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Old 04-17-16, 03:14AM   #1
TA63
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Default Crankshaft prep

Hello, you guys have any tips and tricks for improving the durability of our stock cast cranks in performance motors? I'd like to put together a 400 that will live happily seeing 6k or slightly more on occasion.
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Old 04-17-16, 11:35AM   #2
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Would not worry
Just as long as you have a competent machinist grinding and balancing it.
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Old 04-17-16, 01:50PM   #3
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Quote:
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Would not worry
Just as long as you have a competent machinist grinding and balancing it.
Exactly, and make sure it magged good.. plus take into consideration life history of that crank, how it's been stored was it ever dropped did it see any detonation.
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Old 04-18-16, 10:47PM   #4
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A good radius in the fillets is mandatory. The BBC rod mod is a good thing. Using Icon 428 pistons and shortening the stroke up about .050, the 6.8 rod is just right. Be sure the grinder understands "radii" AND offset-grinding.

The oil chamfer is also a good place to "help". A leading trough on the main and a trailing one on the rod does the trick. No sharp edges.

Jim
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Old 04-19-16, 12:30PM   #5
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U are talking a 400 std engine with hp upgrades right ?
What kinda hp ?

I always have the block line honed to middle spec as the Pontiac block has a tendency to move around in the #2 webbing. Make sure u have .015 side to side in the rods. radius the oil holes in the crank. More info can be posted if more info is supplied.
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Old 04-20-16, 12:25AM   #6
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Make sure the thrust bearing surface on the crankshaft is still usable/does not need repair.
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Old 04-20-16, 01:25AM   #7
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U are talking a 400 std engine with hp upgrades right ?
What kinda hp ?

I always have the block line honed to middle spec as the Pontiac block has a tendency to move around in the #2 webbing. Make sure u have .015 side to side in the rods. radius the oil holes in the crank. More info can be posted if more info is supplied.
Eventually if I find the correct block I would build the lower end to handle 450 hp plus with a set of aluminum heads from SD performance, a roller cam in the mid 230's at .050, and 10.3-1 compression. Short term goal would be 350 or so with ported iron heads, crower 221/229 cam, and a compression just below 9-1.
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Old 10-08-17, 07:01PM   #8
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Have my Scat custom crankshaft form they requested I fill out since I'm paying for a forged 4.0 stroke 400 crank with a custom rod journal. The rod journal is being downsized and offset to increase stroke with a -.020 BBC rod journal size . The diameter of the rod journal I spec'd @ 2.179" for a clearance of .0023-.0025 with the rod bearings that have already been calico'd and professionally measured. I had also spec'd an offset of plus .010 for a stroke of 4.020 as I thought thats all that would be available from a 2.20 rod journal and I wanted strong even fillets. My block has been decked and measured to 10.220.

Scat told me after putting down my deposit they only had a raw forging so I asked them if I might be able to get the additional .010 I had wanted for a 4.040 stroke and zero deck height with my 6.900 rod and 1.300ch piston. They said "maybe" and sent me this form. It specifically asks for rod width and journal width I had professionally measured, rods are .994 so desired journal width of 2.008-2.010 for a .020-.022 side clearance was advised. Im using CB-743HN -.020 rod bearings (FM 113M-std mains).

Was curious about thrust dimension they will grind in and wondered if anyone here might have gotten a Scat crank and measured it and know what they typically run and whether that works well or not? I have the opportunity to have them grind that to my spec since its a raw casting. Possibly make other small beneficial changes too? Thanks for any input ... will be sending form shortly.
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Old 10-08-17, 07:12PM   #9
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Keith can I ask what your paying for it. If you don't want to I understand
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Old 05-07-18, 08:45PM   #10
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Hi Thomas
Didn't really want to quote price as I thought I was getting a good deal, and might possibly want another one but really I'm just paying normal price for a 4340 forged baseline crank plus a rod journal resize. 1136.00 It is a custom new forged crank that may get me my zero-deck, log-rod, light-piston 444ci/400 conversion with parts I had on hand.
Originally I wanted to downsize mains on a factory crank to obtain my zero-deck stroke of 4.040 since the block has been machined for a 10.220 deck height with 1.300CH pistons and 6.900 H-beam rods. Easier with the larger Pontiac rod journal 2.250. But through several years of being in the "the woods" waiting for a crank or getting ripped off, my experience is no one really wants to do that any more .. its hard enough to find someone who wants to modify a rod journal. I found Scat would grind rod journals on their forged Pontiac cranks which all have BBC journals. I decided to grind rod journals to -.020 to obtain room enough for stroke increase. Bought bearings and had them massaged by Paul(thrust surfaces), cryo'd and calico CT-1 treated. Then learned that most crank shops limit offset to 80% of journal reduction when Scat balked at grind time and wanted me to reduce journals further to -.030 for .020 offset. We are now negotiating but to get-r-dun I have suggested they grind to 80% journal reduction(offset of .016 for if they cant get uniform fillet integrity at desired .020) This will leave me down-hole .008 .. not what I want but can live with that if I have to. They had previously told me since this job just so happens to be being done from a raw forging I could specify thrust and journal grinds AND enough excess material would exist to get the offset and stroke I desired(4.040). But I guess in order to know for sure the raw forging has to be jigged and measured, which costs time=money and you don't normally do till you are actually going to grind it ... once on machine if a no-go then you just wasted $. Or grind something unsatisfactory which I don't think Scat would do.

