 inch stroke and use a .030 250 bearing.
4.000 stroke-3.985(258 stroke)=.105 , my crank rod journal is 2.085, so 2.085-2.000(250 journal)=.085. So I need to offset grind .105 to get to a 4.0 stroke, but I only have .085 material to do it. I would need to offset grind .020 more material, so I could get that much from the .030 250 bearing. And that would give me .010 extra material to grind the outside of the crank journal to make sure its all smooth. Is my math correct?

This way no crank welding is required. I am going to talk to my machinist this week to see how much he charges to offset grind a crank.

[Plechtan]

You are correct, that is how it is done. The Chevy 6 cylinder rods can use the V8 bearings, so you can get a performance bearing if you want. Have your machinest put a large radius on the sides of the rod journals, plenty of room for it.

[Cheromaniac]

I remember Alex22 on this board was talking about offset grinding a crank to a 4 inch stroke and using chevy 250 6 cylinder H beam rods, 6.000 long used on a 2.000 crank pin. I have been thinking about my next stroker project, and I am going to look into this. The chevy 250 rods are 1.050 wide at big end, so they are .010 narrower than the 4.0 rods. The rod side gap will be a little wide, but I don't think it will be a big problem. my crank rod journal is 2.085, so 2.085-2.000(250 journal)=.085. So I need to offset grind .105 to get to a 4.0 stroke, but I only have .085 material to do it.

If the rod journals have already been ground 0.010" undersize, their diameter will indeed be 2.085". You could offset grind them by 0.085" to reach the required 2.000" diameter for the Chevy 250 rods, and that would increase the stroke by 0.085" to 3.980". If you overbored the cylinders by +0.060", displacement would be 290.4ci or 4759cc (nearly 4.8L). You'd need pistons with a compression height of ~1.463" to yield zero deck clearance without milling the block deck.
Rod journal width on the 258 crank is 1.073" so the Chevy 250 rods are a bit narrow, but that's a problem that's easy to overcome.

[Plechtan]

Dino:
I think you are missing somthing, he is proposing th grind the journals to 1.970 and use .030 oversize bearings to get a 4" stroke. 2.085 - 1.970 = .115 So the crank grinder will have .015 of material to play with. A .020 oversize bearing could be used, but that you would only give a .005 margin of error.


On a regular crank, with standard 2.095" journals, you could go .060 under offset ground and increase the stroke by .060 assuming you had a standard crank to start with. I like t idea of offset grinding the crank rather then decking the block. In the above example the pistion would sit .030 higher in the hole.

[Cheromaniac]

Dino:
I think you are missing somthing, he is proposing th grind the journals to 1.970 and use .030 oversize bearings to get a 4" stroke. 2.085 - 1.970 = .115 So the crank grinder will have .015 of material to play with. A .020 oversize bearing could be used, but that you would only give a .005 margin of error.

Nah, I understood what he was going after but I suggested a 3.98" stroke so that he could stick to standard size Chevy 250 bearings.

[alexfloresaz]

Hi guys. Old thread. New question. Wouldn't it gain only half in the stroke? Example grind off .060". And the new center will only move .030" thus, pushing the piston up only 0.030". Going from 3.895" to 3.925". If my numbers are correct, To get to 3.980" we would need 0.095" increase, so grind off 0.190", so the new center moves 0.095", pushing the piston up 0.095". And that would mean a rod with a 1.900" to 1.905" journal would be needed. Or really big rod bearings. Please correct me if I am wrong. I am doing the math because I am building my stroker with a mexican VAM 4.6 282 c.i. block that I brought from mexico. It already has the 3.895 stroke, just want to go bigger.

[Cheromaniac]

The Chevy 250 I6 rod journal is 2.000" compared to the 2.095" rod journal diameter of the AMC/Jeep I6. Therefore you could offset grind 0.095" to increase the stroke from 3.895" to 3.943" and use Chevy 250 I6 rod bearings.
A greater degree of offset grinding would require even smaller rod bearings.

[astjp2]

Would any heads flow enough to take advantage of a 4" stroke? Also what pistons would be most appropriate? How much dish to not get detonation or preignition..

[Cheromaniac]

You could do something similar to this:

4.8L stroker (4759cc)

Custom billet 3.98" stroke crank
Jeep 4.0L 6.125" rods
Keith-Black UEM-IC944-060 pistons
10.0:1 CR
CompCams 68-235-4 camshaft
Russ Pottenger ported Edelbrock 60cc aluminum cylinder head
Felpro 0.051" head gasket
0.036" quench height
Custom PCM programming

[astjp2]

Well that would blow any budget away, for what that stroker would cost, I would put in a 5.7 hemi.

[Cheromaniac]

Perhaps this will be more suited to your budget.

4.6L low-buck stroker

Jeep 4.2L 3.895" stroke crank
Jeep 4.0L 6.125" rods
Custom forged +0.030" bore pistons, compression height 1.380", dish volume 18cc
9.6:1 CR
CompCams 68-231-4 206/214 degree camshaft
Russ Pottenger ported HO 2.00"/1.55" 62cc cylinder head
Mopar/Victor 0.043" head gasket
0.043" quench height
Ford 24lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines, Ford 24lb/hr injectors for '96-'04 engines, '99-'00 Chevy LS1 26.2lb/hr injectors for '05-'06 engines
268hp @ 4900rpm, 326lbft @ 3500rpm

[Russ Pottenger]

The minimal performance gains that you would achieve increasing the stroke from a reasonably priced and commonly available 3.895 stroke 4.2/258 crankshaft isn’t worth the time and effort in my opinion.

You’d be further ahead if you put that money in other areas of the build such as the cylinder head and Intake manifold/throttle body.

My .02

[Cheromaniac]

The minimal performance gains that you would achieve increasing the stroke from a reasonably priced and commonly available 3.895 stroke 4.2/258 crankshaft isn’t worth the time and effort in my opinion.

You’d be further ahead if you put that money in other areas of the build such as the cylinder head and Intake manifold/throttle body.

My thoughts exactly. I would also add a header and free-flowing exhaust to the mix.

[Russ Pottenger]

The minimal performance gains that you would achieve increasing the stroke from a reasonably priced and commonly available 3.895 stroke 4.2/258 crankshaft isn’t worth the time and effort in my opinion.

You’d be further ahead if you put that money in other areas of the build such as the cylinder head and Intake manifold/throttle body.

My thoughts exactly. I would also add a header and free-flowing exhaust to the mix.

X2,

Absolutely nothing wrong building custom big stroke engines, But it’s been my experience that 95% of them have overlooked lower-cost upgrades first.

Ok, .... done beating the dead horse.

[astjp2]

I have already done a budget stroker almost 15 years ago, it was a 258 rods and crank, crank was ground .020 offset and block was bored .030. H802CP modified pistons modified by Oleshot. If I was to do it again, I would try to offset it .040 so I would not have to deck the block. I would also use the 5.7 Hemi valves, they are oversize and would not require special valve train. I would't mind doing another one, but if I do, I need to get the PCM reprogrammed. I also do not know what it would take to put a late 90's block and head into an 06 LJ. Tim