Offset crankshaft grinding

[dwg86]

I'm almost affraid to post this. With all the bashing that has been going on here lately. If it turns into a big bitch session, Muad'Dib please lock it.
I talked to the machinist I use for my engine work. He has been in business since 1976. He builds race engines and will machine anything. He has gotten engines sent to him from all over the country. He has shown me a strange looking engine from the early 1900's that was sent to him because nobody else would touch it. The head was cast as part of the engine block and didn't come off. I am saying all of this before someone tells me I am a moron and I don't know what I am doing (I'm not even going to defend my cridentials). I trust Tommy (my machinist) and his opinion. With all that said, here it goes...

Tommy has a crank welding machine, so I asked him about welding and offset grinding a cast 258 crankshaft. He said he wouldn't do it. He said he has seen to many CAST cranks fail because of welding. So I mentioned the chevy 6 cylinder rods. They are 6 inches long and use a 2.000 crank pin. With his professional opinion and over 30 years experience, he saw no reason why the crankshaft couldn't be offset ground and use the chevy rods. I also let him know that the rods were .010 thinner than the Jeep rods(1.050 vs 1.060). He said that it might bleed off a little oil pressure(less than 5 psi), but would not be a problem. So as I am gathering information, and looking at specs. This will be the direction I go in, in building my next stroker. It will be a longer stroke than the stock 3.895, but I'm not sure what the final stroke will be. Price for offset grinding a 6 rod, 7 main crank is $275.00. It will ba a while before I start the next engine. I just thought I would share some info...

[Cheromaniac]

Theoretically it IS possible to offset grind the rod journals of the 258 crank to use the Chevy 250 6" rods, but I'd be concerned about the rod side clearance as well as weakening the rod journals by making them smaller in diameter. It's a gamble that could pay off or end in disaster, and I'm sure you're already aware of that.

[Biggrnjeep97]

I have heard oodles of talk on offest grinding these cranks. Id like to see you pull through and get a running motor on one. Its my understanding (as told to me by my builder) that welding on anything cast, affects the base metal in a way to make it inconsistant and brittle, thus not recommended. Its cool to hear about your engine builder. He sounds like a spitting image of mine. Post up your math and specs as you go....if you go this route.

-Will

[lafrad]

I have a slight-offset grind crank using the 0.040" under bearings. Pushed the piston up on the hole 0.012" (0.023" more stroke overall), just enough so a "cleanup" cut on the head for a good mating surface set the quench to about 0.045".

Its been started and cam breakin has been done, but I haven't gotten it on the road yet... still getting the front clip back together.

[dwg86]

Theoretically it IS possible to offset grind the rod journals of the 258 crank to use the Chevy 250 6" rods, but I'd be concerned about the rod side clearance as well as weakening the rod journals by making them smaller in diameter. It's a gamble that could pay off or end in disaster, and I'm sure you're already aware of that.

Yeah, I know I could be building a granade . I know that offset grinding a cast crank is a lot different than offset grinding a forged or billet crank, strength wise. One of the things I am researching is "crankshaft journal overlap". Crank overlap is the portions of the rod and main journals that overlap each other. The stock 258 crank has .350 crank overlap (main dia + rod dia - stroke)/2). I don't know if there is a minimum amount of crankshaft overlap that needs to be concidered when offset grinding a crank.

Also, using the 6.00 rod on a 4 inch stroke crank gives a 1.50 rod ratio. That would be the same rod ratio as the stock 258 crank and rods. I know using a longer would would benefit from less side loading the piston, but that is the only rod that has a big end wide enough to use on the 258 rod journal.

[YJ_and_Corey]

Grinding a 258 crank to run Chevy rods is a bad idea.

Why?

The journals in a crank are where all the strength is. Simple answer.

[Cheromaniac]

Grinding a 258 crank to run Chevy rods is a bad idea.
Why?
The journals in a crank are where all the strength is. Simple answer.