I want to do a max effort factory crankshaft using the BME rods I have next. I have a nice straight 400N crank. Victor intake and Ultradyne NR100 solid roller possibly. For the little Astre possibly. I believe the fillet strength I can add by grinding full .125 radius rod journal fillets using the rods 2.100 SBC big-end diameter will be more important to the engine's success than additional stroke although 3.89 stroke sounds interesting I would have the factory crank nitrided specially here in MA and entire r/a cryo'd by Paul. Grinder instruction would be ..

1. Dead-on crank, perfect index and uniform .125 fillet radius
2. Offset what you can per #1.

Will order pistons(CH) after
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Old 05-09-18, 11:09AM   #11
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Sounds like a big waste of time and money for no reason or return.
Factory N crankshaft can take more than you think in its factory condition.
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Old 05-09-18, 12:01PM   #12
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Original post was a question on how to increase durability of a factory crank in a performance application. Increasing fillet radius puts more meat/strength in the area most likely to crack on any crankshaft. Bill Miller personally spec'd the .125 radius in a phone conversation I had with him to use with his rods. Nitriding to harden surface and cryo'ing to relieve stress and strengthen seem to make sense to me for the foundation of something I desire to maximize performance in. Was thinking of trying to polish oil passages with a flexible rod polisher as well as manicuring the oil entrance/exit holes. Oil type and bearing clearance important as well. I have 2 bent large journal cranks, one with a trashed rod journal and one just bent, 1 bent 389 crank among my collection of mostly straight cranks. If I do up a factory crank for max performance I will try to lighten my r/a members to load the crank less as well as all I just mentioned. I believe factory cranks are durable and would like to actually push one's limits with forced induction. Not sure I'll get to that, have to finish my Kool-Aid 444 first.
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Old 05-09-18, 05:57PM   #13
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I have BME rods in my engine that's been together and running for 5 years. Nothing special done to the factory N crank.
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Old 05-09-18, 07:20PM   #14
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Different strokes for different folks I guess. I don't have enough experience to preach to anyone Do you pound on motor or is it a streetcar? Just curious. I choose to follow Bill Miller's recommendations to the letter when I get to using his stuff on bearings and clearances. My reading on crankshafts leads me to believe the mods I listed would help most cranks including factory Pontiac. Not experience. The CB743HN bearings I'm using on Kool-Aid may have slightly less load bearing surface area than a standard bearing tho the notch in them might follow the radius enough to bear alittle load, so increased radius strength might be offset by decreased bearing load ability. Still don't know whether rod journals going away is caused more by lack of lubrication or radius failure. I didn't bend the bent large journal cranks I have ... just bought cheap thinking I might be able to bring them back for use with reduced rod journal and/or mains. Also think reducing rod journal to SBC dimension would not have a strength issue as Butler says, SBC's with smaller mains seem to live quite well and are made with the same materials and skinnier spine. They do have a lighter bob-weight but that can be approached using lighter premium components in Pontiac size. My BME 6.66 rods weigh the same as the I-beam SCAT 6.0 rods for my sbc. My Ross piston-pin combination in Kool-Aid is is 100 gms lighter than the Mahle sbc combo I was going to use in my sbc endeavor. Are your BME rods sbc size big-end? Didn't know they made BME in Pontiac big-end size.


My thought was to really beef the bottom so I could do anything on the top and not worry about it. If I take the trouble to rebuild a motor for performance use I will always try to do that.
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Old 05-09-18, 07:40PM   #15
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BME Rods here...BBC big end. 9270 SBC pin.
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