I need a crank and have seen ads that claim the crank has been turned and is ready to drop in.
I always thought that a crank that is turned 10 - 10 is not really ground .010", but is really ground X inches where X = the amount of material to achieve about .0015" after resizing the rods, line boring the main journals, and considering the thickness of the 10 over bearing. Therefore, not all .010/.010 cranks are really the same.
Also, if one uses lighter than stock hyperutetictic pistons, then doesn’t the crank need to be balanced?
What do I need to do to be sure a used and freshly turned crank will work for my engine that had been rebuilt once and had the crank, now dead, turned 10 -10 and internally balanced?
I have always been able to save the existing crank - but this one has been turned once and ate to much metal this time to be saved.
For best results, yes, the rotating assembly needs to be balanced. If there isn’t much difference in weight you might still be ok without balancing.
As far as the rest…
A .010"/.010" crank has had .010" removed from the rod and main bearing surfaces. You will then need .010" oversized bearings for each to make up the difference. It sounded like you knew all of this though, so I’m not really clear what the question was.
If there is anything special about your old crank, it could be saved. Any engine shop that works on cranks should be able to build an old journal back up and regrind it.
For some other advice, it really depends on the engine and what its use is going to be. You can buy new cast iron cranks pretty cheaply depending on the application. Naturally, you can also spend a huge pile of money on them.
My engine builder was never impressed with the hypereutetic pistons. He figures that if you want better pistons, buy a real forged piston. If it’s a stock application, save a little money and use plain cast pistons.
Yeah, I cheeped out on the hyper— (sp?) pistons. I had heard on the net that the bad rap the hyper pistons have are due to one manufacturer (Keith Black).
I found a new 4043 forged steel crank for only $550. Well, we will see if they really have’em after the holiday.
I have always been able to have the machine shop turn and balance the existing crank so I have not had to consider the work necessary to make a different, new or used crank, work in an engine that had already been line bored and had rods resized. It seems that even a new crank would need to be turned to confirm to specific clearance based on the bearing used and the resulting size of the refinished rods and mains.
I am trying to determine what it will take to make it work and some of the subtle differences in finishing the crank like what % counter weight balance…
OK, OK I’m here.
I’m not entirely sure what you mean by:
So to forestall any misunderstanding, let me go over this. Warning, I am not an automotive machinist, and I don’t play one on TV. So if I screw this up, please don’t yell too loud.
OK first off for any crank there has to be a running clearance for the oil
Let me pull some spec’s here for one of the engines I teach on
Rod journal diameter 50.00mm
Max out of round or taper 0.004mm
Undersize diameter 49.750mm
Rod bearing seat diameter 53.00mm
Max out of round 0.006mm
Of course they don’t give a dimension for the thickness of the shells, but simple math will tell you it is under1.5mm. (53-50 /2) How much under is the running clearance for the oil.
So when a crank is standard then the bearing surface is the published size. If the crank is listed as 10/10 (usually by a painted M10/R10) on the front counterweight the bearing surfaces will be the published sizes-0.010". Example Rod bearing journal spec 2.500" New standard crank would measure 2.500"
Main bearing journal spec 3.000" New standard crank would measure 3.000"
A 10/10 crank for that engine would measure Rods 2.490", mains 2.990"
IIRC correctly some V-8 can also be turned to .020, but it has been many many years since I hot-rodded American V-8s, so don’t quote me on this.
Now I have to agree with ** RogueRacer** why are you buying a crank? Unless it is in 2 pieces it can probably be repaired for less money than a new unit.
My second question is: Why if you are buying a crank, are you getting a 10/10 crank instead standard size?
As far as balancing goes, all parts of the rotating assembly need to be balanced, crank pistons and rods.
If I were rebuilding an American V-8 (I’m guessing that this is what you are doing)
I would find the best machine shop around,***** and have them:
[ul]
[li]Check the rods for straightness[/li][li]shot peen the rods[/li][li]size the rods[/li][li]rebush the rods, and fit the pins[/li][/ul]
I would then have them repair or if that is not an option, check the size on the new crank to work with my newly refurbished rods. I would ask the machine shop to supply which ever bearing they recommend (or what ever one you like best) they know the size of the crank, and will get you very close. Again assuming 'merican V-8 you will probably be playing with plasticguage for a while to get the oil clearances perfect. I would then get the whole mess balanced (either by the machine shop or by an outside vendor)****** then and only then start the assembly of the engine.
I hope this helps.
*****Nobody but nobody can screw you up faster than a bad machine shop. Ask around, find the guy that does the best work, it is not always the guy that charges the most money. You can skimp in other areas, but don’t skimp on the machine work or you will be sorry. Ask me how I know this. Once you have a machine shop you can trust, ask him these questions, and follow his advice. This is his business, he will be much better at it than I am.
****** We have not spoken about boring the engine, but you must give the pistons to the machine shop before boring, they will mic them and bore to the size of the piston to get the correct piston to wall clearance.
Hi Rick:
I had a detailed reply ready to submit a couple of days ago that was lost as AOHell dropped me for no good reason - again.
I was looking at a .010/.010 crank because it is hard to find a new stock crank and they are lots less that a new forged crank - not needed for the 500 or so HP this engine will see.
What I was trying to say, and my issue with the already turned crank is that the block has certain tolerances and the crank needs to be ground based on how the line bore and rod resizing turns out. So .010" may not be ground off the crank is the line bore consisted of lots of metal removal. The caps may be machined to bring the line bore back to spec, but after subtracting the 10 over bearing thickness, the crank may only need .008 to achieve a tight .0015" clearance.
The block is at the machine shop. After I showed the main bearings to the crank guy, he said even if it looks good (no spun bearings) the front cap may have been beat out of shape and that will kill the new crank. So hopefully it will just need a crank, going for the new forged unit, and some balance work. Since they still have the balance sheet, they don’t even need any other internal parts, just the crank, balancer, and flex plate.
I have never heard a bad thing about this machine shop - but they are only human.