offset grinding cranks

[amcinstaller]

i havent seen much mention about this on here, only a couple of times. i also seen it mentioned towards the high buck end on dinos site. so far i have:

- a 12cwt crank (ground but not yet polished),
- a 93 block and head (block hot tanked and magnafluxed, head not even stripped yet)
- a set of off the shelf 4.0 pistons, "hypericantspellits" (sorry, had to) that i bought before i knew about the KBs, in 3.905 size.
- a a full set of each flavour of rods, 4.0 and 4.2 (both sets hot tanked and mag'd too)

so my question is, can i offest grind my crank (depending on journal meat) to try to "square up my motor" and if i was to have this done, would i see any real gains? from what i have read, if i offset grind, ill need to use the shorter rods to keep the piston in the block at the top. am i understanding this correctly?

[Plechtan]

If you offset grind the crank, you usually take all the material from one side of the journal, and nothing from the other. If you have a standard crank, and offset ground it .040 undersize, you would increase the stroke by .040 The center Line of the rod would move up .020 at the top of the stroke and down .020 at the bottom of the stroke, so .020 + .020 = .040 You would have to use .040 oversize rod bearings of course.

You could do a mild stroke and just increase your compression ratio, or maybe use a thicker head gasket. I think the KB944 pistons sit about .010 low on a standard block, so a .020 stroke increase here would work great. If you want to stroke more than .050 you probably have to go to special rods with smaller ends on them. 2.0" instead of the Jeep standard 2.1" This would let you stroke the motor .100 or more. This is how you get a 4" stroke out of a 258 crank.

[amcinstaller]

thanks, some good info there. BUT, i dont have the KB's. i already have different ones, and id like to use them.
anyway, if im reading this right, id need to have the machine shop increase my stroke by .010, or by taking off .010 on one side (the inside) of the journal. is this even going to make a difference in my build? what if i had them take off a little more, like .020 or .030? will i need to worry about how far the pistons come out of the block at the bottom?

this is going to be a racing engine, not quite as balls to the walls as some but id like to see the mythical 300hp.

for reference, here are my pistons. not silvolite or anything special i dont think.

[Plechtan]

Look at stroker basics, you need to do some math. you should find all the informatin thaer to calculate your compression ratio and deck height.

[Mgardiner1]

To grind the crank to a 4" stroke, you can't JUST grind off the inside of the journal and keep getting a thcker bearing to make up for it. The journal needs to be welded on the top and ground all around to keep the diameter of the journal the same, but would relocate the centerline

[amcinstaller]

all i wanted to know was if this would be a process worth looking into for my build. i never said anything about bringing it to a full 4" stroke. im only wondering if i would see gains grinding it .030 or less.

[Mgardiner1]

3.875 bore
3.895 stroke

275.6 C.I. or 4.52 liters


3.875 bore
3.955 stroke (.030 offset grind = .060 total stroke)

279.8 C.I or 4.59 liters

I guess it depends more on the cost to do it. If it were to cost the same amount for the machine work, and you could still use standard bearings, then go for it. But if the machining was double, then to me it wouldn't be worth it

[amcinstaller]

thanks, thats more the answer i was looking for. i know the process, just wasnt sure of any gains.

oh, as well, what about how far the piston will come out of the bottom of the bore, is this somethin i should be concerned with as well? after seeing oletshots pictures of rod comparisons with stock style pistons and the two different rods (i have stock style pistons) it doesnt look like much difference. what about with the added stroke? worth worrying about?

[Mgardiner1]

My personal opinion is that there is enough piston in the hole that .030 farther out will not have any adverse effects. .030 is the thickness of about 8 pieces of paper (20lbs copier paper )

[5-90]

To grind the crank to a 4" stroke, you can't JUST grind off the inside of the journal and keep getting a thcker bearing to make up for it. The journal needs to be welded on the top and ground all around to keep the diameter of the journal the same, but would relocate the centerline

Depends. There are two ways to offset grind...

For small changes, and weld-and-grind usually works best. This would be good for use with the OEM rods and pistons (within limits of clearance between the big end and the cam, and the piston and head,) and is probably a bit cheaper.

For larger changes, you go with a straight grind. Instead of adding material to move the centre of the crankpin and keeping it the same size, you reduce the size of the crankpin - then go with a production rod that will give you the measurements you need, or get some "one-off" pieces made (spendy!) to make things work.

Note that either approach may require a modification to production pistons - mainly, alteration of the compression height (the vertical distance between the gudgeon pin centreline and the piston static deck line.)

Either one can be spendy - and the more you want to change things, the more it's going to cost!