I ran into this year's back and ran into it again. This is the second time I've seen a crank machined on the rods unevenly. I don't have pics but I'll try to explain. This last motor I bought ran good, oil pressure normal, etc but I went ahead and tore it apart because they used the inserts and pins on the rear main and I wanted to do it with right stuff instead. I also wanted to verify all the bearings were good. When I took it apart, all bearings were fine, rods turned 10 under, mains std. BUT....I noticed when they cut the rod journals, they weren't cut centered on the journal. Every one was cut more on the inside, to the point where there wasn't much of the rolled fillet left, but almost all the outside fillet still remained. They were cut well, not out of round as far as I could tell. Almost like the jig was off though. After I thought more about it and analyzed the crank, it looks like they tried to get more stroke out of the crank. I went ahead and used a new crank because I had one here that was fresh from the machine shop. But I'm wondering if anyone else had seen this before? I would have just pitched the crank and figured it was a screw up but since it's the second time I've seen it, figured I'd give it to a buddy that's doing a budget build. What are your thoughts? Screw up on setting the jig or trying to get more stroke?
Odd crank machining twice now
- Thread starter jdpolzin
- Start date
This motor has stock rods, regular 10 under bearings. Nothing out if the ordinary. I will say, we've built quite a few 4.1's with hyper pistons and have issues with a large amount of quench. This particular motor is a hot air block with hypers, maybe the machinist was trying to make up for the quench issue?
I'm just wondering if the hot air blocks with hypers have the same issue with quench as the 4.1's do? Maybe they just went ahead and used the difference in the under cut size and tried to move the journal outward as much as possible? More stroke, less quench.
fastblackracing
Well-Known Member
- Joined
- Apr 7, 2009
Not to mention that offset grinding a crank in the order of .008-.010 is only going to give a minimal displacement gain. Prolly less than 1 inch.
Usually when offset grinding is done the one side of the journal is heavily welded to allow stroke increases of 1/8 inch or more.
Usually when offset grinding is done the one side of the journal is heavily welded to allow stroke increases of 1/8 inch or more.
TexasT
Texas, Where are you from
- Joined
- Sep 10, 2002
This offset is pretty popular for a BBB . You use BBC 6.8 or 7.1 inch rods with a 2.20 crank end and cut the crank like the pick above. This usually yeilds a 470 " engine.
With the v6 they might have been taking up the piston in the bore to get it to zero deck and then they wouldn't have to cut the block if the deck was already straight.
As far as the small engine stuff goes, I know my uncle sends cranks somewhere in Fon du Lac , WI to have them spray welded and recut as some of them are nla or very expensive for nos pieces.
Lots of pretty cool nuances can be accomplished by those who are willing to do the homework . Just goes to show how some cars "just seem to run better than others". There is always a reason. It might not be apparent , but there is a reason. Smokey Yunick and Grumpy Jenkins made a career of it.
With the v6 they might have been taking up the piston in the bore to get it to zero deck and then they wouldn't have to cut the block if the deck was already straight.
As far as the small engine stuff goes, I know my uncle sends cranks somewhere in Fon du Lac , WI to have them spray welded and recut as some of them are nla or very expensive for nos pieces.
Lots of pretty cool nuances can be accomplished by those who are willing to do the homework . Just goes to show how some cars "just seem to run better than others". There is always a reason. It might not be apparent , but there is a reason. Smokey Yunick and Grumpy Jenkins made a career of it.
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The depth of rolled radii in the cast crankshafts is not consistently even with the rod journals - suggesting the rolled radii was present in its finished state prior to the rod journals being indexed and finish ground. As others have suggested, offset grinding when there is only .010 of material to remove won't amount to much of anything, and it's more likely that the rods being ground .010 made it easier to discern the different stroke of the rolled radii as compared to the journals. Back in the day, prior to 3.625 stroke crankshafts, we would take a 3.400 crankshaft and take the rod journal from 2.2495 to 2.100 and use a custom rod to achieve the desired increase in stroke.