If he offset ground the crank to use Eagle 6.15" rods instead, the journals would need to be welded on the opposite side to restore the original 2.09" diameter. Do you think that would take care of the strength issue or should he not bother? Otherwise it's a choice between a standard 258 crank (cheap) or a custom billet crank costing an arm and a leg.

[lafrad]

do NOT weld a crank.
the strength is all had on the INSIDE.. the overlap of the rod and main journals, and the radius of the journal's grind. offset grinding smaller is stronger than welding up the outside, which causes internal stress on all of the things I have mentioned.

ONLY weld it if you are willing to spend all the money to do it "properly" which... I think is more than the cost of a new billet crank to start with. LOL

[YJ_and_Corey]

do NOT weld a crank.
the strength is all had on the INSIDE.. the overlap of the rod and main journals, and the radius of the journal's grind. offset grinding smaller is stronger than welding up the outside, which causes internal stress on all of the things I have mentioned.

ONLY weld it if you are willing to spend all the money to do it "properly" which... I think is more than the cost of a new billet crank to start with. LOL

EXACTLY, if you desire that extra stroke that badly - spend the dough and call SCAT and get that billet crank started

[dwg86]

I'm still looking into journal overlap(how much is safe for a cast crank). With the 1.050 width rods, I can maybe run a larger radius to keep some strength there. I really don't think there is going to be a big issue in doing this. I am not building an 8000 rpm engine. With the 12 wt crank, the counter weights should help balancing out some of the tortional vibrations. Hopefully the rods and pistons will be lighter than stock(haven't looked into that yet), and that will help take some load off the crank. I only wish I could get a longer off the shelf rod.

[lafrad]

the 12CW crank is definately a better candidate for a "severe" offset grind... the extra counterweights reduce almost all of the worst cranksaft vibration stress.

With a better journal radius, and the knowledge that stock 4.2 cranks can take turbos/blowers/nitrous... I would expect that a reasonable compression N/A motor would be JUST FINE on the Chevy rods and an offset grind... NO WELDING.

[dwg86]

Yes I agree, no welding

[Plechtan]

Grinding a 258 crank to run Chevy rods is a bad idea.

Why?

The journals in a crank are where all the strength is. Simple answer.

I guess those Nascar guys shouldn't be running those Honda Journals and rods. Many of the turbocharged imports have more HP than the strokers we build and have smaller rod journals. Is bigger better? sure Well unless you are looking to save weight, and have less friction. That is why the Nascar guys go with the small rods. I aggree that grinding a crank offset for 2" rods will make it weaker, but i have never seen a broken Jeep Crank. If someone has one, could they pleas post and tell us how it happend.

All true 5.0L strokers have either a offset ground crank or a billet crank. Since a billet crank cost $3,500 or more, most of them are using an offset ground crank.

[lafrad]

Grinding a 258 crank to run Chevy rods is a bad idea.

Why?

The journals in a crank are where all the strength is. Simple answer.

I guess those Nascar guys shouldn't be running those Honda Journals and rods. Many of the turbocharged imports have more HP than the strokers we build and have smaller rod journals. Is bigger better? sure Well unless you are looking to save weight, and have less friction. That is why the Nascar guys go with the small rods. I aggree that grinding a crank offset for 2" rods will make it weaker, but i have never seen a broken Jeep Crank. If someone has one, could they pleas post and tell us how it happend.

All true 5.0L strokers have either a offset ground crank or a billet crank. Since a billet crank cost $3,500 or more, most of them are using an offset ground crank.

Agreed. I don't think you will have any trouble with a NON WELDED offset ground crank.

[Cheromaniac]

So we all agree that:

1. Welding the crank journals is a bad idea.
2. Offset grinding the crank journals will weaken them but the risk of failure will be reduced if the crank has 12cwts to dampen the harmonics and if lighter rods/pistons are used.

The crank could also be cryogenically treated to add more strength and it only costs about $100.