Grinding a stroker crank

[Plechtan]

I'm new to this, and had a question on stroker cranks, I get most of this, but i do not understand how you can take a 4.2 crank and grind the pins off center and get a stroke of 4.00" or more. If i took a stock crank and ground the pin .040 off center ( the largest undersize bearing i could find) , that would give me about 3.955 The way i figure it, you would have 3.875" + .080 = 3.955. and that would assume a stock size crank to start with. do people use undersize rods?

Pete

[dwg86]

You do not grind the 4.2 crank pins off center. The 4.2 crank has a longer stoke. You just install it in the 4.0 block. The only crankshaft grinding that needs to be done, would to clean up scored journals and install oversize bearings.

[seanyb505]

Not sure on how the journals are changed to get a longer stroke, but if you clean off the journals and take .040 off, and then use .040 oversized bearings, you will still have the same stroke. If you were able to use the same thickness as stock bearings with a small connecting rod you could take advantage of a longer stroke. With oversized bearings that possibility is eliminated. Im also curious how offset cranks are produced...

[kris]

Offset grinding requires a reduction in journal size for that increase in stroke, and an appropriate rod to match the journal.
Small block Chevy and Honda rods are the most popular, but youd need to check alot of specs (especially the width on the large end) before commiting to it.


k.

[dwg86]

Most V8 rods are narrower than the jeep 6 rod. So that would require welding the sides of the journals, and custom pistons to fit your application.

[seanyb505]

Gosh that sounds like fun

[Plechtan]

Not sure on how the journals are changed to get a longer stroke, but if you clean off the journals and take .040 off, and then use .040 oversized bearings, you will still have the same stroke. If you were able to use the same thickness as stock bearings with a small connecting rod you could take advantage of a longer stroke. With oversized bearings that possibility is eliminated. Im also curious how offset cranks are produced...

My understanding is that you take almost nothing of the "top" side of the( rod) journal, and .040 off the the bottom. This assumes that the crank is almost new size. Using custom rods with an offset ground crank looks like the only way to get a longer stroke, but what rod bearings would you use?

Pete

[dwg86]

The bearing for the rod that you use...If you use a sbc rod you would use the bearing for the sbc rod. The crank pin would be ground the size of a sbc pin, just ground offset for a longer stroke.

[Plechtan]

I have a question as to how this might relate to using the KB944 pistons, it looks llke the pin hight on the 944 is about .025 less than a stock piston, so if you offset ground the crank correctly, it looks like you could get a stock Quench hight without milling the block, correct?

Pete

[Alex22]

Its typically cheaper to deck the block .025 than to offset grind and index a crankshaft. I'm still looking for another 4.2 12 counterweight crank so I can have it offset ground .010 under to give me a 4 inch stroke and use chevy straight 6 rods.
And to offset grind a crank you were correct by saying that most of the material would be removed from the inside edge of the rod journal while barley removing any from the outside edge.

~Alex

[Plechtan]

Are you using standard chevy 6 rods? I believe they are designed to run on a 2" journal, where the Jeep rod runs on a 2.1". I am not familiar with the standard chevy 6 rods, would these be custom made or off the shelf? What motor would they have been used in? What pistons would you use with them?

Thanks
Pete

[Cheromaniac]

I'm new to this, and had a question on stroker cranks, I get most of this, but i do not understand how you can take a 4.2 crank and grind the pins off center and get a stroke of 4.00" or more. If i took a stock crank and ground the pin .040 off center ( the largest undersize bearing i could find) , that would give me about 3.955 The way i figure it, you would have 3.875" + .080 = 3.955. and that would assume a stock size crank to start with. do people use undersize rods?

Pete

There seems to be some confusion here so let me try to clear it up.

Stock 4.0 crank has a stroke of 3.413"
Stock 258 crank has a stroke of 3.895"

To increase the stroke of either crank, you need to offset-grind the rod journals. The inside of the rod journal is ground down while material is welded on the outside so that the same rod journal diameter is maintained (and you can use the same stock rod bearings that are common to both the 4.0 and 258). If you offset-grind the crank, you'll need custom pistons.
The KB944 pistons are ~0.25" shorter than the stock 4.0 pistons and are intended for use with the 4.0 rods in a stroker that has a standard (not offset-ground) 258 crank. You could offset-grind the 258 crank to a stroke of 3.955" and have zero deck clearance with those pistons without milling the block deck. If you go for +0.060" pistons, this would yield a displacement of 288.6ci or 4729cc.

[Alex22]

Are you using standard chevy 6 rods? I believe they are designed to run on a 2" journal, where the Jeep rod runs on a 2.1". I am not familiar with the standard chevy 6 rods, would these be custom made or off the shelf? What motor would they have been used in? What pistons would you use with them?

Thanks
Pete

Correct, the rod journal is about .100 smaller on the Chevy 6. In order to use them with a 4.2 crankshaft the 4.2 crank will need its rod journals ground to 2.000 or 1.990. If you offset the crank before grinding it you can have a stroke of 4 inches. That combined with a .020 overbore will give about 4.7L displacement. I already had my first 4.2 crank ground .010 and nitrided so i'm looking for another virgin 4.2 crank to have reground and nitrided for this purpose. The great part about using a Chevy rod is the fact that SCAT makes a Forged H-beam rod with a 6inch center to center and it uses a .927 wrist pin (jeep pin: .931) and it is full floating. I am going to be using a custom forged piston most likely from Diamond or JE/SRP. I'm not sure how the clearance will be for the pan or girdle but the H-beam rods generally have a thinner cap and use rod bolts so there should be plenty.

~Alex

[Alex22]

Since its too late to edit the last post...

Welding and offset grinding of a stock cast crankshaft That's a real big hand grenade tossed in your crank case.

~Alex

[seanyb505]

So then which crankshafts are good to offset grind?