Crank thrust opinions
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Crank thrust opinions

  1. #1
    Senior Member jetboatperformance's Avatar
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    Default Crank thrust opinions

    Yeah I know I seem to have the worst luck with machine shops , thought this latest one was the answer to my woes ! I honestly want candid opinions from you folks and truly respect your what you have to say here , not only on the machining and technical aspects of this issue but also on the "are we being too picky" aspect ..... here goes this is a BBF stroker Alum head roller deal we're building for a customer 3k miles away , our machine shop rejected the first block for a casting flaw we agreed , they (new shop) had this BBF block for us we said go ahead lets mag check, bore hone and line bore! ,we get the motor bare block prepped and ready to go and this week Josh starts to mock up the crank (mind you they (new shop) knew it was a marine motor ) we found the main brg clearance were way too skinny sooooo we bought .001 + US mains Now we're where we want to be clearance wise and we move forward (they said sorry on the line hone "we put the LB at med spec " ............ Now a whole new issue, little to no crank thrust ??? wtf ?? so we measure and check and measure and check and measure and check.... heres what we got today IF you lay the crank in the uppers and simply check (dial indicate) its perfect BUT when you bolt up and torque the lowers you have little to no thrust clearance (dial indicated) maybe .003+ "pried" forward , so we measure some more heres what we found , we belive the center thrust main cap was mismachined during line bore (at very slight forward angle) please excuse my crappy drawing below , the owner of the machine shop came up today dicked with this deal and declared it "normal" we disagree.
    The way this looks on this block is quite simply that the thrust WILL NOT be controlled 360 degrees (or even close to it) as was designed but rather by that one high spot at 6 oclock position (possibly .003) retorically speaking was not the thrust control on this engine/crank not designed to work 360 degrees of rotation ?? We say the thrust clearance between this rather expensive crank and the thrust bearing should be consistant all around , shop says were nuts any thoughts Tom






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    The thrust cap was not ground square to the mating surface. When we grind the caps we machine the side of the cap square to the mating surface before we grind the mating surface. This is even more pronounced on the 460 because the main is so big. There is also less side surface on the cap than on a chevy. Now that it is not square it is tough to repair. You cant just keep grinding the caps wthout moving the center line of the crank. I agree you have a problem and it isnt good...
    Mike

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    i'm back!! 1QuickCP's Avatar
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    Yup I agree, they ground the cap surface not 90* with the sides. Lay cap on surface plate and run good dial indicator up side..needle should NOT move if its square. You can do this in a bridgeport as well, clamp in vise and mount indicator on quill, id use a starret best test, .0001 lol bet that needle jumps a bit.

    anyway...not good at all. Now looking at your drawing if its .002 off at the top, id very carefully cut each cap side .001..should correct it. dykem the cap, bolt to angle plate in mill, indicate and flycut till it cleans up. its not going to take much at all and you should end up with .006 everywhere You cant cut the cap base as its 90 to the linebore now, just the sides are messed up. .002 is not much to cut/grind off.


    im also assuming you snugged up the cap and gave the crank a good smack with the old rubber hammer then torqued. and checked.
    Last edited by 1QuickCP; 07-24-2010 at 09:33 PM.

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  6. #4
    gn7
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    Thats my take. Its either the block(doubtfull) or the cap mating surface isn't square with the sides/bore. Compared to a early SBC, a BBF is way more critical due to the hieght of the cap, and the size of the bore. Maybe an umolested replacement cap at best, or an aftermaket cap will save it. But no matter, it will need to be re-bored. Possibly honed if using a stock cap that is close. Even if it didn't lean, the two thrust surfaces are not aligned. Just a bad situation all around.



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    When you installed the center main cap, did you force the crank forward and back before tightening, to center the cap? I see you measured one side of the thrust on the main cap, Did you measure the other side to verify the same condition exists on both sides? How are you measuring these clearances you're describing?
    Before you start playing the blame game, make sure you have all your variables accounted for. Take a C-mic (you know how to use a mic, right?) and measure the thickness of the flange on the thrust bearing. You might be surprised at how much it varies. It's not uncommon to have to work on the thrust surface. Your method of checking really proves nothing about the cap or how it sits in the block.
    One more thing...bearing clearances are not marine application sensitive. Other clearances are, but not bearing clearances.

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    gn7
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    Quote Originally Posted by 1QuickCP View Post
    Yup I agree, they ground the cap surface not 90* with the sides. Lay cap on surface plate and run good dial indicator up side..needle should NOT move if its square. You can do this in a bridgeport as well, clamp in vise and mount indicator on quill, id use a starret best test, .0001 lol bet that needle jumps a bit.

    anyway...not good at all.
    I'll bet he can see that with the cap on a surface plate a good machinist square.



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    Quote Originally Posted by 1QuickCP View Post
    Yup I agree, they ground the cap surface not 90* with the sides. Lay cap on surface plate and run good dial indicator up side..needle should NOT move if its square. You can do this in a bridgeport as well, clamp in vise and mount indicator on quill, id use a starret best test, .0001 lol bet that needle jumps a bit.

    anyway...not good at all. Now looking at your drawing if its .002 off at the top, id very carefully cut each cap side .001..should correct it. dykem the cap, bolt to angle plate in mill, indicate and flycut till it cleans up. its not going to take much at all and you should end up with .006 everywhere.
    LOL...you're kidding, right?

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    you could pull .001 off each side, takes a good machine tho. also how much thrust does it have with the cap off?

    sorry, brain fart, .002 on 1 side then .002 opposite side and thats to much so scratch my idea
    Last edited by 1QuickCP; 07-24-2010 at 09:38 PM.

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    Its sounds like the cap is not 90 degrees to the mating surface. Its ovious the cap has to be faced before the mating surface is ground, BUT first you have know your cap grinder is set up correct, this done with the horse shoe and shims and a magic marker. And since you asked you better debur the bottom of the bores.

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    Quote Originally Posted by gn7 View Post
    I'll bet he can see that with the cap on a surface plate a good machinist square.
    We pulled the crank reinstalled the center main cap with bearing and just for kicks we used our "Carb part" plate glass surfacer plate on the front and rear of the "installed thrust" surfaces and read the same results as posted (actually had visable space/gap at the sweep from about 4 to 8 oclock Tom

  13. #11
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    I'll say it agian...you're all jumping to huge conclusions.
    I'll ask this...how many of you have actually checked the radial clearance on a thrust surface? How many have actually measured the thickness of the thrust flange with a mic? (not calipers). I've never bothered...if I get the thrust clearance I need, I usually don't give it any more thought, but this last engine I did was a thrust bearing nightmare. I will never look at thrust bearings the same again.

    If this was my engine, as long as the main bore was correct, I'd take the thrust bearing to some 360g sand paper on a piece of glass and flat sand that thrust surface till I got the clearances I needed, and not lose a minute's sleep over it.
    Knowinf the machine shop that did this, it's not likely to be the cap.
    Last edited by scott foxwell; 07-24-2010 at 09:49 PM.

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    The shop was nice enough to come to our shop and verify all this as we watched (brought thier own precision instruments) Tom

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    Quote Originally Posted by jetboatperformance View Post
    The shop was nice enough to come to our shop and verify all this and then suggested (brought thier own instruments) Tom
    What was their suggestion?

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    gn7
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    I have to agree with Steel on one resect. If the crank only rode on the block half of the thrust, would you really care. Problem here is, it is being limited at the top of the cap on one side, and the block on the other. If the block clearence are fine, you could just sand the cap bearing on the side causing the problem. The bearing will still control thrust for and aft. Its not as if this thing is ever going to see a 3000 lb clutch.



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